xref: /llvm-project/llvm/lib/Transforms/Scalar/JumpThreading.cpp (revision 3c603024bbaaeb41c105aa0aa922e5b08208533c)
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/IntrinsicInst.h"
17 #include "llvm/LLVMContext.h"
18 #include "llvm/Pass.h"
19 #include "llvm/Analysis/InstructionSimplify.h"
20 #include "llvm/Analysis/LazyValueInfo.h"
21 #include "llvm/Analysis/Loads.h"
22 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
23 #include "llvm/Transforms/Utils/Local.h"
24 #include "llvm/Transforms/Utils/SSAUpdater.h"
25 #include "llvm/Target/TargetData.h"
26 #include "llvm/ADT/DenseMap.h"
27 #include "llvm/ADT/Statistic.h"
28 #include "llvm/ADT/STLExtras.h"
29 #include "llvm/ADT/SmallPtrSet.h"
30 #include "llvm/ADT/SmallSet.h"
31 #include "llvm/Support/CommandLine.h"
32 #include "llvm/Support/Debug.h"
33 #include "llvm/Support/ValueHandle.h"
34 #include "llvm/Support/raw_ostream.h"
35 using namespace llvm;
36 
37 STATISTIC(NumThreads, "Number of jumps threaded");
38 STATISTIC(NumFolds,   "Number of terminators folded");
39 STATISTIC(NumDupes,   "Number of branch blocks duplicated to eliminate phi");
40 
41 static cl::opt<unsigned>
42 Threshold("jump-threading-threshold",
43           cl::desc("Max block size to duplicate for jump threading"),
44           cl::init(6), cl::Hidden);
45 
46 // Turn on use of LazyValueInfo.
47 static cl::opt<bool>
48 EnableLVI("enable-jump-threading-lvi",
49           cl::desc("Use LVI for jump threading"),
50           cl::init(false),
51           cl::ReallyHidden);
52 
53 
54 
55 namespace {
56   /// This pass performs 'jump threading', which looks at blocks that have
57   /// multiple predecessors and multiple successors.  If one or more of the
58   /// predecessors of the block can be proven to always jump to one of the
59   /// successors, we forward the edge from the predecessor to the successor by
60   /// duplicating the contents of this block.
61   ///
62   /// An example of when this can occur is code like this:
63   ///
64   ///   if () { ...
65   ///     X = 4;
66   ///   }
67   ///   if (X < 3) {
68   ///
69   /// In this case, the unconditional branch at the end of the first if can be
70   /// revectored to the false side of the second if.
71   ///
72   class JumpThreading : public FunctionPass {
73     TargetData *TD;
74     LazyValueInfo *LVI;
75 #ifdef NDEBUG
76     SmallPtrSet<BasicBlock*, 16> LoopHeaders;
77 #else
78     SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
79 #endif
80   public:
81     static char ID; // Pass identification
82     JumpThreading() : FunctionPass(ID) {}
83 
84     bool runOnFunction(Function &F);
85 
86     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
87       if (EnableLVI)
88         AU.addRequired<LazyValueInfo>();
89     }
90 
91     void FindLoopHeaders(Function &F);
92     bool ProcessBlock(BasicBlock *BB);
93     bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
94                     BasicBlock *SuccBB);
95     bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
96                                   const SmallVectorImpl<BasicBlock *> &PredBBs);
97 
98     typedef SmallVectorImpl<std::pair<ConstantInt*,
99                                       BasicBlock*> > PredValueInfo;
100 
101     bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
102                                          PredValueInfo &Result);
103     bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB);
104 
105 
106     bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
107     bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
108 
109     bool ProcessBranchOnPHI(PHINode *PN);
110     bool ProcessBranchOnXOR(BinaryOperator *BO);
111 
112     bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
113   };
114 }
115 
116 char JumpThreading::ID = 0;
117 INITIALIZE_PASS(JumpThreading, "jump-threading",
118                 "Jump Threading", false, false);
119 
120 // Public interface to the Jump Threading pass
121 FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
122 
123 /// runOnFunction - Top level algorithm.
124 ///
125 bool JumpThreading::runOnFunction(Function &F) {
126   DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
127   TD = getAnalysisIfAvailable<TargetData>();
128   LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0;
129 
130   FindLoopHeaders(F);
131 
132   bool Changed, EverChanged = false;
133   do {
134     Changed = false;
135     for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
136       BasicBlock *BB = I;
137       // Thread all of the branches we can over this block.
138       while (ProcessBlock(BB))
139         Changed = true;
140 
141       ++I;
142 
143       // If the block is trivially dead, zap it.  This eliminates the successor
144       // edges which simplifies the CFG.
145       if (pred_begin(BB) == pred_end(BB) &&
146           BB != &BB->getParent()->getEntryBlock()) {
147         DEBUG(dbgs() << "  JT: Deleting dead block '" << BB->getName()
148               << "' with terminator: " << *BB->getTerminator() << '\n');
149         LoopHeaders.erase(BB);
150         DeleteDeadBlock(BB);
151         Changed = true;
152       } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
153         // Can't thread an unconditional jump, but if the block is "almost
154         // empty", we can replace uses of it with uses of the successor and make
155         // this dead.
156         if (BI->isUnconditional() &&
157             BB != &BB->getParent()->getEntryBlock()) {
158           BasicBlock::iterator BBI = BB->getFirstNonPHI();
159           // Ignore dbg intrinsics.
160           while (isa<DbgInfoIntrinsic>(BBI))
161             ++BBI;
162           // If the terminator is the only non-phi instruction, try to nuke it.
163           if (BBI->isTerminator()) {
164             // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
165             // block, we have to make sure it isn't in the LoopHeaders set.  We
166             // reinsert afterward if needed.
167             bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
168             BasicBlock *Succ = BI->getSuccessor(0);
169 
170             if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
171               Changed = true;
172               // If we deleted BB and BB was the header of a loop, then the
173               // successor is now the header of the loop.
174               BB = Succ;
175             }
176 
177             if (ErasedFromLoopHeaders)
178               LoopHeaders.insert(BB);
179           }
180         }
181       }
182     }
183     EverChanged |= Changed;
184   } while (Changed);
185 
186   LoopHeaders.clear();
187   return EverChanged;
188 }
189 
190 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
191 /// thread across it.
192 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
193   /// Ignore PHI nodes, these will be flattened when duplication happens.
194   BasicBlock::const_iterator I = BB->getFirstNonPHI();
195 
196   // FIXME: THREADING will delete values that are just used to compute the
197   // branch, so they shouldn't count against the duplication cost.
198 
199 
200   // Sum up the cost of each instruction until we get to the terminator.  Don't
201   // include the terminator because the copy won't include it.
202   unsigned Size = 0;
203   for (; !isa<TerminatorInst>(I); ++I) {
204     // Debugger intrinsics don't incur code size.
205     if (isa<DbgInfoIntrinsic>(I)) continue;
206 
207     // If this is a pointer->pointer bitcast, it is free.
208     if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
209       continue;
210 
211     // All other instructions count for at least one unit.
212     ++Size;
213 
214     // Calls are more expensive.  If they are non-intrinsic calls, we model them
215     // as having cost of 4.  If they are a non-vector intrinsic, we model them
216     // as having cost of 2 total, and if they are a vector intrinsic, we model
217     // them as having cost 1.
218     if (const CallInst *CI = dyn_cast<CallInst>(I)) {
219       if (!isa<IntrinsicInst>(CI))
220         Size += 3;
221       else if (!CI->getType()->isVectorTy())
222         Size += 1;
223     }
224   }
225 
226   // Threading through a switch statement is particularly profitable.  If this
227   // block ends in a switch, decrease its cost to make it more likely to happen.
228   if (isa<SwitchInst>(I))
229     Size = Size > 6 ? Size-6 : 0;
230 
231   return Size;
232 }
233 
234 /// FindLoopHeaders - We do not want jump threading to turn proper loop
235 /// structures into irreducible loops.  Doing this breaks up the loop nesting
236 /// hierarchy and pessimizes later transformations.  To prevent this from
237 /// happening, we first have to find the loop headers.  Here we approximate this
238 /// by finding targets of backedges in the CFG.
239 ///
240 /// Note that there definitely are cases when we want to allow threading of
241 /// edges across a loop header.  For example, threading a jump from outside the
242 /// loop (the preheader) to an exit block of the loop is definitely profitable.
243 /// It is also almost always profitable to thread backedges from within the loop
244 /// to exit blocks, and is often profitable to thread backedges to other blocks
245 /// within the loop (forming a nested loop).  This simple analysis is not rich
246 /// enough to track all of these properties and keep it up-to-date as the CFG
247 /// mutates, so we don't allow any of these transformations.
248 ///
249 void JumpThreading::FindLoopHeaders(Function &F) {
250   SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
251   FindFunctionBackedges(F, Edges);
252 
253   for (unsigned i = 0, e = Edges.size(); i != e; ++i)
254     LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
255 }
256 
257 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
258 /// if we can infer that the value is a known ConstantInt in any of our
259 /// predecessors.  If so, return the known list of value and pred BB in the
260 /// result vector.  If a value is known to be undef, it is returned as null.
261 ///
262 /// This returns true if there were any known values.
263 ///
264 bool JumpThreading::
265 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
266   // If V is a constantint, then it is known in all predecessors.
267   if (isa<ConstantInt>(V) || isa<UndefValue>(V)) {
268     ConstantInt *CI = dyn_cast<ConstantInt>(V);
269 
270     for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
271       Result.push_back(std::make_pair(CI, *PI));
272     return true;
273   }
274 
275   // If V is a non-instruction value, or an instruction in a different block,
276   // then it can't be derived from a PHI.
277   Instruction *I = dyn_cast<Instruction>(V);
278   if (I == 0 || I->getParent() != BB) {
279 
280     // Okay, if this is a live-in value, see if it has a known value at the end
281     // of any of our predecessors.
282     //
283     // FIXME: This should be an edge property, not a block end property.
284     /// TODO: Per PR2563, we could infer value range information about a
285     /// predecessor based on its terminator.
286     //
287     if (LVI) {
288       // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
289       // "I" is a non-local compare-with-a-constant instruction.  This would be
290       // able to handle value inequalities better, for example if the compare is
291       // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
292       // Perhaps getConstantOnEdge should be smart enough to do this?
293 
294       for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
295         BasicBlock *P = *PI;
296         // If the value is known by LazyValueInfo to be a constant in a
297         // predecessor, use that information to try to thread this block.
298         Constant *PredCst = LVI->getConstantOnEdge(V, P, BB);
299         if (PredCst == 0 ||
300             (!isa<ConstantInt>(PredCst) && !isa<UndefValue>(PredCst)))
301           continue;
302 
303         Result.push_back(std::make_pair(dyn_cast<ConstantInt>(PredCst), P));
304       }
305 
306       return !Result.empty();
307     }
308 
309     return false;
310   }
311 
312   /// If I is a PHI node, then we know the incoming values for any constants.
313   if (PHINode *PN = dyn_cast<PHINode>(I)) {
314     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
315       Value *InVal = PN->getIncomingValue(i);
316       if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) {
317         ConstantInt *CI = dyn_cast<ConstantInt>(InVal);
318         Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i)));
319       }
320     }
321     return !Result.empty();
322   }
323 
324   SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals;
325 
326   // Handle some boolean conditions.
327   if (I->getType()->getPrimitiveSizeInBits() == 1) {
328     // X | true -> true
329     // X & false -> false
330     if (I->getOpcode() == Instruction::Or ||
331         I->getOpcode() == Instruction::And) {
332       ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals);
333       ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals);
334 
335       if (LHSVals.empty() && RHSVals.empty())
336         return false;
337 
338       ConstantInt *InterestingVal;
339       if (I->getOpcode() == Instruction::Or)
340         InterestingVal = ConstantInt::getTrue(I->getContext());
341       else
342         InterestingVal = ConstantInt::getFalse(I->getContext());
343 
344       SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
345 
346       // Scan for the sentinel.  If we find an undef, force it to the
347       // interesting value: x|undef -> true and x&undef -> false.
348       for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
349         if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0) {
350           Result.push_back(LHSVals[i]);
351           Result.back().first = InterestingVal;
352           LHSKnownBBs.insert(LHSVals[i].second);
353         }
354       for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
355         if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0) {
356           // If we already inferred a value for this block on the LHS, don't
357           // re-add it.
358           if (!LHSKnownBBs.count(RHSVals[i].second)) {
359             Result.push_back(RHSVals[i]);
360             Result.back().first = InterestingVal;
361           }
362         }
363       return !Result.empty();
364     }
365 
366     // Handle the NOT form of XOR.
367     if (I->getOpcode() == Instruction::Xor &&
368         isa<ConstantInt>(I->getOperand(1)) &&
369         cast<ConstantInt>(I->getOperand(1))->isOne()) {
370       ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result);
371       if (Result.empty())
372         return false;
373 
374       // Invert the known values.
375       for (unsigned i = 0, e = Result.size(); i != e; ++i)
376         if (Result[i].first)
377           Result[i].first =
378             cast<ConstantInt>(ConstantExpr::getNot(Result[i].first));
379       return true;
380     }
381   }
382 
383   // Handle compare with phi operand, where the PHI is defined in this block.
384   if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
385     PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
386     if (PN && PN->getParent() == BB) {
387       // We can do this simplification if any comparisons fold to true or false.
388       // See if any do.
389       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
390         BasicBlock *PredBB = PN->getIncomingBlock(i);
391         Value *LHS = PN->getIncomingValue(i);
392         Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
393 
394         Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD);
395         if (Res == 0) {
396           if (!LVI || !isa<Constant>(RHS))
397             continue;
398 
399           LazyValueInfo::Tristate
400             ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
401                                            cast<Constant>(RHS), PredBB, BB);
402           if (ResT == LazyValueInfo::Unknown)
403             continue;
404           Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
405         }
406 
407         if (isa<UndefValue>(Res))
408           Result.push_back(std::make_pair((ConstantInt*)0, PredBB));
409         else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res))
410           Result.push_back(std::make_pair(CI, PredBB));
411       }
412 
413       return !Result.empty();
414     }
415 
416 
417     // If comparing a live-in value against a constant, see if we know the
418     // live-in value on any predecessors.
419     if (LVI && isa<Constant>(Cmp->getOperand(1)) &&
420         Cmp->getType()->isIntegerTy() && // Not vector compare.
421         (!isa<Instruction>(Cmp->getOperand(0)) ||
422          cast<Instruction>(Cmp->getOperand(0))->getParent() != BB)) {
423       Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
424 
425       for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
426         BasicBlock *P = *PI;
427         // If the value is known by LazyValueInfo to be a constant in a
428         // predecessor, use that information to try to thread this block.
429         LazyValueInfo::Tristate
430           Res = LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
431                                         RHSCst, P, BB);
432         if (Res == LazyValueInfo::Unknown)
433           continue;
434 
435         Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
436         Result.push_back(std::make_pair(cast<ConstantInt>(ResC), P));
437       }
438 
439       return !Result.empty();
440     }
441   }
442   return false;
443 }
444 
445 
446 
447 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
448 /// in an undefined jump, decide which block is best to revector to.
449 ///
450 /// Since we can pick an arbitrary destination, we pick the successor with the
451 /// fewest predecessors.  This should reduce the in-degree of the others.
452 ///
453 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
454   TerminatorInst *BBTerm = BB->getTerminator();
455   unsigned MinSucc = 0;
456   BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
457   // Compute the successor with the minimum number of predecessors.
458   unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
459   for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
460     TestBB = BBTerm->getSuccessor(i);
461     unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
462     if (NumPreds < MinNumPreds)
463       MinSucc = i;
464   }
465 
466   return MinSucc;
467 }
468 
469 /// ProcessBlock - If there are any predecessors whose control can be threaded
470 /// through to a successor, transform them now.
471 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
472   // If the block is trivially dead, just return and let the caller nuke it.
473   // This simplifies other transformations.
474   if (pred_begin(BB) == pred_end(BB) &&
475       BB != &BB->getParent()->getEntryBlock())
476     return false;
477 
478   // If this block has a single predecessor, and if that pred has a single
479   // successor, merge the blocks.  This encourages recursive jump threading
480   // because now the condition in this block can be threaded through
481   // predecessors of our predecessor block.
482   if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
483     if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
484         SinglePred != BB) {
485       // If SinglePred was a loop header, BB becomes one.
486       if (LoopHeaders.erase(SinglePred))
487         LoopHeaders.insert(BB);
488 
489       // Remember if SinglePred was the entry block of the function.  If so, we
490       // will need to move BB back to the entry position.
491       bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
492       MergeBasicBlockIntoOnlyPred(BB);
493 
494       if (isEntry && BB != &BB->getParent()->getEntryBlock())
495         BB->moveBefore(&BB->getParent()->getEntryBlock());
496       return true;
497     }
498   }
499 
500   // Look to see if the terminator is a branch of switch, if not we can't thread
501   // it.
502   Value *Condition;
503   if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
504     // Can't thread an unconditional jump.
505     if (BI->isUnconditional()) return false;
506     Condition = BI->getCondition();
507   } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
508     Condition = SI->getCondition();
509   else
510     return false; // Must be an invoke.
511 
512   // If the terminator of this block is branching on a constant, simplify the
513   // terminator to an unconditional branch.  This can occur due to threading in
514   // other blocks.
515   if (isa<ConstantInt>(Condition)) {
516     DEBUG(dbgs() << "  In block '" << BB->getName()
517           << "' folding terminator: " << *BB->getTerminator() << '\n');
518     ++NumFolds;
519     ConstantFoldTerminator(BB);
520     return true;
521   }
522 
523   // If the terminator is branching on an undef, we can pick any of the
524   // successors to branch to.  Let GetBestDestForJumpOnUndef decide.
525   if (isa<UndefValue>(Condition)) {
526     unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
527 
528     // Fold the branch/switch.
529     TerminatorInst *BBTerm = BB->getTerminator();
530     for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
531       if (i == BestSucc) continue;
532       RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD);
533     }
534 
535     DEBUG(dbgs() << "  In block '" << BB->getName()
536           << "' folding undef terminator: " << *BBTerm << '\n');
537     BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
538     BBTerm->eraseFromParent();
539     return true;
540   }
541 
542   Instruction *CondInst = dyn_cast<Instruction>(Condition);
543 
544   // If the condition is an instruction defined in another block, see if a
545   // predecessor has the same condition:
546   //     br COND, BBX, BBY
547   //  BBX:
548   //     br COND, BBZ, BBW
549   if (!LVI &&
550       !Condition->hasOneUse() && // Multiple uses.
551       (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
552     pred_iterator PI = pred_begin(BB), E = pred_end(BB);
553     if (isa<BranchInst>(BB->getTerminator())) {
554       for (; PI != E; ++PI) {
555         BasicBlock *P = *PI;
556         if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator()))
557           if (PBI->isConditional() && PBI->getCondition() == Condition &&
558               ProcessBranchOnDuplicateCond(P, BB))
559             return true;
560       }
561     } else {
562       assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
563       for (; PI != E; ++PI) {
564         BasicBlock *P = *PI;
565         if (SwitchInst *PSI = dyn_cast<SwitchInst>(P->getTerminator()))
566           if (PSI->getCondition() == Condition &&
567               ProcessSwitchOnDuplicateCond(P, BB))
568             return true;
569       }
570     }
571   }
572 
573   // All the rest of our checks depend on the condition being an instruction.
574   if (CondInst == 0) {
575     // FIXME: Unify this with code below.
576     if (LVI && ProcessThreadableEdges(Condition, BB))
577       return true;
578     return false;
579   }
580 
581 
582   if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
583     if (!LVI &&
584         (!isa<PHINode>(CondCmp->getOperand(0)) ||
585          cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB)) {
586       // If we have a comparison, loop over the predecessors to see if there is
587       // a condition with a lexically identical value.
588       pred_iterator PI = pred_begin(BB), E = pred_end(BB);
589       for (; PI != E; ++PI) {
590         BasicBlock *P = *PI;
591         if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator()))
592           if (PBI->isConditional() && P != BB) {
593             if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) {
594               if (CI->getOperand(0) == CondCmp->getOperand(0) &&
595                   CI->getOperand(1) == CondCmp->getOperand(1) &&
596                   CI->getPredicate() == CondCmp->getPredicate()) {
597                 // TODO: Could handle things like (x != 4) --> (x == 17)
598                 if (ProcessBranchOnDuplicateCond(P, BB))
599                   return true;
600               }
601             }
602           }
603       }
604     }
605   }
606 
607   // Check for some cases that are worth simplifying.  Right now we want to look
608   // for loads that are used by a switch or by the condition for the branch.  If
609   // we see one, check to see if it's partially redundant.  If so, insert a PHI
610   // which can then be used to thread the values.
611   //
612   Value *SimplifyValue = CondInst;
613   if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
614     if (isa<Constant>(CondCmp->getOperand(1)))
615       SimplifyValue = CondCmp->getOperand(0);
616 
617   // TODO: There are other places where load PRE would be profitable, such as
618   // more complex comparisons.
619   if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
620     if (SimplifyPartiallyRedundantLoad(LI))
621       return true;
622 
623 
624   // Handle a variety of cases where we are branching on something derived from
625   // a PHI node in the current block.  If we can prove that any predecessors
626   // compute a predictable value based on a PHI node, thread those predecessors.
627   //
628   if (ProcessThreadableEdges(CondInst, BB))
629     return true;
630 
631   // If this is an otherwise-unfoldable branch on a phi node in the current
632   // block, see if we can simplify.
633   if (PHINode *PN = dyn_cast<PHINode>(CondInst))
634     if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
635       return ProcessBranchOnPHI(PN);
636 
637 
638   // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
639   if (CondInst->getOpcode() == Instruction::Xor &&
640       CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
641     return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
642 
643 
644   // TODO: If we have: "br (X > 0)"  and we have a predecessor where we know
645   // "(X == 4)", thread through this block.
646 
647   return false;
648 }
649 
650 /// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
651 /// block that jump on exactly the same condition.  This means that we almost
652 /// always know the direction of the edge in the DESTBB:
653 ///  PREDBB:
654 ///     br COND, DESTBB, BBY
655 ///  DESTBB:
656 ///     br COND, BBZ, BBW
657 ///
658 /// If DESTBB has multiple predecessors, we can't just constant fold the branch
659 /// in DESTBB, we have to thread over it.
660 bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
661                                                  BasicBlock *BB) {
662   BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
663 
664   // If both successors of PredBB go to DESTBB, we don't know anything.  We can
665   // fold the branch to an unconditional one, which allows other recursive
666   // simplifications.
667   bool BranchDir;
668   if (PredBI->getSuccessor(1) != BB)
669     BranchDir = true;
670   else if (PredBI->getSuccessor(0) != BB)
671     BranchDir = false;
672   else {
673     DEBUG(dbgs() << "  In block '" << PredBB->getName()
674           << "' folding terminator: " << *PredBB->getTerminator() << '\n');
675     ++NumFolds;
676     ConstantFoldTerminator(PredBB);
677     return true;
678   }
679 
680   BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
681 
682   // If the dest block has one predecessor, just fix the branch condition to a
683   // constant and fold it.
684   if (BB->getSinglePredecessor()) {
685     DEBUG(dbgs() << "  In block '" << BB->getName()
686           << "' folding condition to '" << BranchDir << "': "
687           << *BB->getTerminator() << '\n');
688     ++NumFolds;
689     Value *OldCond = DestBI->getCondition();
690     DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
691                                           BranchDir));
692     // Delete dead instructions before we fold the branch.  Folding the branch
693     // can eliminate edges from the CFG which can end up deleting OldCond.
694     RecursivelyDeleteTriviallyDeadInstructions(OldCond);
695     ConstantFoldTerminator(BB);
696     return true;
697   }
698 
699 
700   // Next, figure out which successor we are threading to.
701   BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
702 
703   SmallVector<BasicBlock*, 2> Preds;
704   Preds.push_back(PredBB);
705 
706   // Ok, try to thread it!
707   return ThreadEdge(BB, Preds, SuccBB);
708 }
709 
710 /// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
711 /// block that switch on exactly the same condition.  This means that we almost
712 /// always know the direction of the edge in the DESTBB:
713 ///  PREDBB:
714 ///     switch COND [... DESTBB, BBY ... ]
715 ///  DESTBB:
716 ///     switch COND [... BBZ, BBW ]
717 ///
718 /// Optimizing switches like this is very important, because simplifycfg builds
719 /// switches out of repeated 'if' conditions.
720 bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
721                                                  BasicBlock *DestBB) {
722   // Can't thread edge to self.
723   if (PredBB == DestBB)
724     return false;
725 
726   SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
727   SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
728 
729   // There are a variety of optimizations that we can potentially do on these
730   // blocks: we order them from most to least preferable.
731 
732   // If DESTBB *just* contains the switch, then we can forward edges from PREDBB
733   // directly to their destination.  This does not introduce *any* code size
734   // growth.  Skip debug info first.
735   BasicBlock::iterator BBI = DestBB->begin();
736   while (isa<DbgInfoIntrinsic>(BBI))
737     BBI++;
738 
739   // FIXME: Thread if it just contains a PHI.
740   if (isa<SwitchInst>(BBI)) {
741     bool MadeChange = false;
742     // Ignore the default edge for now.
743     for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
744       ConstantInt *DestVal = DestSI->getCaseValue(i);
745       BasicBlock *DestSucc = DestSI->getSuccessor(i);
746 
747       // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'.  See if
748       // PredSI has an explicit case for it.  If so, forward.  If it is covered
749       // by the default case, we can't update PredSI.
750       unsigned PredCase = PredSI->findCaseValue(DestVal);
751       if (PredCase == 0) continue;
752 
753       // If PredSI doesn't go to DestBB on this value, then it won't reach the
754       // case on this condition.
755       if (PredSI->getSuccessor(PredCase) != DestBB &&
756           DestSI->getSuccessor(i) != DestBB)
757         continue;
758 
759       // Do not forward this if it already goes to this destination, this would
760       // be an infinite loop.
761       if (PredSI->getSuccessor(PredCase) == DestSucc)
762         continue;
763 
764       // Otherwise, we're safe to make the change.  Make sure that the edge from
765       // DestSI to DestSucc is not critical and has no PHI nodes.
766       DEBUG(dbgs() << "FORWARDING EDGE " << *DestVal << "   FROM: " << *PredSI);
767       DEBUG(dbgs() << "THROUGH: " << *DestSI);
768 
769       // If the destination has PHI nodes, just split the edge for updating
770       // simplicity.
771       if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
772         SplitCriticalEdge(DestSI, i, this);
773         DestSucc = DestSI->getSuccessor(i);
774       }
775       FoldSingleEntryPHINodes(DestSucc);
776       PredSI->setSuccessor(PredCase, DestSucc);
777       MadeChange = true;
778     }
779 
780     if (MadeChange)
781       return true;
782   }
783 
784   return false;
785 }
786 
787 
788 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
789 /// load instruction, eliminate it by replacing it with a PHI node.  This is an
790 /// important optimization that encourages jump threading, and needs to be run
791 /// interlaced with other jump threading tasks.
792 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
793   // Don't hack volatile loads.
794   if (LI->isVolatile()) return false;
795 
796   // If the load is defined in a block with exactly one predecessor, it can't be
797   // partially redundant.
798   BasicBlock *LoadBB = LI->getParent();
799   if (LoadBB->getSinglePredecessor())
800     return false;
801 
802   Value *LoadedPtr = LI->getOperand(0);
803 
804   // If the loaded operand is defined in the LoadBB, it can't be available.
805   // TODO: Could do simple PHI translation, that would be fun :)
806   if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
807     if (PtrOp->getParent() == LoadBB)
808       return false;
809 
810   // Scan a few instructions up from the load, to see if it is obviously live at
811   // the entry to its block.
812   BasicBlock::iterator BBIt = LI;
813 
814   if (Value *AvailableVal =
815         FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
816     // If the value if the load is locally available within the block, just use
817     // it.  This frequently occurs for reg2mem'd allocas.
818     //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
819 
820     // If the returned value is the load itself, replace with an undef. This can
821     // only happen in dead loops.
822     if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
823     LI->replaceAllUsesWith(AvailableVal);
824     LI->eraseFromParent();
825     return true;
826   }
827 
828   // Otherwise, if we scanned the whole block and got to the top of the block,
829   // we know the block is locally transparent to the load.  If not, something
830   // might clobber its value.
831   if (BBIt != LoadBB->begin())
832     return false;
833 
834 
835   SmallPtrSet<BasicBlock*, 8> PredsScanned;
836   typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
837   AvailablePredsTy AvailablePreds;
838   BasicBlock *OneUnavailablePred = 0;
839 
840   // If we got here, the loaded value is transparent through to the start of the
841   // block.  Check to see if it is available in any of the predecessor blocks.
842   for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
843        PI != PE; ++PI) {
844     BasicBlock *PredBB = *PI;
845 
846     // If we already scanned this predecessor, skip it.
847     if (!PredsScanned.insert(PredBB))
848       continue;
849 
850     // Scan the predecessor to see if the value is available in the pred.
851     BBIt = PredBB->end();
852     Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6);
853     if (!PredAvailable) {
854       OneUnavailablePred = PredBB;
855       continue;
856     }
857 
858     // If so, this load is partially redundant.  Remember this info so that we
859     // can create a PHI node.
860     AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
861   }
862 
863   // If the loaded value isn't available in any predecessor, it isn't partially
864   // redundant.
865   if (AvailablePreds.empty()) return false;
866 
867   // Okay, the loaded value is available in at least one (and maybe all!)
868   // predecessors.  If the value is unavailable in more than one unique
869   // predecessor, we want to insert a merge block for those common predecessors.
870   // This ensures that we only have to insert one reload, thus not increasing
871   // code size.
872   BasicBlock *UnavailablePred = 0;
873 
874   // If there is exactly one predecessor where the value is unavailable, the
875   // already computed 'OneUnavailablePred' block is it.  If it ends in an
876   // unconditional branch, we know that it isn't a critical edge.
877   if (PredsScanned.size() == AvailablePreds.size()+1 &&
878       OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
879     UnavailablePred = OneUnavailablePred;
880   } else if (PredsScanned.size() != AvailablePreds.size()) {
881     // Otherwise, we had multiple unavailable predecessors or we had a critical
882     // edge from the one.
883     SmallVector<BasicBlock*, 8> PredsToSplit;
884     SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
885 
886     for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
887       AvailablePredSet.insert(AvailablePreds[i].first);
888 
889     // Add all the unavailable predecessors to the PredsToSplit list.
890     for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
891          PI != PE; ++PI) {
892       BasicBlock *P = *PI;
893       // If the predecessor is an indirect goto, we can't split the edge.
894       if (isa<IndirectBrInst>(P->getTerminator()))
895         return false;
896 
897       if (!AvailablePredSet.count(P))
898         PredsToSplit.push_back(P);
899     }
900 
901     // Split them out to their own block.
902     UnavailablePred =
903       SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
904                              "thread-pre-split", this);
905   }
906 
907   // If the value isn't available in all predecessors, then there will be
908   // exactly one where it isn't available.  Insert a load on that edge and add
909   // it to the AvailablePreds list.
910   if (UnavailablePred) {
911     assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
912            "Can't handle critical edge here!");
913     Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
914                                  LI->getAlignment(),
915                                  UnavailablePred->getTerminator());
916     AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
917   }
918 
919   // Now we know that each predecessor of this block has a value in
920   // AvailablePreds, sort them for efficient access as we're walking the preds.
921   array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
922 
923   // Create a PHI node at the start of the block for the PRE'd load value.
924   PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
925   PN->takeName(LI);
926 
927   // Insert new entries into the PHI for each predecessor.  A single block may
928   // have multiple entries here.
929   for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
930        ++PI) {
931     BasicBlock *P = *PI;
932     AvailablePredsTy::iterator I =
933       std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
934                        std::make_pair(P, (Value*)0));
935 
936     assert(I != AvailablePreds.end() && I->first == P &&
937            "Didn't find entry for predecessor!");
938 
939     PN->addIncoming(I->second, I->first);
940   }
941 
942   //cerr << "PRE: " << *LI << *PN << "\n";
943 
944   LI->replaceAllUsesWith(PN);
945   LI->eraseFromParent();
946 
947   return true;
948 }
949 
950 /// FindMostPopularDest - The specified list contains multiple possible
951 /// threadable destinations.  Pick the one that occurs the most frequently in
952 /// the list.
953 static BasicBlock *
954 FindMostPopularDest(BasicBlock *BB,
955                     const SmallVectorImpl<std::pair<BasicBlock*,
956                                   BasicBlock*> > &PredToDestList) {
957   assert(!PredToDestList.empty());
958 
959   // Determine popularity.  If there are multiple possible destinations, we
960   // explicitly choose to ignore 'undef' destinations.  We prefer to thread
961   // blocks with known and real destinations to threading undef.  We'll handle
962   // them later if interesting.
963   DenseMap<BasicBlock*, unsigned> DestPopularity;
964   for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
965     if (PredToDestList[i].second)
966       DestPopularity[PredToDestList[i].second]++;
967 
968   // Find the most popular dest.
969   DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
970   BasicBlock *MostPopularDest = DPI->first;
971   unsigned Popularity = DPI->second;
972   SmallVector<BasicBlock*, 4> SamePopularity;
973 
974   for (++DPI; DPI != DestPopularity.end(); ++DPI) {
975     // If the popularity of this entry isn't higher than the popularity we've
976     // seen so far, ignore it.
977     if (DPI->second < Popularity)
978       ; // ignore.
979     else if (DPI->second == Popularity) {
980       // If it is the same as what we've seen so far, keep track of it.
981       SamePopularity.push_back(DPI->first);
982     } else {
983       // If it is more popular, remember it.
984       SamePopularity.clear();
985       MostPopularDest = DPI->first;
986       Popularity = DPI->second;
987     }
988   }
989 
990   // Okay, now we know the most popular destination.  If there is more than
991   // destination, we need to determine one.  This is arbitrary, but we need
992   // to make a deterministic decision.  Pick the first one that appears in the
993   // successor list.
994   if (!SamePopularity.empty()) {
995     SamePopularity.push_back(MostPopularDest);
996     TerminatorInst *TI = BB->getTerminator();
997     for (unsigned i = 0; ; ++i) {
998       assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
999 
1000       if (std::find(SamePopularity.begin(), SamePopularity.end(),
1001                     TI->getSuccessor(i)) == SamePopularity.end())
1002         continue;
1003 
1004       MostPopularDest = TI->getSuccessor(i);
1005       break;
1006     }
1007   }
1008 
1009   // Okay, we have finally picked the most popular destination.
1010   return MostPopularDest;
1011 }
1012 
1013 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) {
1014   // If threading this would thread across a loop header, don't even try to
1015   // thread the edge.
1016   if (LoopHeaders.count(BB))
1017     return false;
1018 
1019   SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues;
1020   if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues))
1021     return false;
1022   assert(!PredValues.empty() &&
1023          "ComputeValueKnownInPredecessors returned true with no values");
1024 
1025   DEBUG(dbgs() << "IN BB: " << *BB;
1026         for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1027           dbgs() << "  BB '" << BB->getName() << "': FOUND condition = ";
1028           if (PredValues[i].first)
1029             dbgs() << *PredValues[i].first;
1030           else
1031             dbgs() << "UNDEF";
1032           dbgs() << " for pred '" << PredValues[i].second->getName()
1033           << "'.\n";
1034         });
1035 
1036   // Decide what we want to thread through.  Convert our list of known values to
1037   // a list of known destinations for each pred.  This also discards duplicate
1038   // predecessors and keeps track of the undefined inputs (which are represented
1039   // as a null dest in the PredToDestList).
1040   SmallPtrSet<BasicBlock*, 16> SeenPreds;
1041   SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
1042 
1043   BasicBlock *OnlyDest = 0;
1044   BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1045 
1046   for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1047     BasicBlock *Pred = PredValues[i].second;
1048     if (!SeenPreds.insert(Pred))
1049       continue;  // Duplicate predecessor entry.
1050 
1051     // If the predecessor ends with an indirect goto, we can't change its
1052     // destination.
1053     if (isa<IndirectBrInst>(Pred->getTerminator()))
1054       continue;
1055 
1056     ConstantInt *Val = PredValues[i].first;
1057 
1058     BasicBlock *DestBB;
1059     if (Val == 0)      // Undef.
1060       DestBB = 0;
1061     else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1062       DestBB = BI->getSuccessor(Val->isZero());
1063     else {
1064       SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
1065       DestBB = SI->getSuccessor(SI->findCaseValue(Val));
1066     }
1067 
1068     // If we have exactly one destination, remember it for efficiency below.
1069     if (i == 0)
1070       OnlyDest = DestBB;
1071     else if (OnlyDest != DestBB)
1072       OnlyDest = MultipleDestSentinel;
1073 
1074     PredToDestList.push_back(std::make_pair(Pred, DestBB));
1075   }
1076 
1077   // If all edges were unthreadable, we fail.
1078   if (PredToDestList.empty())
1079     return false;
1080 
1081   // Determine which is the most common successor.  If we have many inputs and
1082   // this block is a switch, we want to start by threading the batch that goes
1083   // to the most popular destination first.  If we only know about one
1084   // threadable destination (the common case) we can avoid this.
1085   BasicBlock *MostPopularDest = OnlyDest;
1086 
1087   if (MostPopularDest == MultipleDestSentinel)
1088     MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1089 
1090   // Now that we know what the most popular destination is, factor all
1091   // predecessors that will jump to it into a single predecessor.
1092   SmallVector<BasicBlock*, 16> PredsToFactor;
1093   for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1094     if (PredToDestList[i].second == MostPopularDest) {
1095       BasicBlock *Pred = PredToDestList[i].first;
1096 
1097       // This predecessor may be a switch or something else that has multiple
1098       // edges to the block.  Factor each of these edges by listing them
1099       // according to # occurrences in PredsToFactor.
1100       TerminatorInst *PredTI = Pred->getTerminator();
1101       for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
1102         if (PredTI->getSuccessor(i) == BB)
1103           PredsToFactor.push_back(Pred);
1104     }
1105 
1106   // If the threadable edges are branching on an undefined value, we get to pick
1107   // the destination that these predecessors should get to.
1108   if (MostPopularDest == 0)
1109     MostPopularDest = BB->getTerminator()->
1110                             getSuccessor(GetBestDestForJumpOnUndef(BB));
1111 
1112   // Ok, try to thread it!
1113   return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1114 }
1115 
1116 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1117 /// a PHI node in the current block.  See if there are any simplifications we
1118 /// can do based on inputs to the phi node.
1119 ///
1120 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
1121   BasicBlock *BB = PN->getParent();
1122 
1123   // TODO: We could make use of this to do it once for blocks with common PHI
1124   // values.
1125   SmallVector<BasicBlock*, 1> PredBBs;
1126   PredBBs.resize(1);
1127 
1128   // If any of the predecessor blocks end in an unconditional branch, we can
1129   // *duplicate* the conditional branch into that block in order to further
1130   // encourage jump threading and to eliminate cases where we have branch on a
1131   // phi of an icmp (branch on icmp is much better).
1132   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1133     BasicBlock *PredBB = PN->getIncomingBlock(i);
1134     if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1135       if (PredBr->isUnconditional()) {
1136         PredBBs[0] = PredBB;
1137         // Try to duplicate BB into PredBB.
1138         if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
1139           return true;
1140       }
1141   }
1142 
1143   return false;
1144 }
1145 
1146 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
1147 /// a xor instruction in the current block.  See if there are any
1148 /// simplifications we can do based on inputs to the xor.
1149 ///
1150 bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
1151   BasicBlock *BB = BO->getParent();
1152 
1153   // If either the LHS or RHS of the xor is a constant, don't do this
1154   // optimization.
1155   if (isa<ConstantInt>(BO->getOperand(0)) ||
1156       isa<ConstantInt>(BO->getOperand(1)))
1157     return false;
1158 
1159   // If the first instruction in BB isn't a phi, we won't be able to infer
1160   // anything special about any particular predecessor.
1161   if (!isa<PHINode>(BB->front()))
1162     return false;
1163 
1164   // If we have a xor as the branch input to this block, and we know that the
1165   // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1166   // the condition into the predecessor and fix that value to true, saving some
1167   // logical ops on that path and encouraging other paths to simplify.
1168   //
1169   // This copies something like this:
1170   //
1171   //  BB:
1172   //    %X = phi i1 [1],  [%X']
1173   //    %Y = icmp eq i32 %A, %B
1174   //    %Z = xor i1 %X, %Y
1175   //    br i1 %Z, ...
1176   //
1177   // Into:
1178   //  BB':
1179   //    %Y = icmp ne i32 %A, %B
1180   //    br i1 %Z, ...
1181 
1182   SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> XorOpValues;
1183   bool isLHS = true;
1184   if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) {
1185     assert(XorOpValues.empty());
1186     if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues))
1187       return false;
1188     isLHS = false;
1189   }
1190 
1191   assert(!XorOpValues.empty() &&
1192          "ComputeValueKnownInPredecessors returned true with no values");
1193 
1194   // Scan the information to see which is most popular: true or false.  The
1195   // predecessors can be of the set true, false, or undef.
1196   unsigned NumTrue = 0, NumFalse = 0;
1197   for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1198     if (!XorOpValues[i].first) continue;  // Ignore undefs for the count.
1199     if (XorOpValues[i].first->isZero())
1200       ++NumFalse;
1201     else
1202       ++NumTrue;
1203   }
1204 
1205   // Determine which value to split on, true, false, or undef if neither.
1206   ConstantInt *SplitVal = 0;
1207   if (NumTrue > NumFalse)
1208     SplitVal = ConstantInt::getTrue(BB->getContext());
1209   else if (NumTrue != 0 || NumFalse != 0)
1210     SplitVal = ConstantInt::getFalse(BB->getContext());
1211 
1212   // Collect all of the blocks that this can be folded into so that we can
1213   // factor this once and clone it once.
1214   SmallVector<BasicBlock*, 8> BlocksToFoldInto;
1215   for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1216     if (XorOpValues[i].first != SplitVal && XorOpValues[i].first != 0) continue;
1217 
1218     BlocksToFoldInto.push_back(XorOpValues[i].second);
1219   }
1220 
1221   // If we inferred a value for all of the predecessors, then duplication won't
1222   // help us.  However, we can just replace the LHS or RHS with the constant.
1223   if (BlocksToFoldInto.size() ==
1224       cast<PHINode>(BB->front()).getNumIncomingValues()) {
1225     if (SplitVal == 0) {
1226       // If all preds provide undef, just nuke the xor, because it is undef too.
1227       BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
1228       BO->eraseFromParent();
1229     } else if (SplitVal->isZero()) {
1230       // If all preds provide 0, replace the xor with the other input.
1231       BO->replaceAllUsesWith(BO->getOperand(isLHS));
1232       BO->eraseFromParent();
1233     } else {
1234       // If all preds provide 1, set the computed value to 1.
1235       BO->setOperand(!isLHS, SplitVal);
1236     }
1237 
1238     return true;
1239   }
1240 
1241   // Try to duplicate BB into PredBB.
1242   return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
1243 }
1244 
1245 
1246 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1247 /// predecessor to the PHIBB block.  If it has PHI nodes, add entries for
1248 /// NewPred using the entries from OldPred (suitably mapped).
1249 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1250                                             BasicBlock *OldPred,
1251                                             BasicBlock *NewPred,
1252                                      DenseMap<Instruction*, Value*> &ValueMap) {
1253   for (BasicBlock::iterator PNI = PHIBB->begin();
1254        PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1255     // Ok, we have a PHI node.  Figure out what the incoming value was for the
1256     // DestBlock.
1257     Value *IV = PN->getIncomingValueForBlock(OldPred);
1258 
1259     // Remap the value if necessary.
1260     if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1261       DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1262       if (I != ValueMap.end())
1263         IV = I->second;
1264     }
1265 
1266     PN->addIncoming(IV, NewPred);
1267   }
1268 }
1269 
1270 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1271 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1272 /// across BB.  Transform the IR to reflect this change.
1273 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1274                                const SmallVectorImpl<BasicBlock*> &PredBBs,
1275                                BasicBlock *SuccBB) {
1276   // If threading to the same block as we come from, we would infinite loop.
1277   if (SuccBB == BB) {
1278     DEBUG(dbgs() << "  Not threading across BB '" << BB->getName()
1279           << "' - would thread to self!\n");
1280     return false;
1281   }
1282 
1283   // If threading this would thread across a loop header, don't thread the edge.
1284   // See the comments above FindLoopHeaders for justifications and caveats.
1285   if (LoopHeaders.count(BB)) {
1286     DEBUG(dbgs() << "  Not threading across loop header BB '" << BB->getName()
1287           << "' to dest BB '" << SuccBB->getName()
1288           << "' - it might create an irreducible loop!\n");
1289     return false;
1290   }
1291 
1292   unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
1293   if (JumpThreadCost > Threshold) {
1294     DEBUG(dbgs() << "  Not threading BB '" << BB->getName()
1295           << "' - Cost is too high: " << JumpThreadCost << "\n");
1296     return false;
1297   }
1298 
1299   // And finally, do it!  Start by factoring the predecessors is needed.
1300   BasicBlock *PredBB;
1301   if (PredBBs.size() == 1)
1302     PredBB = PredBBs[0];
1303   else {
1304     DEBUG(dbgs() << "  Factoring out " << PredBBs.size()
1305           << " common predecessors.\n");
1306     PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1307                                     ".thr_comm", this);
1308   }
1309 
1310   // And finally, do it!
1311   DEBUG(dbgs() << "  Threading edge from '" << PredBB->getName() << "' to '"
1312         << SuccBB->getName() << "' with cost: " << JumpThreadCost
1313         << ", across block:\n    "
1314         << *BB << "\n");
1315 
1316   if (LVI)
1317     LVI->threadEdge(PredBB, BB, SuccBB);
1318 
1319   // We are going to have to map operands from the original BB block to the new
1320   // copy of the block 'NewBB'.  If there are PHI nodes in BB, evaluate them to
1321   // account for entry from PredBB.
1322   DenseMap<Instruction*, Value*> ValueMapping;
1323 
1324   BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1325                                          BB->getName()+".thread",
1326                                          BB->getParent(), BB);
1327   NewBB->moveAfter(PredBB);
1328 
1329   BasicBlock::iterator BI = BB->begin();
1330   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1331     ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1332 
1333   // Clone the non-phi instructions of BB into NewBB, keeping track of the
1334   // mapping and using it to remap operands in the cloned instructions.
1335   for (; !isa<TerminatorInst>(BI); ++BI) {
1336     Instruction *New = BI->clone();
1337     New->setName(BI->getName());
1338     NewBB->getInstList().push_back(New);
1339     ValueMapping[BI] = New;
1340 
1341     // Remap operands to patch up intra-block references.
1342     for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1343       if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1344         DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1345         if (I != ValueMapping.end())
1346           New->setOperand(i, I->second);
1347       }
1348   }
1349 
1350   // We didn't copy the terminator from BB over to NewBB, because there is now
1351   // an unconditional jump to SuccBB.  Insert the unconditional jump.
1352   BranchInst::Create(SuccBB, NewBB);
1353 
1354   // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1355   // PHI nodes for NewBB now.
1356   AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1357 
1358   // If there were values defined in BB that are used outside the block, then we
1359   // now have to update all uses of the value to use either the original value,
1360   // the cloned value, or some PHI derived value.  This can require arbitrary
1361   // PHI insertion, of which we are prepared to do, clean these up now.
1362   SSAUpdater SSAUpdate;
1363   SmallVector<Use*, 16> UsesToRename;
1364   for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1365     // Scan all uses of this instruction to see if it is used outside of its
1366     // block, and if so, record them in UsesToRename.
1367     for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1368          ++UI) {
1369       Instruction *User = cast<Instruction>(*UI);
1370       if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1371         if (UserPN->getIncomingBlock(UI) == BB)
1372           continue;
1373       } else if (User->getParent() == BB)
1374         continue;
1375 
1376       UsesToRename.push_back(&UI.getUse());
1377     }
1378 
1379     // If there are no uses outside the block, we're done with this instruction.
1380     if (UsesToRename.empty())
1381       continue;
1382 
1383     DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1384 
1385     // We found a use of I outside of BB.  Rename all uses of I that are outside
1386     // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
1387     // with the two values we know.
1388     SSAUpdate.Initialize(I);
1389     SSAUpdate.AddAvailableValue(BB, I);
1390     SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1391 
1392     while (!UsesToRename.empty())
1393       SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1394     DEBUG(dbgs() << "\n");
1395   }
1396 
1397 
1398   // Ok, NewBB is good to go.  Update the terminator of PredBB to jump to
1399   // NewBB instead of BB.  This eliminates predecessors from BB, which requires
1400   // us to simplify any PHI nodes in BB.
1401   TerminatorInst *PredTerm = PredBB->getTerminator();
1402   for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1403     if (PredTerm->getSuccessor(i) == BB) {
1404       RemovePredecessorAndSimplify(BB, PredBB, TD);
1405       PredTerm->setSuccessor(i, NewBB);
1406     }
1407 
1408   // At this point, the IR is fully up to date and consistent.  Do a quick scan
1409   // over the new instructions and zap any that are constants or dead.  This
1410   // frequently happens because of phi translation.
1411   SimplifyInstructionsInBlock(NewBB, TD);
1412 
1413   // Threaded an edge!
1414   ++NumThreads;
1415   return true;
1416 }
1417 
1418 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1419 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1420 /// If we can duplicate the contents of BB up into PredBB do so now, this
1421 /// improves the odds that the branch will be on an analyzable instruction like
1422 /// a compare.
1423 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1424                                  const SmallVectorImpl<BasicBlock *> &PredBBs) {
1425   assert(!PredBBs.empty() && "Can't handle an empty set");
1426 
1427   // If BB is a loop header, then duplicating this block outside the loop would
1428   // cause us to transform this into an irreducible loop, don't do this.
1429   // See the comments above FindLoopHeaders for justifications and caveats.
1430   if (LoopHeaders.count(BB)) {
1431     DEBUG(dbgs() << "  Not duplicating loop header '" << BB->getName()
1432           << "' into predecessor block '" << PredBBs[0]->getName()
1433           << "' - it might create an irreducible loop!\n");
1434     return false;
1435   }
1436 
1437   unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
1438   if (DuplicationCost > Threshold) {
1439     DEBUG(dbgs() << "  Not duplicating BB '" << BB->getName()
1440           << "' - Cost is too high: " << DuplicationCost << "\n");
1441     return false;
1442   }
1443 
1444   // And finally, do it!  Start by factoring the predecessors is needed.
1445   BasicBlock *PredBB;
1446   if (PredBBs.size() == 1)
1447     PredBB = PredBBs[0];
1448   else {
1449     DEBUG(dbgs() << "  Factoring out " << PredBBs.size()
1450           << " common predecessors.\n");
1451     PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1452                                     ".thr_comm", this);
1453   }
1454 
1455   // Okay, we decided to do this!  Clone all the instructions in BB onto the end
1456   // of PredBB.
1457   DEBUG(dbgs() << "  Duplicating block '" << BB->getName() << "' into end of '"
1458         << PredBB->getName() << "' to eliminate branch on phi.  Cost: "
1459         << DuplicationCost << " block is:" << *BB << "\n");
1460 
1461   // Unless PredBB ends with an unconditional branch, split the edge so that we
1462   // can just clone the bits from BB into the end of the new PredBB.
1463   BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
1464 
1465   if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) {
1466     PredBB = SplitEdge(PredBB, BB, this);
1467     OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1468   }
1469 
1470   // We are going to have to map operands from the original BB block into the
1471   // PredBB block.  Evaluate PHI nodes in BB.
1472   DenseMap<Instruction*, Value*> ValueMapping;
1473 
1474   BasicBlock::iterator BI = BB->begin();
1475   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1476     ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1477 
1478   // Clone the non-phi instructions of BB into PredBB, keeping track of the
1479   // mapping and using it to remap operands in the cloned instructions.
1480   for (; BI != BB->end(); ++BI) {
1481     Instruction *New = BI->clone();
1482 
1483     // Remap operands to patch up intra-block references.
1484     for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1485       if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1486         DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1487         if (I != ValueMapping.end())
1488           New->setOperand(i, I->second);
1489       }
1490 
1491     // If this instruction can be simplified after the operands are updated,
1492     // just use the simplified value instead.  This frequently happens due to
1493     // phi translation.
1494     if (Value *IV = SimplifyInstruction(New, TD)) {
1495       delete New;
1496       ValueMapping[BI] = IV;
1497     } else {
1498       // Otherwise, insert the new instruction into the block.
1499       New->setName(BI->getName());
1500       PredBB->getInstList().insert(OldPredBranch, New);
1501       ValueMapping[BI] = New;
1502     }
1503   }
1504 
1505   // Check to see if the targets of the branch had PHI nodes. If so, we need to
1506   // add entries to the PHI nodes for branch from PredBB now.
1507   BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1508   AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1509                                   ValueMapping);
1510   AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1511                                   ValueMapping);
1512 
1513   // If there were values defined in BB that are used outside the block, then we
1514   // now have to update all uses of the value to use either the original value,
1515   // the cloned value, or some PHI derived value.  This can require arbitrary
1516   // PHI insertion, of which we are prepared to do, clean these up now.
1517   SSAUpdater SSAUpdate;
1518   SmallVector<Use*, 16> UsesToRename;
1519   for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1520     // Scan all uses of this instruction to see if it is used outside of its
1521     // block, and if so, record them in UsesToRename.
1522     for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1523          ++UI) {
1524       Instruction *User = cast<Instruction>(*UI);
1525       if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1526         if (UserPN->getIncomingBlock(UI) == BB)
1527           continue;
1528       } else if (User->getParent() == BB)
1529         continue;
1530 
1531       UsesToRename.push_back(&UI.getUse());
1532     }
1533 
1534     // If there are no uses outside the block, we're done with this instruction.
1535     if (UsesToRename.empty())
1536       continue;
1537 
1538     DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1539 
1540     // We found a use of I outside of BB.  Rename all uses of I that are outside
1541     // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
1542     // with the two values we know.
1543     SSAUpdate.Initialize(I);
1544     SSAUpdate.AddAvailableValue(BB, I);
1545     SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1546 
1547     while (!UsesToRename.empty())
1548       SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1549     DEBUG(dbgs() << "\n");
1550   }
1551 
1552   // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1553   // that we nuked.
1554   RemovePredecessorAndSimplify(BB, PredBB, TD);
1555 
1556   // Remove the unconditional branch at the end of the PredBB block.
1557   OldPredBranch->eraseFromParent();
1558 
1559   ++NumDupes;
1560   return true;
1561 }
1562 
1563 
1564