xref: /llvm-project/llvm/lib/Transforms/Scalar/JumpThreading.cpp (revision 0ada5b0d14ec6111da461ddec9f10c4fb5f595d1)
1 //===- JumpThreading.cpp - Thread control through conditional blocks ------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements the Jump Threading pass.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Transforms/Scalar.h"
15 #include "llvm/ADT/DenseMap.h"
16 #include "llvm/ADT/DenseSet.h"
17 #include "llvm/ADT/STLExtras.h"
18 #include "llvm/ADT/SmallPtrSet.h"
19 #include "llvm/ADT/SmallSet.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/Analysis/GlobalsModRef.h"
22 #include "llvm/Analysis/CFG.h"
23 #include "llvm/Analysis/BlockFrequencyInfo.h"
24 #include "llvm/Analysis/BlockFrequencyInfoImpl.h"
25 #include "llvm/Analysis/BranchProbabilityInfo.h"
26 #include "llvm/Analysis/ConstantFolding.h"
27 #include "llvm/Analysis/InstructionSimplify.h"
28 #include "llvm/Analysis/LazyValueInfo.h"
29 #include "llvm/Analysis/Loads.h"
30 #include "llvm/Analysis/LoopInfo.h"
31 #include "llvm/Analysis/TargetLibraryInfo.h"
32 #include "llvm/Analysis/ValueTracking.h"
33 #include "llvm/IR/DataLayout.h"
34 #include "llvm/IR/IntrinsicInst.h"
35 #include "llvm/IR/LLVMContext.h"
36 #include "llvm/IR/MDBuilder.h"
37 #include "llvm/IR/Metadata.h"
38 #include "llvm/IR/ValueHandle.h"
39 #include "llvm/Pass.h"
40 #include "llvm/Support/CommandLine.h"
41 #include "llvm/Support/Debug.h"
42 #include "llvm/Support/raw_ostream.h"
43 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
44 #include "llvm/Transforms/Utils/Local.h"
45 #include "llvm/Transforms/Utils/SSAUpdater.h"
46 #include <algorithm>
47 #include <memory>
48 using namespace llvm;
49 
50 #define DEBUG_TYPE "jump-threading"
51 
52 STATISTIC(NumThreads, "Number of jumps threaded");
53 STATISTIC(NumFolds,   "Number of terminators folded");
54 STATISTIC(NumDupes,   "Number of branch blocks duplicated to eliminate phi");
55 
56 static cl::opt<unsigned>
57 BBDuplicateThreshold("jump-threading-threshold",
58           cl::desc("Max block size to duplicate for jump threading"),
59           cl::init(6), cl::Hidden);
60 
61 static cl::opt<unsigned>
62 ImplicationSearchThreshold(
63   "jump-threading-implication-search-threshold",
64   cl::desc("The number of predecessors to search for a stronger "
65            "condition to use to thread over a weaker condition"),
66   cl::init(3), cl::Hidden);
67 
68 namespace {
69   // These are at global scope so static functions can use them too.
70   typedef SmallVectorImpl<std::pair<Constant*, BasicBlock*> > PredValueInfo;
71   typedef SmallVector<std::pair<Constant*, BasicBlock*>, 8> PredValueInfoTy;
72 
73   // This is used to keep track of what kind of constant we're currently hoping
74   // to find.
75   enum ConstantPreference {
76     WantInteger,
77     WantBlockAddress
78   };
79 
80   /// This pass performs 'jump threading', which looks at blocks that have
81   /// multiple predecessors and multiple successors.  If one or more of the
82   /// predecessors of the block can be proven to always jump to one of the
83   /// successors, we forward the edge from the predecessor to the successor by
84   /// duplicating the contents of this block.
85   ///
86   /// An example of when this can occur is code like this:
87   ///
88   ///   if () { ...
89   ///     X = 4;
90   ///   }
91   ///   if (X < 3) {
92   ///
93   /// In this case, the unconditional branch at the end of the first if can be
94   /// revectored to the false side of the second if.
95   ///
96   class JumpThreading : public FunctionPass {
97     TargetLibraryInfo *TLI;
98     LazyValueInfo *LVI;
99     std::unique_ptr<BlockFrequencyInfo> BFI;
100     std::unique_ptr<BranchProbabilityInfo> BPI;
101     bool HasProfileData;
102 #ifdef NDEBUG
103     SmallPtrSet<const BasicBlock *, 16> LoopHeaders;
104 #else
105     SmallSet<AssertingVH<const BasicBlock>, 16> LoopHeaders;
106 #endif
107     DenseSet<std::pair<Value*, BasicBlock*> > RecursionSet;
108 
109     unsigned BBDupThreshold;
110 
111     // RAII helper for updating the recursion stack.
112     struct RecursionSetRemover {
113       DenseSet<std::pair<Value*, BasicBlock*> > &TheSet;
114       std::pair<Value*, BasicBlock*> ThePair;
115 
116       RecursionSetRemover(DenseSet<std::pair<Value*, BasicBlock*> > &S,
117                           std::pair<Value*, BasicBlock*> P)
118         : TheSet(S), ThePair(P) { }
119 
120       ~RecursionSetRemover() {
121         TheSet.erase(ThePair);
122       }
123     };
124   public:
125     static char ID; // Pass identification
126     JumpThreading(int T = -1) : FunctionPass(ID) {
127       BBDupThreshold = (T == -1) ? BBDuplicateThreshold : unsigned(T);
128       initializeJumpThreadingPass(*PassRegistry::getPassRegistry());
129     }
130 
131     bool runOnFunction(Function &F) override;
132 
133     void getAnalysisUsage(AnalysisUsage &AU) const override {
134       AU.addRequired<LazyValueInfo>();
135       AU.addPreserved<LazyValueInfo>();
136       AU.addPreserved<GlobalsAAWrapperPass>();
137       AU.addRequired<TargetLibraryInfoWrapperPass>();
138     }
139 
140     void releaseMemory() override {
141       BFI.reset();
142       BPI.reset();
143     }
144 
145     void FindLoopHeaders(Function &F);
146     bool ProcessBlock(BasicBlock *BB);
147     bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
148                     BasicBlock *SuccBB);
149     bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
150                                   const SmallVectorImpl<BasicBlock *> &PredBBs);
151 
152     bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
153                                          PredValueInfo &Result,
154                                          ConstantPreference Preference,
155                                          Instruction *CxtI = nullptr);
156     bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
157                                 ConstantPreference Preference,
158                                 Instruction *CxtI = nullptr);
159 
160     bool ProcessBranchOnPHI(PHINode *PN);
161     bool ProcessBranchOnXOR(BinaryOperator *BO);
162     bool ProcessImpliedCondition(BasicBlock *BB);
163 
164     bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
165     bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
166     bool TryToUnfoldSelectInCurrBB(BasicBlock *BB);
167 
168   private:
169     BasicBlock *SplitBlockPreds(BasicBlock *BB, ArrayRef<BasicBlock *> Preds,
170                                 const char *Suffix);
171     void UpdateBlockFreqAndEdgeWeight(BasicBlock *PredBB, BasicBlock *BB,
172                                       BasicBlock *NewBB, BasicBlock *SuccBB);
173   };
174 }
175 
176 char JumpThreading::ID = 0;
177 INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading",
178                 "Jump Threading", false, false)
179 INITIALIZE_PASS_DEPENDENCY(LazyValueInfo)
180 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
181 INITIALIZE_PASS_END(JumpThreading, "jump-threading",
182                 "Jump Threading", false, false)
183 
184 // Public interface to the Jump Threading pass
185 FunctionPass *llvm::createJumpThreadingPass(int Threshold) { return new JumpThreading(Threshold); }
186 
187 /// runOnFunction - Top level algorithm.
188 ///
189 bool JumpThreading::runOnFunction(Function &F) {
190   if (skipOptnoneFunction(F))
191     return false;
192 
193   DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
194   TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
195   LVI = &getAnalysis<LazyValueInfo>();
196   BFI.reset();
197   BPI.reset();
198   // When profile data is available, we need to update edge weights after
199   // successful jump threading, which requires both BPI and BFI being available.
200   HasProfileData = F.getEntryCount().hasValue();
201   if (HasProfileData) {
202     LoopInfo LI{DominatorTree(F)};
203     BPI.reset(new BranchProbabilityInfo(F, LI));
204     BFI.reset(new BlockFrequencyInfo(F, *BPI, LI));
205   }
206 
207   // Remove unreachable blocks from function as they may result in infinite
208   // loop. We do threading if we found something profitable. Jump threading a
209   // branch can create other opportunities. If these opportunities form a cycle
210   // i.e. if any jump threading is undoing previous threading in the path, then
211   // we will loop forever. We take care of this issue by not jump threading for
212   // back edges. This works for normal cases but not for unreachable blocks as
213   // they may have cycle with no back edge.
214   bool EverChanged = false;
215   EverChanged |= removeUnreachableBlocks(F, LVI);
216 
217   FindLoopHeaders(F);
218 
219   bool Changed;
220   do {
221     Changed = false;
222     for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
223       BasicBlock *BB = &*I;
224       // Thread all of the branches we can over this block.
225       while (ProcessBlock(BB))
226         Changed = true;
227 
228       ++I;
229 
230       // If the block is trivially dead, zap it.  This eliminates the successor
231       // edges which simplifies the CFG.
232       if (pred_empty(BB) &&
233           BB != &BB->getParent()->getEntryBlock()) {
234         DEBUG(dbgs() << "  JT: Deleting dead block '" << BB->getName()
235               << "' with terminator: " << *BB->getTerminator() << '\n');
236         LoopHeaders.erase(BB);
237         LVI->eraseBlock(BB);
238         DeleteDeadBlock(BB);
239         Changed = true;
240         continue;
241       }
242 
243       BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
244 
245       // Can't thread an unconditional jump, but if the block is "almost
246       // empty", we can replace uses of it with uses of the successor and make
247       // this dead.
248       // We should not eliminate the loop header either, because eliminating
249       // a loop header might later prevent LoopSimplify from transforming nested
250       // loops into simplified form.
251       if (BI && BI->isUnconditional() &&
252           BB != &BB->getParent()->getEntryBlock() &&
253           // If the terminator is the only non-phi instruction, try to nuke it.
254           BB->getFirstNonPHIOrDbg()->isTerminator() && !LoopHeaders.count(BB)) {
255         // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
256         // block, we have to make sure it isn't in the LoopHeaders set.  We
257         // reinsert afterward if needed.
258         bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
259         BasicBlock *Succ = BI->getSuccessor(0);
260 
261         // FIXME: It is always conservatively correct to drop the info
262         // for a block even if it doesn't get erased.  This isn't totally
263         // awesome, but it allows us to use AssertingVH to prevent nasty
264         // dangling pointer issues within LazyValueInfo.
265         LVI->eraseBlock(BB);
266         if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
267           Changed = true;
268           // If we deleted BB and BB was the header of a loop, then the
269           // successor is now the header of the loop.
270           BB = Succ;
271         }
272 
273         if (ErasedFromLoopHeaders)
274           LoopHeaders.insert(BB);
275       }
276     }
277     EverChanged |= Changed;
278   } while (Changed);
279 
280   LoopHeaders.clear();
281   return EverChanged;
282 }
283 
284 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
285 /// thread across it. Stop scanning the block when passing the threshold.
286 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB,
287                                              unsigned Threshold) {
288   /// Ignore PHI nodes, these will be flattened when duplication happens.
289   BasicBlock::const_iterator I(BB->getFirstNonPHI());
290 
291   // FIXME: THREADING will delete values that are just used to compute the
292   // branch, so they shouldn't count against the duplication cost.
293 
294   unsigned Bonus = 0;
295   const TerminatorInst *BBTerm = BB->getTerminator();
296   // Threading through a switch statement is particularly profitable.  If this
297   // block ends in a switch, decrease its cost to make it more likely to happen.
298   if (isa<SwitchInst>(BBTerm))
299     Bonus = 6;
300 
301   // The same holds for indirect branches, but slightly more so.
302   if (isa<IndirectBrInst>(BBTerm))
303     Bonus = 8;
304 
305   // Bump the threshold up so the early exit from the loop doesn't skip the
306   // terminator-based Size adjustment at the end.
307   Threshold += Bonus;
308 
309   // Sum up the cost of each instruction until we get to the terminator.  Don't
310   // include the terminator because the copy won't include it.
311   unsigned Size = 0;
312   for (; !isa<TerminatorInst>(I); ++I) {
313 
314     // Stop scanning the block if we've reached the threshold.
315     if (Size > Threshold)
316       return Size;
317 
318     // Debugger intrinsics don't incur code size.
319     if (isa<DbgInfoIntrinsic>(I)) continue;
320 
321     // If this is a pointer->pointer bitcast, it is free.
322     if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
323       continue;
324 
325     // Bail out if this instruction gives back a token type, it is not possible
326     // to duplicate it if it is used outside this BB.
327     if (I->getType()->isTokenTy() && I->isUsedOutsideOfBlock(BB))
328       return ~0U;
329 
330     // All other instructions count for at least one unit.
331     ++Size;
332 
333     // Calls are more expensive.  If they are non-intrinsic calls, we model them
334     // as having cost of 4.  If they are a non-vector intrinsic, we model them
335     // as having cost of 2 total, and if they are a vector intrinsic, we model
336     // them as having cost 1.
337     if (const CallInst *CI = dyn_cast<CallInst>(I)) {
338       if (CI->cannotDuplicate() || CI->isConvergent())
339         // Blocks with NoDuplicate are modelled as having infinite cost, so they
340         // are never duplicated.
341         return ~0U;
342       else if (!isa<IntrinsicInst>(CI))
343         Size += 3;
344       else if (!CI->getType()->isVectorTy())
345         Size += 1;
346     }
347   }
348 
349   return Size > Bonus ? Size - Bonus : 0;
350 }
351 
352 /// FindLoopHeaders - We do not want jump threading to turn proper loop
353 /// structures into irreducible loops.  Doing this breaks up the loop nesting
354 /// hierarchy and pessimizes later transformations.  To prevent this from
355 /// happening, we first have to find the loop headers.  Here we approximate this
356 /// by finding targets of backedges in the CFG.
357 ///
358 /// Note that there definitely are cases when we want to allow threading of
359 /// edges across a loop header.  For example, threading a jump from outside the
360 /// loop (the preheader) to an exit block of the loop is definitely profitable.
361 /// It is also almost always profitable to thread backedges from within the loop
362 /// to exit blocks, and is often profitable to thread backedges to other blocks
363 /// within the loop (forming a nested loop).  This simple analysis is not rich
364 /// enough to track all of these properties and keep it up-to-date as the CFG
365 /// mutates, so we don't allow any of these transformations.
366 ///
367 void JumpThreading::FindLoopHeaders(Function &F) {
368   SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
369   FindFunctionBackedges(F, Edges);
370 
371   for (const auto &Edge : Edges)
372     LoopHeaders.insert(Edge.second);
373 }
374 
375 /// getKnownConstant - Helper method to determine if we can thread over a
376 /// terminator with the given value as its condition, and if so what value to
377 /// use for that. What kind of value this is depends on whether we want an
378 /// integer or a block address, but an undef is always accepted.
379 /// Returns null if Val is null or not an appropriate constant.
380 static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
381   if (!Val)
382     return nullptr;
383 
384   // Undef is "known" enough.
385   if (UndefValue *U = dyn_cast<UndefValue>(Val))
386     return U;
387 
388   if (Preference == WantBlockAddress)
389     return dyn_cast<BlockAddress>(Val->stripPointerCasts());
390 
391   return dyn_cast<ConstantInt>(Val);
392 }
393 
394 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
395 /// if we can infer that the value is a known ConstantInt/BlockAddress or undef
396 /// in any of our predecessors.  If so, return the known list of value and pred
397 /// BB in the result vector.
398 ///
399 /// This returns true if there were any known values.
400 ///
401 bool JumpThreading::
402 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
403                                 ConstantPreference Preference,
404                                 Instruction *CxtI) {
405   // This method walks up use-def chains recursively.  Because of this, we could
406   // get into an infinite loop going around loops in the use-def chain.  To
407   // prevent this, keep track of what (value, block) pairs we've already visited
408   // and terminate the search if we loop back to them
409   if (!RecursionSet.insert(std::make_pair(V, BB)).second)
410     return false;
411 
412   // An RAII help to remove this pair from the recursion set once the recursion
413   // stack pops back out again.
414   RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB));
415 
416   // If V is a constant, then it is known in all predecessors.
417   if (Constant *KC = getKnownConstant(V, Preference)) {
418     for (BasicBlock *Pred : predecessors(BB))
419       Result.push_back(std::make_pair(KC, Pred));
420 
421     return !Result.empty();
422   }
423 
424   // If V is a non-instruction value, or an instruction in a different block,
425   // then it can't be derived from a PHI.
426   Instruction *I = dyn_cast<Instruction>(V);
427   if (!I || I->getParent() != BB) {
428 
429     // Okay, if this is a live-in value, see if it has a known value at the end
430     // of any of our predecessors.
431     //
432     // FIXME: This should be an edge property, not a block end property.
433     /// TODO: Per PR2563, we could infer value range information about a
434     /// predecessor based on its terminator.
435     //
436     // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
437     // "I" is a non-local compare-with-a-constant instruction.  This would be
438     // able to handle value inequalities better, for example if the compare is
439     // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
440     // Perhaps getConstantOnEdge should be smart enough to do this?
441 
442     for (BasicBlock *P : predecessors(BB)) {
443       // If the value is known by LazyValueInfo to be a constant in a
444       // predecessor, use that information to try to thread this block.
445       Constant *PredCst = LVI->getConstantOnEdge(V, P, BB, CxtI);
446       if (Constant *KC = getKnownConstant(PredCst, Preference))
447         Result.push_back(std::make_pair(KC, P));
448     }
449 
450     return !Result.empty();
451   }
452 
453   /// If I is a PHI node, then we know the incoming values for any constants.
454   if (PHINode *PN = dyn_cast<PHINode>(I)) {
455     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
456       Value *InVal = PN->getIncomingValue(i);
457       if (Constant *KC = getKnownConstant(InVal, Preference)) {
458         Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
459       } else {
460         Constant *CI = LVI->getConstantOnEdge(InVal,
461                                               PN->getIncomingBlock(i),
462                                               BB, CxtI);
463         if (Constant *KC = getKnownConstant(CI, Preference))
464           Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
465       }
466     }
467 
468     return !Result.empty();
469   }
470 
471   // Handle Cast instructions.  Only see through Cast when the source operand is
472   // PHI or Cmp and the source type is i1 to save the compilation time.
473   if (CastInst *CI = dyn_cast<CastInst>(I)) {
474     Value *Source = CI->getOperand(0);
475     if (!Source->getType()->isIntegerTy(1))
476       return false;
477     if (!isa<PHINode>(Source) && !isa<CmpInst>(Source))
478       return false;
479     ComputeValueKnownInPredecessors(Source, BB, Result, Preference, CxtI);
480     if (Result.empty())
481       return false;
482 
483     // Convert the known values.
484     for (auto &R : Result)
485       R.first = ConstantExpr::getCast(CI->getOpcode(), R.first, CI->getType());
486 
487     return true;
488   }
489 
490   PredValueInfoTy LHSVals, RHSVals;
491 
492   // Handle some boolean conditions.
493   if (I->getType()->getPrimitiveSizeInBits() == 1) {
494     assert(Preference == WantInteger && "One-bit non-integer type?");
495     // X | true -> true
496     // X & false -> false
497     if (I->getOpcode() == Instruction::Or ||
498         I->getOpcode() == Instruction::And) {
499       ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
500                                       WantInteger, CxtI);
501       ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals,
502                                       WantInteger, CxtI);
503 
504       if (LHSVals.empty() && RHSVals.empty())
505         return false;
506 
507       ConstantInt *InterestingVal;
508       if (I->getOpcode() == Instruction::Or)
509         InterestingVal = ConstantInt::getTrue(I->getContext());
510       else
511         InterestingVal = ConstantInt::getFalse(I->getContext());
512 
513       SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
514 
515       // Scan for the sentinel.  If we find an undef, force it to the
516       // interesting value: x|undef -> true and x&undef -> false.
517       for (const auto &LHSVal : LHSVals)
518         if (LHSVal.first == InterestingVal || isa<UndefValue>(LHSVal.first)) {
519           Result.emplace_back(InterestingVal, LHSVal.second);
520           LHSKnownBBs.insert(LHSVal.second);
521         }
522       for (const auto &RHSVal : RHSVals)
523         if (RHSVal.first == InterestingVal || isa<UndefValue>(RHSVal.first)) {
524           // If we already inferred a value for this block on the LHS, don't
525           // re-add it.
526           if (!LHSKnownBBs.count(RHSVal.second))
527             Result.emplace_back(InterestingVal, RHSVal.second);
528         }
529 
530       return !Result.empty();
531     }
532 
533     // Handle the NOT form of XOR.
534     if (I->getOpcode() == Instruction::Xor &&
535         isa<ConstantInt>(I->getOperand(1)) &&
536         cast<ConstantInt>(I->getOperand(1))->isOne()) {
537       ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result,
538                                       WantInteger, CxtI);
539       if (Result.empty())
540         return false;
541 
542       // Invert the known values.
543       for (auto &R : Result)
544         R.first = ConstantExpr::getNot(R.first);
545 
546       return true;
547     }
548 
549   // Try to simplify some other binary operator values.
550   } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
551     assert(Preference != WantBlockAddress
552             && "A binary operator creating a block address?");
553     if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
554       PredValueInfoTy LHSVals;
555       ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals,
556                                       WantInteger, CxtI);
557 
558       // Try to use constant folding to simplify the binary operator.
559       for (const auto &LHSVal : LHSVals) {
560         Constant *V = LHSVal.first;
561         Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI);
562 
563         if (Constant *KC = getKnownConstant(Folded, WantInteger))
564           Result.push_back(std::make_pair(KC, LHSVal.second));
565       }
566     }
567 
568     return !Result.empty();
569   }
570 
571   // Handle compare with phi operand, where the PHI is defined in this block.
572   if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
573     assert(Preference == WantInteger && "Compares only produce integers");
574     PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
575     if (PN && PN->getParent() == BB) {
576       const DataLayout &DL = PN->getModule()->getDataLayout();
577       // We can do this simplification if any comparisons fold to true or false.
578       // See if any do.
579       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
580         BasicBlock *PredBB = PN->getIncomingBlock(i);
581         Value *LHS = PN->getIncomingValue(i);
582         Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
583 
584         Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, DL);
585         if (!Res) {
586           if (!isa<Constant>(RHS))
587             continue;
588 
589           LazyValueInfo::Tristate
590             ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
591                                            cast<Constant>(RHS), PredBB, BB,
592                                            CxtI ? CxtI : Cmp);
593           if (ResT == LazyValueInfo::Unknown)
594             continue;
595           Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
596         }
597 
598         if (Constant *KC = getKnownConstant(Res, WantInteger))
599           Result.push_back(std::make_pair(KC, PredBB));
600       }
601 
602       return !Result.empty();
603     }
604 
605     // If comparing a live-in value against a constant, see if we know the
606     // live-in value on any predecessors.
607     if (isa<Constant>(Cmp->getOperand(1)) && Cmp->getType()->isIntegerTy()) {
608       if (!isa<Instruction>(Cmp->getOperand(0)) ||
609           cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) {
610         Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
611 
612         for (BasicBlock *P : predecessors(BB)) {
613           // If the value is known by LazyValueInfo to be a constant in a
614           // predecessor, use that information to try to thread this block.
615           LazyValueInfo::Tristate Res =
616             LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
617                                     RHSCst, P, BB, CxtI ? CxtI : Cmp);
618           if (Res == LazyValueInfo::Unknown)
619             continue;
620 
621           Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
622           Result.push_back(std::make_pair(ResC, P));
623         }
624 
625         return !Result.empty();
626       }
627 
628       // Try to find a constant value for the LHS of a comparison,
629       // and evaluate it statically if we can.
630       if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) {
631         PredValueInfoTy LHSVals;
632         ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
633                                         WantInteger, CxtI);
634 
635         for (const auto &LHSVal : LHSVals) {
636           Constant *V = LHSVal.first;
637           Constant *Folded = ConstantExpr::getCompare(Cmp->getPredicate(),
638                                                       V, CmpConst);
639           if (Constant *KC = getKnownConstant(Folded, WantInteger))
640             Result.push_back(std::make_pair(KC, LHSVal.second));
641         }
642 
643         return !Result.empty();
644       }
645     }
646   }
647 
648   if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
649     // Handle select instructions where at least one operand is a known constant
650     // and we can figure out the condition value for any predecessor block.
651     Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
652     Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
653     PredValueInfoTy Conds;
654     if ((TrueVal || FalseVal) &&
655         ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
656                                         WantInteger, CxtI)) {
657       for (auto &C : Conds) {
658         Constant *Cond = C.first;
659 
660         // Figure out what value to use for the condition.
661         bool KnownCond;
662         if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
663           // A known boolean.
664           KnownCond = CI->isOne();
665         } else {
666           assert(isa<UndefValue>(Cond) && "Unexpected condition value");
667           // Either operand will do, so be sure to pick the one that's a known
668           // constant.
669           // FIXME: Do this more cleverly if both values are known constants?
670           KnownCond = (TrueVal != nullptr);
671         }
672 
673         // See if the select has a known constant value for this predecessor.
674         if (Constant *Val = KnownCond ? TrueVal : FalseVal)
675           Result.push_back(std::make_pair(Val, C.second));
676       }
677 
678       return !Result.empty();
679     }
680   }
681 
682   // If all else fails, see if LVI can figure out a constant value for us.
683   Constant *CI = LVI->getConstant(V, BB, CxtI);
684   if (Constant *KC = getKnownConstant(CI, Preference)) {
685     for (BasicBlock *Pred : predecessors(BB))
686       Result.push_back(std::make_pair(KC, Pred));
687   }
688 
689   return !Result.empty();
690 }
691 
692 
693 
694 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
695 /// in an undefined jump, decide which block is best to revector to.
696 ///
697 /// Since we can pick an arbitrary destination, we pick the successor with the
698 /// fewest predecessors.  This should reduce the in-degree of the others.
699 ///
700 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
701   TerminatorInst *BBTerm = BB->getTerminator();
702   unsigned MinSucc = 0;
703   BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
704   // Compute the successor with the minimum number of predecessors.
705   unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
706   for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
707     TestBB = BBTerm->getSuccessor(i);
708     unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
709     if (NumPreds < MinNumPreds) {
710       MinSucc = i;
711       MinNumPreds = NumPreds;
712     }
713   }
714 
715   return MinSucc;
716 }
717 
718 static bool hasAddressTakenAndUsed(BasicBlock *BB) {
719   if (!BB->hasAddressTaken()) return false;
720 
721   // If the block has its address taken, it may be a tree of dead constants
722   // hanging off of it.  These shouldn't keep the block alive.
723   BlockAddress *BA = BlockAddress::get(BB);
724   BA->removeDeadConstantUsers();
725   return !BA->use_empty();
726 }
727 
728 /// ProcessBlock - If there are any predecessors whose control can be threaded
729 /// through to a successor, transform them now.
730 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
731   // If the block is trivially dead, just return and let the caller nuke it.
732   // This simplifies other transformations.
733   if (pred_empty(BB) &&
734       BB != &BB->getParent()->getEntryBlock())
735     return false;
736 
737   // If this block has a single predecessor, and if that pred has a single
738   // successor, merge the blocks.  This encourages recursive jump threading
739   // because now the condition in this block can be threaded through
740   // predecessors of our predecessor block.
741   if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
742     const TerminatorInst *TI = SinglePred->getTerminator();
743     if (!TI->isExceptional() && TI->getNumSuccessors() == 1 &&
744         SinglePred != BB && !hasAddressTakenAndUsed(BB)) {
745       // If SinglePred was a loop header, BB becomes one.
746       if (LoopHeaders.erase(SinglePred))
747         LoopHeaders.insert(BB);
748 
749       LVI->eraseBlock(SinglePred);
750       MergeBasicBlockIntoOnlyPred(BB);
751 
752       return true;
753     }
754   }
755 
756   if (TryToUnfoldSelectInCurrBB(BB))
757     return true;
758 
759   // What kind of constant we're looking for.
760   ConstantPreference Preference = WantInteger;
761 
762   // Look to see if the terminator is a conditional branch, switch or indirect
763   // branch, if not we can't thread it.
764   Value *Condition;
765   Instruction *Terminator = BB->getTerminator();
766   if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) {
767     // Can't thread an unconditional jump.
768     if (BI->isUnconditional()) return false;
769     Condition = BI->getCondition();
770   } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) {
771     Condition = SI->getCondition();
772   } else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) {
773     // Can't thread indirect branch with no successors.
774     if (IB->getNumSuccessors() == 0) return false;
775     Condition = IB->getAddress()->stripPointerCasts();
776     Preference = WantBlockAddress;
777   } else {
778     return false; // Must be an invoke.
779   }
780 
781   // Run constant folding to see if we can reduce the condition to a simple
782   // constant.
783   if (Instruction *I = dyn_cast<Instruction>(Condition)) {
784     Value *SimpleVal =
785         ConstantFoldInstruction(I, BB->getModule()->getDataLayout(), TLI);
786     if (SimpleVal) {
787       I->replaceAllUsesWith(SimpleVal);
788       I->eraseFromParent();
789       Condition = SimpleVal;
790     }
791   }
792 
793   // If the terminator is branching on an undef, we can pick any of the
794   // successors to branch to.  Let GetBestDestForJumpOnUndef decide.
795   if (isa<UndefValue>(Condition)) {
796     unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
797 
798     // Fold the branch/switch.
799     TerminatorInst *BBTerm = BB->getTerminator();
800     for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
801       if (i == BestSucc) continue;
802       BBTerm->getSuccessor(i)->removePredecessor(BB, true);
803     }
804 
805     DEBUG(dbgs() << "  In block '" << BB->getName()
806           << "' folding undef terminator: " << *BBTerm << '\n');
807     BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
808     BBTerm->eraseFromParent();
809     return true;
810   }
811 
812   // If the terminator of this block is branching on a constant, simplify the
813   // terminator to an unconditional branch.  This can occur due to threading in
814   // other blocks.
815   if (getKnownConstant(Condition, Preference)) {
816     DEBUG(dbgs() << "  In block '" << BB->getName()
817           << "' folding terminator: " << *BB->getTerminator() << '\n');
818     ++NumFolds;
819     ConstantFoldTerminator(BB, true);
820     return true;
821   }
822 
823   Instruction *CondInst = dyn_cast<Instruction>(Condition);
824 
825   // All the rest of our checks depend on the condition being an instruction.
826   if (!CondInst) {
827     // FIXME: Unify this with code below.
828     if (ProcessThreadableEdges(Condition, BB, Preference, Terminator))
829       return true;
830     return false;
831   }
832 
833 
834   if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
835     // If we're branching on a conditional, LVI might be able to determine
836     // it's value at the branch instruction.  We only handle comparisons
837     // against a constant at this time.
838     // TODO: This should be extended to handle switches as well.
839     BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
840     Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
841     if (CondBr && CondConst && CondBr->isConditional()) {
842       LazyValueInfo::Tristate Ret =
843         LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0),
844                             CondConst, CondBr);
845       if (Ret != LazyValueInfo::Unknown) {
846         unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0;
847         unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1;
848         CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
849         BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
850         CondBr->eraseFromParent();
851         if (CondCmp->use_empty())
852           CondCmp->eraseFromParent();
853         else if (CondCmp->getParent() == BB) {
854           // If the fact we just learned is true for all uses of the
855           // condition, replace it with a constant value
856           auto *CI = Ret == LazyValueInfo::True ?
857             ConstantInt::getTrue(CondCmp->getType()) :
858             ConstantInt::getFalse(CondCmp->getType());
859           CondCmp->replaceAllUsesWith(CI);
860           CondCmp->eraseFromParent();
861         }
862         return true;
863       }
864     }
865 
866     if (CondBr && CondConst && TryToUnfoldSelect(CondCmp, BB))
867       return true;
868   }
869 
870   // Check for some cases that are worth simplifying.  Right now we want to look
871   // for loads that are used by a switch or by the condition for the branch.  If
872   // we see one, check to see if it's partially redundant.  If so, insert a PHI
873   // which can then be used to thread the values.
874   //
875   Value *SimplifyValue = CondInst;
876   if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
877     if (isa<Constant>(CondCmp->getOperand(1)))
878       SimplifyValue = CondCmp->getOperand(0);
879 
880   // TODO: There are other places where load PRE would be profitable, such as
881   // more complex comparisons.
882   if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
883     if (SimplifyPartiallyRedundantLoad(LI))
884       return true;
885 
886 
887   // Handle a variety of cases where we are branching on something derived from
888   // a PHI node in the current block.  If we can prove that any predecessors
889   // compute a predictable value based on a PHI node, thread those predecessors.
890   //
891   if (ProcessThreadableEdges(CondInst, BB, Preference, Terminator))
892     return true;
893 
894   // If this is an otherwise-unfoldable branch on a phi node in the current
895   // block, see if we can simplify.
896   if (PHINode *PN = dyn_cast<PHINode>(CondInst))
897     if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
898       return ProcessBranchOnPHI(PN);
899 
900 
901   // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
902   if (CondInst->getOpcode() == Instruction::Xor &&
903       CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
904     return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
905 
906   // Search for a stronger dominating condition that can be used to simplify a
907   // conditional branch leaving BB.
908   if (ProcessImpliedCondition(BB))
909     return true;
910 
911   return false;
912 }
913 
914 bool JumpThreading::ProcessImpliedCondition(BasicBlock *BB) {
915   auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
916   if (!BI || !BI->isConditional())
917     return false;
918 
919   Value *Cond = BI->getCondition();
920   BasicBlock *CurrentBB = BB;
921   BasicBlock *CurrentPred = BB->getSinglePredecessor();
922   unsigned Iter = 0;
923 
924   auto &DL = BB->getModule()->getDataLayout();
925 
926   while (CurrentPred && Iter++ < ImplicationSearchThreshold) {
927     auto *PBI = dyn_cast<BranchInst>(CurrentPred->getTerminator());
928     if (!PBI || !PBI->isConditional() || PBI->getSuccessor(0) != CurrentBB)
929       return false;
930 
931     if (isImpliedCondition(PBI->getCondition(), Cond, DL)) {
932       BI->getSuccessor(1)->removePredecessor(BB);
933       BranchInst::Create(BI->getSuccessor(0), BI);
934       BI->eraseFromParent();
935       return true;
936     }
937     CurrentBB = CurrentPred;
938     CurrentPred = CurrentBB->getSinglePredecessor();
939   }
940 
941   return false;
942 }
943 
944 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
945 /// load instruction, eliminate it by replacing it with a PHI node.  This is an
946 /// important optimization that encourages jump threading, and needs to be run
947 /// interlaced with other jump threading tasks.
948 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
949   // Don't hack volatile/atomic loads.
950   if (!LI->isSimple()) return false;
951 
952   // If the load is defined in a block with exactly one predecessor, it can't be
953   // partially redundant.
954   BasicBlock *LoadBB = LI->getParent();
955   if (LoadBB->getSinglePredecessor())
956     return false;
957 
958   // If the load is defined in an EH pad, it can't be partially redundant,
959   // because the edges between the invoke and the EH pad cannot have other
960   // instructions between them.
961   if (LoadBB->isEHPad())
962     return false;
963 
964   Value *LoadedPtr = LI->getOperand(0);
965 
966   // If the loaded operand is defined in the LoadBB, it can't be available.
967   // TODO: Could do simple PHI translation, that would be fun :)
968   if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
969     if (PtrOp->getParent() == LoadBB)
970       return false;
971 
972   // Scan a few instructions up from the load, to see if it is obviously live at
973   // the entry to its block.
974   BasicBlock::iterator BBIt(LI);
975 
976   if (Value *AvailableVal =
977         FindAvailableLoadedValue(LI, LoadBB, BBIt, DefMaxInstsToScan)) {
978     // If the value of the load is locally available within the block, just use
979     // it.  This frequently occurs for reg2mem'd allocas.
980     //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
981 
982     // If the returned value is the load itself, replace with an undef. This can
983     // only happen in dead loops.
984     if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
985     if (AvailableVal->getType() != LI->getType())
986       AvailableVal =
987           CastInst::CreateBitOrPointerCast(AvailableVal, LI->getType(), "", LI);
988     LI->replaceAllUsesWith(AvailableVal);
989     LI->eraseFromParent();
990     return true;
991   }
992 
993   // Otherwise, if we scanned the whole block and got to the top of the block,
994   // we know the block is locally transparent to the load.  If not, something
995   // might clobber its value.
996   if (BBIt != LoadBB->begin())
997     return false;
998 
999   // If all of the loads and stores that feed the value have the same AA tags,
1000   // then we can propagate them onto any newly inserted loads.
1001   AAMDNodes AATags;
1002   LI->getAAMetadata(AATags);
1003 
1004   SmallPtrSet<BasicBlock*, 8> PredsScanned;
1005   typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
1006   AvailablePredsTy AvailablePreds;
1007   BasicBlock *OneUnavailablePred = nullptr;
1008 
1009   // If we got here, the loaded value is transparent through to the start of the
1010   // block.  Check to see if it is available in any of the predecessor blocks.
1011   for (BasicBlock *PredBB : predecessors(LoadBB)) {
1012     // If we already scanned this predecessor, skip it.
1013     if (!PredsScanned.insert(PredBB).second)
1014       continue;
1015 
1016     // Scan the predecessor to see if the value is available in the pred.
1017     BBIt = PredBB->end();
1018     AAMDNodes ThisAATags;
1019     Value *PredAvailable = FindAvailableLoadedValue(LI, PredBB, BBIt,
1020                                                     DefMaxInstsToScan,
1021                                                     nullptr, &ThisAATags);
1022     if (!PredAvailable) {
1023       OneUnavailablePred = PredBB;
1024       continue;
1025     }
1026 
1027     // If AA tags disagree or are not present, forget about them.
1028     if (AATags != ThisAATags) AATags = AAMDNodes();
1029 
1030     // If so, this load is partially redundant.  Remember this info so that we
1031     // can create a PHI node.
1032     AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
1033   }
1034 
1035   // If the loaded value isn't available in any predecessor, it isn't partially
1036   // redundant.
1037   if (AvailablePreds.empty()) return false;
1038 
1039   // Okay, the loaded value is available in at least one (and maybe all!)
1040   // predecessors.  If the value is unavailable in more than one unique
1041   // predecessor, we want to insert a merge block for those common predecessors.
1042   // This ensures that we only have to insert one reload, thus not increasing
1043   // code size.
1044   BasicBlock *UnavailablePred = nullptr;
1045 
1046   // If there is exactly one predecessor where the value is unavailable, the
1047   // already computed 'OneUnavailablePred' block is it.  If it ends in an
1048   // unconditional branch, we know that it isn't a critical edge.
1049   if (PredsScanned.size() == AvailablePreds.size()+1 &&
1050       OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
1051     UnavailablePred = OneUnavailablePred;
1052   } else if (PredsScanned.size() != AvailablePreds.size()) {
1053     // Otherwise, we had multiple unavailable predecessors or we had a critical
1054     // edge from the one.
1055     SmallVector<BasicBlock*, 8> PredsToSplit;
1056     SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
1057 
1058     for (const auto &AvailablePred : AvailablePreds)
1059       AvailablePredSet.insert(AvailablePred.first);
1060 
1061     // Add all the unavailable predecessors to the PredsToSplit list.
1062     for (BasicBlock *P : predecessors(LoadBB)) {
1063       // If the predecessor is an indirect goto, we can't split the edge.
1064       if (isa<IndirectBrInst>(P->getTerminator()))
1065         return false;
1066 
1067       if (!AvailablePredSet.count(P))
1068         PredsToSplit.push_back(P);
1069     }
1070 
1071     // Split them out to their own block.
1072     UnavailablePred = SplitBlockPreds(LoadBB, PredsToSplit, "thread-pre-split");
1073   }
1074 
1075   // If the value isn't available in all predecessors, then there will be
1076   // exactly one where it isn't available.  Insert a load on that edge and add
1077   // it to the AvailablePreds list.
1078   if (UnavailablePred) {
1079     assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
1080            "Can't handle critical edge here!");
1081     LoadInst *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
1082                                  LI->getAlignment(),
1083                                  UnavailablePred->getTerminator());
1084     NewVal->setDebugLoc(LI->getDebugLoc());
1085     if (AATags)
1086       NewVal->setAAMetadata(AATags);
1087 
1088     AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
1089   }
1090 
1091   // Now we know that each predecessor of this block has a value in
1092   // AvailablePreds, sort them for efficient access as we're walking the preds.
1093   array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
1094 
1095   // Create a PHI node at the start of the block for the PRE'd load value.
1096   pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB);
1097   PHINode *PN = PHINode::Create(LI->getType(), std::distance(PB, PE), "",
1098                                 &LoadBB->front());
1099   PN->takeName(LI);
1100   PN->setDebugLoc(LI->getDebugLoc());
1101 
1102   // Insert new entries into the PHI for each predecessor.  A single block may
1103   // have multiple entries here.
1104   for (pred_iterator PI = PB; PI != PE; ++PI) {
1105     BasicBlock *P = *PI;
1106     AvailablePredsTy::iterator I =
1107       std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
1108                        std::make_pair(P, (Value*)nullptr));
1109 
1110     assert(I != AvailablePreds.end() && I->first == P &&
1111            "Didn't find entry for predecessor!");
1112 
1113     // If we have an available predecessor but it requires casting, insert the
1114     // cast in the predecessor and use the cast. Note that we have to update the
1115     // AvailablePreds vector as we go so that all of the PHI entries for this
1116     // predecessor use the same bitcast.
1117     Value *&PredV = I->second;
1118     if (PredV->getType() != LI->getType())
1119       PredV = CastInst::CreateBitOrPointerCast(PredV, LI->getType(), "",
1120                                                P->getTerminator());
1121 
1122     PN->addIncoming(PredV, I->first);
1123   }
1124 
1125   //cerr << "PRE: " << *LI << *PN << "\n";
1126 
1127   LI->replaceAllUsesWith(PN);
1128   LI->eraseFromParent();
1129 
1130   return true;
1131 }
1132 
1133 /// FindMostPopularDest - The specified list contains multiple possible
1134 /// threadable destinations.  Pick the one that occurs the most frequently in
1135 /// the list.
1136 static BasicBlock *
1137 FindMostPopularDest(BasicBlock *BB,
1138                     const SmallVectorImpl<std::pair<BasicBlock*,
1139                                   BasicBlock*> > &PredToDestList) {
1140   assert(!PredToDestList.empty());
1141 
1142   // Determine popularity.  If there are multiple possible destinations, we
1143   // explicitly choose to ignore 'undef' destinations.  We prefer to thread
1144   // blocks with known and real destinations to threading undef.  We'll handle
1145   // them later if interesting.
1146   DenseMap<BasicBlock*, unsigned> DestPopularity;
1147   for (const auto &PredToDest : PredToDestList)
1148     if (PredToDest.second)
1149       DestPopularity[PredToDest.second]++;
1150 
1151   // Find the most popular dest.
1152   DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
1153   BasicBlock *MostPopularDest = DPI->first;
1154   unsigned Popularity = DPI->second;
1155   SmallVector<BasicBlock*, 4> SamePopularity;
1156 
1157   for (++DPI; DPI != DestPopularity.end(); ++DPI) {
1158     // If the popularity of this entry isn't higher than the popularity we've
1159     // seen so far, ignore it.
1160     if (DPI->second < Popularity)
1161       ; // ignore.
1162     else if (DPI->second == Popularity) {
1163       // If it is the same as what we've seen so far, keep track of it.
1164       SamePopularity.push_back(DPI->first);
1165     } else {
1166       // If it is more popular, remember it.
1167       SamePopularity.clear();
1168       MostPopularDest = DPI->first;
1169       Popularity = DPI->second;
1170     }
1171   }
1172 
1173   // Okay, now we know the most popular destination.  If there is more than one
1174   // destination, we need to determine one.  This is arbitrary, but we need
1175   // to make a deterministic decision.  Pick the first one that appears in the
1176   // successor list.
1177   if (!SamePopularity.empty()) {
1178     SamePopularity.push_back(MostPopularDest);
1179     TerminatorInst *TI = BB->getTerminator();
1180     for (unsigned i = 0; ; ++i) {
1181       assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
1182 
1183       if (std::find(SamePopularity.begin(), SamePopularity.end(),
1184                     TI->getSuccessor(i)) == SamePopularity.end())
1185         continue;
1186 
1187       MostPopularDest = TI->getSuccessor(i);
1188       break;
1189     }
1190   }
1191 
1192   // Okay, we have finally picked the most popular destination.
1193   return MostPopularDest;
1194 }
1195 
1196 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
1197                                            ConstantPreference Preference,
1198                                            Instruction *CxtI) {
1199   // If threading this would thread across a loop header, don't even try to
1200   // thread the edge.
1201   if (LoopHeaders.count(BB))
1202     return false;
1203 
1204   PredValueInfoTy PredValues;
1205   if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference, CxtI))
1206     return false;
1207 
1208   assert(!PredValues.empty() &&
1209          "ComputeValueKnownInPredecessors returned true with no values");
1210 
1211   DEBUG(dbgs() << "IN BB: " << *BB;
1212         for (const auto &PredValue : PredValues) {
1213           dbgs() << "  BB '" << BB->getName() << "': FOUND condition = "
1214             << *PredValue.first
1215             << " for pred '" << PredValue.second->getName() << "'.\n";
1216         });
1217 
1218   // Decide what we want to thread through.  Convert our list of known values to
1219   // a list of known destinations for each pred.  This also discards duplicate
1220   // predecessors and keeps track of the undefined inputs (which are represented
1221   // as a null dest in the PredToDestList).
1222   SmallPtrSet<BasicBlock*, 16> SeenPreds;
1223   SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
1224 
1225   BasicBlock *OnlyDest = nullptr;
1226   BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1227 
1228   for (const auto &PredValue : PredValues) {
1229     BasicBlock *Pred = PredValue.second;
1230     if (!SeenPreds.insert(Pred).second)
1231       continue;  // Duplicate predecessor entry.
1232 
1233     // If the predecessor ends with an indirect goto, we can't change its
1234     // destination.
1235     if (isa<IndirectBrInst>(Pred->getTerminator()))
1236       continue;
1237 
1238     Constant *Val = PredValue.first;
1239 
1240     BasicBlock *DestBB;
1241     if (isa<UndefValue>(Val))
1242       DestBB = nullptr;
1243     else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1244       DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero());
1245     else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
1246       DestBB = SI->findCaseValue(cast<ConstantInt>(Val)).getCaseSuccessor();
1247     } else {
1248       assert(isa<IndirectBrInst>(BB->getTerminator())
1249               && "Unexpected terminator");
1250       DestBB = cast<BlockAddress>(Val)->getBasicBlock();
1251     }
1252 
1253     // If we have exactly one destination, remember it for efficiency below.
1254     if (PredToDestList.empty())
1255       OnlyDest = DestBB;
1256     else if (OnlyDest != DestBB)
1257       OnlyDest = MultipleDestSentinel;
1258 
1259     PredToDestList.push_back(std::make_pair(Pred, DestBB));
1260   }
1261 
1262   // If all edges were unthreadable, we fail.
1263   if (PredToDestList.empty())
1264     return false;
1265 
1266   // Determine which is the most common successor.  If we have many inputs and
1267   // this block is a switch, we want to start by threading the batch that goes
1268   // to the most popular destination first.  If we only know about one
1269   // threadable destination (the common case) we can avoid this.
1270   BasicBlock *MostPopularDest = OnlyDest;
1271 
1272   if (MostPopularDest == MultipleDestSentinel)
1273     MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1274 
1275   // Now that we know what the most popular destination is, factor all
1276   // predecessors that will jump to it into a single predecessor.
1277   SmallVector<BasicBlock*, 16> PredsToFactor;
1278   for (const auto &PredToDest : PredToDestList)
1279     if (PredToDest.second == MostPopularDest) {
1280       BasicBlock *Pred = PredToDest.first;
1281 
1282       // This predecessor may be a switch or something else that has multiple
1283       // edges to the block.  Factor each of these edges by listing them
1284       // according to # occurrences in PredsToFactor.
1285       for (BasicBlock *Succ : successors(Pred))
1286         if (Succ == BB)
1287           PredsToFactor.push_back(Pred);
1288     }
1289 
1290   // If the threadable edges are branching on an undefined value, we get to pick
1291   // the destination that these predecessors should get to.
1292   if (!MostPopularDest)
1293     MostPopularDest = BB->getTerminator()->
1294                             getSuccessor(GetBestDestForJumpOnUndef(BB));
1295 
1296   // Ok, try to thread it!
1297   return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1298 }
1299 
1300 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1301 /// a PHI node in the current block.  See if there are any simplifications we
1302 /// can do based on inputs to the phi node.
1303 ///
1304 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
1305   BasicBlock *BB = PN->getParent();
1306 
1307   // TODO: We could make use of this to do it once for blocks with common PHI
1308   // values.
1309   SmallVector<BasicBlock*, 1> PredBBs;
1310   PredBBs.resize(1);
1311 
1312   // If any of the predecessor blocks end in an unconditional branch, we can
1313   // *duplicate* the conditional branch into that block in order to further
1314   // encourage jump threading and to eliminate cases where we have branch on a
1315   // phi of an icmp (branch on icmp is much better).
1316   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1317     BasicBlock *PredBB = PN->getIncomingBlock(i);
1318     if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1319       if (PredBr->isUnconditional()) {
1320         PredBBs[0] = PredBB;
1321         // Try to duplicate BB into PredBB.
1322         if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
1323           return true;
1324       }
1325   }
1326 
1327   return false;
1328 }
1329 
1330 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
1331 /// a xor instruction in the current block.  See if there are any
1332 /// simplifications we can do based on inputs to the xor.
1333 ///
1334 bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
1335   BasicBlock *BB = BO->getParent();
1336 
1337   // If either the LHS or RHS of the xor is a constant, don't do this
1338   // optimization.
1339   if (isa<ConstantInt>(BO->getOperand(0)) ||
1340       isa<ConstantInt>(BO->getOperand(1)))
1341     return false;
1342 
1343   // If the first instruction in BB isn't a phi, we won't be able to infer
1344   // anything special about any particular predecessor.
1345   if (!isa<PHINode>(BB->front()))
1346     return false;
1347 
1348   // If we have a xor as the branch input to this block, and we know that the
1349   // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1350   // the condition into the predecessor and fix that value to true, saving some
1351   // logical ops on that path and encouraging other paths to simplify.
1352   //
1353   // This copies something like this:
1354   //
1355   //  BB:
1356   //    %X = phi i1 [1],  [%X']
1357   //    %Y = icmp eq i32 %A, %B
1358   //    %Z = xor i1 %X, %Y
1359   //    br i1 %Z, ...
1360   //
1361   // Into:
1362   //  BB':
1363   //    %Y = icmp ne i32 %A, %B
1364   //    br i1 %Y, ...
1365 
1366   PredValueInfoTy XorOpValues;
1367   bool isLHS = true;
1368   if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
1369                                        WantInteger, BO)) {
1370     assert(XorOpValues.empty());
1371     if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
1372                                          WantInteger, BO))
1373       return false;
1374     isLHS = false;
1375   }
1376 
1377   assert(!XorOpValues.empty() &&
1378          "ComputeValueKnownInPredecessors returned true with no values");
1379 
1380   // Scan the information to see which is most popular: true or false.  The
1381   // predecessors can be of the set true, false, or undef.
1382   unsigned NumTrue = 0, NumFalse = 0;
1383   for (const auto &XorOpValue : XorOpValues) {
1384     if (isa<UndefValue>(XorOpValue.first))
1385       // Ignore undefs for the count.
1386       continue;
1387     if (cast<ConstantInt>(XorOpValue.first)->isZero())
1388       ++NumFalse;
1389     else
1390       ++NumTrue;
1391   }
1392 
1393   // Determine which value to split on, true, false, or undef if neither.
1394   ConstantInt *SplitVal = nullptr;
1395   if (NumTrue > NumFalse)
1396     SplitVal = ConstantInt::getTrue(BB->getContext());
1397   else if (NumTrue != 0 || NumFalse != 0)
1398     SplitVal = ConstantInt::getFalse(BB->getContext());
1399 
1400   // Collect all of the blocks that this can be folded into so that we can
1401   // factor this once and clone it once.
1402   SmallVector<BasicBlock*, 8> BlocksToFoldInto;
1403   for (const auto &XorOpValue : XorOpValues) {
1404     if (XorOpValue.first != SplitVal && !isa<UndefValue>(XorOpValue.first))
1405       continue;
1406 
1407     BlocksToFoldInto.push_back(XorOpValue.second);
1408   }
1409 
1410   // If we inferred a value for all of the predecessors, then duplication won't
1411   // help us.  However, we can just replace the LHS or RHS with the constant.
1412   if (BlocksToFoldInto.size() ==
1413       cast<PHINode>(BB->front()).getNumIncomingValues()) {
1414     if (!SplitVal) {
1415       // If all preds provide undef, just nuke the xor, because it is undef too.
1416       BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
1417       BO->eraseFromParent();
1418     } else if (SplitVal->isZero()) {
1419       // If all preds provide 0, replace the xor with the other input.
1420       BO->replaceAllUsesWith(BO->getOperand(isLHS));
1421       BO->eraseFromParent();
1422     } else {
1423       // If all preds provide 1, set the computed value to 1.
1424       BO->setOperand(!isLHS, SplitVal);
1425     }
1426 
1427     return true;
1428   }
1429 
1430   // Try to duplicate BB into PredBB.
1431   return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
1432 }
1433 
1434 
1435 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1436 /// predecessor to the PHIBB block.  If it has PHI nodes, add entries for
1437 /// NewPred using the entries from OldPred (suitably mapped).
1438 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1439                                             BasicBlock *OldPred,
1440                                             BasicBlock *NewPred,
1441                                      DenseMap<Instruction*, Value*> &ValueMap) {
1442   for (BasicBlock::iterator PNI = PHIBB->begin();
1443        PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1444     // Ok, we have a PHI node.  Figure out what the incoming value was for the
1445     // DestBlock.
1446     Value *IV = PN->getIncomingValueForBlock(OldPred);
1447 
1448     // Remap the value if necessary.
1449     if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1450       DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1451       if (I != ValueMap.end())
1452         IV = I->second;
1453     }
1454 
1455     PN->addIncoming(IV, NewPred);
1456   }
1457 }
1458 
1459 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1460 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1461 /// across BB.  Transform the IR to reflect this change.
1462 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1463                                const SmallVectorImpl<BasicBlock*> &PredBBs,
1464                                BasicBlock *SuccBB) {
1465   // If threading to the same block as we come from, we would infinite loop.
1466   if (SuccBB == BB) {
1467     DEBUG(dbgs() << "  Not threading across BB '" << BB->getName()
1468           << "' - would thread to self!\n");
1469     return false;
1470   }
1471 
1472   // If threading this would thread across a loop header, don't thread the edge.
1473   // See the comments above FindLoopHeaders for justifications and caveats.
1474   if (LoopHeaders.count(BB)) {
1475     DEBUG(dbgs() << "  Not threading across loop header BB '" << BB->getName()
1476           << "' to dest BB '" << SuccBB->getName()
1477           << "' - it might create an irreducible loop!\n");
1478     return false;
1479   }
1480 
1481   unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB, BBDupThreshold);
1482   if (JumpThreadCost > BBDupThreshold) {
1483     DEBUG(dbgs() << "  Not threading BB '" << BB->getName()
1484           << "' - Cost is too high: " << JumpThreadCost << "\n");
1485     return false;
1486   }
1487 
1488   // And finally, do it!  Start by factoring the predecessors if needed.
1489   BasicBlock *PredBB;
1490   if (PredBBs.size() == 1)
1491     PredBB = PredBBs[0];
1492   else {
1493     DEBUG(dbgs() << "  Factoring out " << PredBBs.size()
1494           << " common predecessors.\n");
1495     PredBB = SplitBlockPreds(BB, PredBBs, ".thr_comm");
1496   }
1497 
1498   // And finally, do it!
1499   DEBUG(dbgs() << "  Threading edge from '" << PredBB->getName() << "' to '"
1500         << SuccBB->getName() << "' with cost: " << JumpThreadCost
1501         << ", across block:\n    "
1502         << *BB << "\n");
1503 
1504   LVI->threadEdge(PredBB, BB, SuccBB);
1505 
1506   // We are going to have to map operands from the original BB block to the new
1507   // copy of the block 'NewBB'.  If there are PHI nodes in BB, evaluate them to
1508   // account for entry from PredBB.
1509   DenseMap<Instruction*, Value*> ValueMapping;
1510 
1511   BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1512                                          BB->getName()+".thread",
1513                                          BB->getParent(), BB);
1514   NewBB->moveAfter(PredBB);
1515 
1516   // Set the block frequency of NewBB.
1517   if (HasProfileData) {
1518     auto NewBBFreq =
1519         BFI->getBlockFreq(PredBB) * BPI->getEdgeProbability(PredBB, BB);
1520     BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
1521   }
1522 
1523   BasicBlock::iterator BI = BB->begin();
1524   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1525     ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1526 
1527   // Clone the non-phi instructions of BB into NewBB, keeping track of the
1528   // mapping and using it to remap operands in the cloned instructions.
1529   for (; !isa<TerminatorInst>(BI); ++BI) {
1530     Instruction *New = BI->clone();
1531     New->setName(BI->getName());
1532     NewBB->getInstList().push_back(New);
1533     ValueMapping[&*BI] = New;
1534 
1535     // Remap operands to patch up intra-block references.
1536     for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1537       if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1538         DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1539         if (I != ValueMapping.end())
1540           New->setOperand(i, I->second);
1541       }
1542   }
1543 
1544   // We didn't copy the terminator from BB over to NewBB, because there is now
1545   // an unconditional jump to SuccBB.  Insert the unconditional jump.
1546   BranchInst *NewBI = BranchInst::Create(SuccBB, NewBB);
1547   NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
1548 
1549   // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1550   // PHI nodes for NewBB now.
1551   AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1552 
1553   // If there were values defined in BB that are used outside the block, then we
1554   // now have to update all uses of the value to use either the original value,
1555   // the cloned value, or some PHI derived value.  This can require arbitrary
1556   // PHI insertion, of which we are prepared to do, clean these up now.
1557   SSAUpdater SSAUpdate;
1558   SmallVector<Use*, 16> UsesToRename;
1559   for (Instruction &I : *BB) {
1560     // Scan all uses of this instruction to see if it is used outside of its
1561     // block, and if so, record them in UsesToRename.
1562     for (Use &U : I.uses()) {
1563       Instruction *User = cast<Instruction>(U.getUser());
1564       if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1565         if (UserPN->getIncomingBlock(U) == BB)
1566           continue;
1567       } else if (User->getParent() == BB)
1568         continue;
1569 
1570       UsesToRename.push_back(&U);
1571     }
1572 
1573     // If there are no uses outside the block, we're done with this instruction.
1574     if (UsesToRename.empty())
1575       continue;
1576 
1577     DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I << "\n");
1578 
1579     // We found a use of I outside of BB.  Rename all uses of I that are outside
1580     // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
1581     // with the two values we know.
1582     SSAUpdate.Initialize(I.getType(), I.getName());
1583     SSAUpdate.AddAvailableValue(BB, &I);
1584     SSAUpdate.AddAvailableValue(NewBB, ValueMapping[&I]);
1585 
1586     while (!UsesToRename.empty())
1587       SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1588     DEBUG(dbgs() << "\n");
1589   }
1590 
1591 
1592   // Ok, NewBB is good to go.  Update the terminator of PredBB to jump to
1593   // NewBB instead of BB.  This eliminates predecessors from BB, which requires
1594   // us to simplify any PHI nodes in BB.
1595   TerminatorInst *PredTerm = PredBB->getTerminator();
1596   for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1597     if (PredTerm->getSuccessor(i) == BB) {
1598       BB->removePredecessor(PredBB, true);
1599       PredTerm->setSuccessor(i, NewBB);
1600     }
1601 
1602   // At this point, the IR is fully up to date and consistent.  Do a quick scan
1603   // over the new instructions and zap any that are constants or dead.  This
1604   // frequently happens because of phi translation.
1605   SimplifyInstructionsInBlock(NewBB, TLI);
1606 
1607   // Update the edge weight from BB to SuccBB, which should be less than before.
1608   UpdateBlockFreqAndEdgeWeight(PredBB, BB, NewBB, SuccBB);
1609 
1610   // Threaded an edge!
1611   ++NumThreads;
1612   return true;
1613 }
1614 
1615 /// Create a new basic block that will be the predecessor of BB and successor of
1616 /// all blocks in Preds. When profile data is availble, update the frequency of
1617 /// this new block.
1618 BasicBlock *JumpThreading::SplitBlockPreds(BasicBlock *BB,
1619                                            ArrayRef<BasicBlock *> Preds,
1620                                            const char *Suffix) {
1621   // Collect the frequencies of all predecessors of BB, which will be used to
1622   // update the edge weight on BB->SuccBB.
1623   BlockFrequency PredBBFreq(0);
1624   if (HasProfileData)
1625     for (auto Pred : Preds)
1626       PredBBFreq += BFI->getBlockFreq(Pred) * BPI->getEdgeProbability(Pred, BB);
1627 
1628   BasicBlock *PredBB = SplitBlockPredecessors(BB, Preds, Suffix);
1629 
1630   // Set the block frequency of the newly created PredBB, which is the sum of
1631   // frequencies of Preds.
1632   if (HasProfileData)
1633     BFI->setBlockFreq(PredBB, PredBBFreq.getFrequency());
1634   return PredBB;
1635 }
1636 
1637 /// Update the block frequency of BB and branch weight and the metadata on the
1638 /// edge BB->SuccBB. This is done by scaling the weight of BB->SuccBB by 1 -
1639 /// Freq(PredBB->BB) / Freq(BB->SuccBB).
1640 void JumpThreading::UpdateBlockFreqAndEdgeWeight(BasicBlock *PredBB,
1641                                                  BasicBlock *BB,
1642                                                  BasicBlock *NewBB,
1643                                                  BasicBlock *SuccBB) {
1644   if (!HasProfileData)
1645     return;
1646 
1647   assert(BFI && BPI && "BFI & BPI should have been created here");
1648 
1649   // As the edge from PredBB to BB is deleted, we have to update the block
1650   // frequency of BB.
1651   auto BBOrigFreq = BFI->getBlockFreq(BB);
1652   auto NewBBFreq = BFI->getBlockFreq(NewBB);
1653   auto BB2SuccBBFreq = BBOrigFreq * BPI->getEdgeProbability(BB, SuccBB);
1654   auto BBNewFreq = BBOrigFreq - NewBBFreq;
1655   BFI->setBlockFreq(BB, BBNewFreq.getFrequency());
1656 
1657   // Collect updated outgoing edges' frequencies from BB and use them to update
1658   // edge probabilities.
1659   SmallVector<uint64_t, 4> BBSuccFreq;
1660   for (BasicBlock *Succ : successors(BB)) {
1661     auto SuccFreq = (Succ == SuccBB)
1662                         ? BB2SuccBBFreq - NewBBFreq
1663                         : BBOrigFreq * BPI->getEdgeProbability(BB, Succ);
1664     BBSuccFreq.push_back(SuccFreq.getFrequency());
1665   }
1666 
1667   uint64_t MaxBBSuccFreq =
1668       *std::max_element(BBSuccFreq.begin(), BBSuccFreq.end());
1669 
1670   SmallVector<BranchProbability, 4> BBSuccProbs;
1671   if (MaxBBSuccFreq == 0)
1672     BBSuccProbs.assign(BBSuccFreq.size(),
1673                        {1, static_cast<unsigned>(BBSuccFreq.size())});
1674   else {
1675     for (uint64_t Freq : BBSuccFreq)
1676       BBSuccProbs.push_back(
1677           BranchProbability::getBranchProbability(Freq, MaxBBSuccFreq));
1678     // Normalize edge probabilities so that they sum up to one.
1679     BranchProbability::normalizeProbabilities(BBSuccProbs.begin(),
1680                                               BBSuccProbs.end());
1681   }
1682 
1683   // Update edge probabilities in BPI.
1684   for (int I = 0, E = BBSuccProbs.size(); I < E; I++)
1685     BPI->setEdgeProbability(BB, I, BBSuccProbs[I]);
1686 
1687   if (BBSuccProbs.size() >= 2) {
1688     SmallVector<uint32_t, 4> Weights;
1689     for (auto Prob : BBSuccProbs)
1690       Weights.push_back(Prob.getNumerator());
1691 
1692     auto TI = BB->getTerminator();
1693     TI->setMetadata(
1694         LLVMContext::MD_prof,
1695         MDBuilder(TI->getParent()->getContext()).createBranchWeights(Weights));
1696   }
1697 }
1698 
1699 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1700 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1701 /// If we can duplicate the contents of BB up into PredBB do so now, this
1702 /// improves the odds that the branch will be on an analyzable instruction like
1703 /// a compare.
1704 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1705                                  const SmallVectorImpl<BasicBlock *> &PredBBs) {
1706   assert(!PredBBs.empty() && "Can't handle an empty set");
1707 
1708   // If BB is a loop header, then duplicating this block outside the loop would
1709   // cause us to transform this into an irreducible loop, don't do this.
1710   // See the comments above FindLoopHeaders for justifications and caveats.
1711   if (LoopHeaders.count(BB)) {
1712     DEBUG(dbgs() << "  Not duplicating loop header '" << BB->getName()
1713           << "' into predecessor block '" << PredBBs[0]->getName()
1714           << "' - it might create an irreducible loop!\n");
1715     return false;
1716   }
1717 
1718   unsigned DuplicationCost = getJumpThreadDuplicationCost(BB, BBDupThreshold);
1719   if (DuplicationCost > BBDupThreshold) {
1720     DEBUG(dbgs() << "  Not duplicating BB '" << BB->getName()
1721           << "' - Cost is too high: " << DuplicationCost << "\n");
1722     return false;
1723   }
1724 
1725   // And finally, do it!  Start by factoring the predecessors if needed.
1726   BasicBlock *PredBB;
1727   if (PredBBs.size() == 1)
1728     PredBB = PredBBs[0];
1729   else {
1730     DEBUG(dbgs() << "  Factoring out " << PredBBs.size()
1731           << " common predecessors.\n");
1732     PredBB = SplitBlockPreds(BB, PredBBs, ".thr_comm");
1733   }
1734 
1735   // Okay, we decided to do this!  Clone all the instructions in BB onto the end
1736   // of PredBB.
1737   DEBUG(dbgs() << "  Duplicating block '" << BB->getName() << "' into end of '"
1738         << PredBB->getName() << "' to eliminate branch on phi.  Cost: "
1739         << DuplicationCost << " block is:" << *BB << "\n");
1740 
1741   // Unless PredBB ends with an unconditional branch, split the edge so that we
1742   // can just clone the bits from BB into the end of the new PredBB.
1743   BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
1744 
1745   if (!OldPredBranch || !OldPredBranch->isUnconditional()) {
1746     PredBB = SplitEdge(PredBB, BB);
1747     OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1748   }
1749 
1750   // We are going to have to map operands from the original BB block into the
1751   // PredBB block.  Evaluate PHI nodes in BB.
1752   DenseMap<Instruction*, Value*> ValueMapping;
1753 
1754   BasicBlock::iterator BI = BB->begin();
1755   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1756     ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1757   // Clone the non-phi instructions of BB into PredBB, keeping track of the
1758   // mapping and using it to remap operands in the cloned instructions.
1759   for (; BI != BB->end(); ++BI) {
1760     Instruction *New = BI->clone();
1761 
1762     // Remap operands to patch up intra-block references.
1763     for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1764       if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1765         DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1766         if (I != ValueMapping.end())
1767           New->setOperand(i, I->second);
1768       }
1769 
1770     // If this instruction can be simplified after the operands are updated,
1771     // just use the simplified value instead.  This frequently happens due to
1772     // phi translation.
1773     if (Value *IV =
1774             SimplifyInstruction(New, BB->getModule()->getDataLayout())) {
1775       delete New;
1776       ValueMapping[&*BI] = IV;
1777     } else {
1778       // Otherwise, insert the new instruction into the block.
1779       New->setName(BI->getName());
1780       PredBB->getInstList().insert(OldPredBranch->getIterator(), New);
1781       ValueMapping[&*BI] = New;
1782     }
1783   }
1784 
1785   // Check to see if the targets of the branch had PHI nodes. If so, we need to
1786   // add entries to the PHI nodes for branch from PredBB now.
1787   BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1788   AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1789                                   ValueMapping);
1790   AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1791                                   ValueMapping);
1792 
1793   // If there were values defined in BB that are used outside the block, then we
1794   // now have to update all uses of the value to use either the original value,
1795   // the cloned value, or some PHI derived value.  This can require arbitrary
1796   // PHI insertion, of which we are prepared to do, clean these up now.
1797   SSAUpdater SSAUpdate;
1798   SmallVector<Use*, 16> UsesToRename;
1799   for (Instruction &I : *BB) {
1800     // Scan all uses of this instruction to see if it is used outside of its
1801     // block, and if so, record them in UsesToRename.
1802     for (Use &U : I.uses()) {
1803       Instruction *User = cast<Instruction>(U.getUser());
1804       if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1805         if (UserPN->getIncomingBlock(U) == BB)
1806           continue;
1807       } else if (User->getParent() == BB)
1808         continue;
1809 
1810       UsesToRename.push_back(&U);
1811     }
1812 
1813     // If there are no uses outside the block, we're done with this instruction.
1814     if (UsesToRename.empty())
1815       continue;
1816 
1817     DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I << "\n");
1818 
1819     // We found a use of I outside of BB.  Rename all uses of I that are outside
1820     // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
1821     // with the two values we know.
1822     SSAUpdate.Initialize(I.getType(), I.getName());
1823     SSAUpdate.AddAvailableValue(BB, &I);
1824     SSAUpdate.AddAvailableValue(PredBB, ValueMapping[&I]);
1825 
1826     while (!UsesToRename.empty())
1827       SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1828     DEBUG(dbgs() << "\n");
1829   }
1830 
1831   // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1832   // that we nuked.
1833   BB->removePredecessor(PredBB, true);
1834 
1835   // Remove the unconditional branch at the end of the PredBB block.
1836   OldPredBranch->eraseFromParent();
1837 
1838   ++NumDupes;
1839   return true;
1840 }
1841 
1842 /// TryToUnfoldSelect - Look for blocks of the form
1843 /// bb1:
1844 ///   %a = select
1845 ///   br bb
1846 ///
1847 /// bb2:
1848 ///   %p = phi [%a, %bb] ...
1849 ///   %c = icmp %p
1850 ///   br i1 %c
1851 ///
1852 /// And expand the select into a branch structure if one of its arms allows %c
1853 /// to be folded. This later enables threading from bb1 over bb2.
1854 bool JumpThreading::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
1855   BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
1856   PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0));
1857   Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1));
1858 
1859   if (!CondBr || !CondBr->isConditional() || !CondLHS ||
1860       CondLHS->getParent() != BB)
1861     return false;
1862 
1863   for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) {
1864     BasicBlock *Pred = CondLHS->getIncomingBlock(I);
1865     SelectInst *SI = dyn_cast<SelectInst>(CondLHS->getIncomingValue(I));
1866 
1867     // Look if one of the incoming values is a select in the corresponding
1868     // predecessor.
1869     if (!SI || SI->getParent() != Pred || !SI->hasOneUse())
1870       continue;
1871 
1872     BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
1873     if (!PredTerm || !PredTerm->isUnconditional())
1874       continue;
1875 
1876     // Now check if one of the select values would allow us to constant fold the
1877     // terminator in BB. We don't do the transform if both sides fold, those
1878     // cases will be threaded in any case.
1879     LazyValueInfo::Tristate LHSFolds =
1880         LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(1),
1881                                 CondRHS, Pred, BB, CondCmp);
1882     LazyValueInfo::Tristate RHSFolds =
1883         LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(2),
1884                                 CondRHS, Pred, BB, CondCmp);
1885     if ((LHSFolds != LazyValueInfo::Unknown ||
1886          RHSFolds != LazyValueInfo::Unknown) &&
1887         LHSFolds != RHSFolds) {
1888       // Expand the select.
1889       //
1890       // Pred --
1891       //  |    v
1892       //  |  NewBB
1893       //  |    |
1894       //  |-----
1895       //  v
1896       // BB
1897       BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold",
1898                                              BB->getParent(), BB);
1899       // Move the unconditional branch to NewBB.
1900       PredTerm->removeFromParent();
1901       NewBB->getInstList().insert(NewBB->end(), PredTerm);
1902       // Create a conditional branch and update PHI nodes.
1903       BranchInst::Create(NewBB, BB, SI->getCondition(), Pred);
1904       CondLHS->setIncomingValue(I, SI->getFalseValue());
1905       CondLHS->addIncoming(SI->getTrueValue(), NewBB);
1906       // The select is now dead.
1907       SI->eraseFromParent();
1908 
1909       // Update any other PHI nodes in BB.
1910       for (BasicBlock::iterator BI = BB->begin();
1911            PHINode *Phi = dyn_cast<PHINode>(BI); ++BI)
1912         if (Phi != CondLHS)
1913           Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB);
1914       return true;
1915     }
1916   }
1917   return false;
1918 }
1919 
1920 /// TryToUnfoldSelectInCurrBB - Look for PHI/Select in the same BB of the form
1921 /// bb:
1922 ///   %p = phi [false, %bb1], [true, %bb2], [false, %bb3], [true, %bb4], ...
1923 ///   %s = select p, trueval, falseval
1924 ///
1925 /// And expand the select into a branch structure. This later enables
1926 /// jump-threading over bb in this pass.
1927 ///
1928 /// Using the similar approach of SimplifyCFG::FoldCondBranchOnPHI(), unfold
1929 /// select if the associated PHI has at least one constant.  If the unfolded
1930 /// select is not jump-threaded, it will be folded again in the later
1931 /// optimizations.
1932 bool JumpThreading::TryToUnfoldSelectInCurrBB(BasicBlock *BB) {
1933   // If threading this would thread across a loop header, don't thread the edge.
1934   // See the comments above FindLoopHeaders for justifications and caveats.
1935   if (LoopHeaders.count(BB))
1936     return false;
1937 
1938   // Look for a Phi/Select pair in the same basic block.  The Phi feeds the
1939   // condition of the Select and at least one of the incoming values is a
1940   // constant.
1941   for (BasicBlock::iterator BI = BB->begin();
1942        PHINode *PN = dyn_cast<PHINode>(BI); ++BI) {
1943     unsigned NumPHIValues = PN->getNumIncomingValues();
1944     if (NumPHIValues == 0 || !PN->hasOneUse())
1945       continue;
1946 
1947     SelectInst *SI = dyn_cast<SelectInst>(PN->user_back());
1948     if (!SI || SI->getParent() != BB)
1949       continue;
1950 
1951     Value *Cond = SI->getCondition();
1952     if (!Cond || Cond != PN || !Cond->getType()->isIntegerTy(1))
1953       continue;
1954 
1955     bool HasConst = false;
1956     for (unsigned i = 0; i != NumPHIValues; ++i) {
1957       if (PN->getIncomingBlock(i) == BB)
1958         return false;
1959       if (isa<ConstantInt>(PN->getIncomingValue(i)))
1960         HasConst = true;
1961     }
1962 
1963     if (HasConst) {
1964       // Expand the select.
1965       TerminatorInst *Term =
1966           SplitBlockAndInsertIfThen(SI->getCondition(), SI, false);
1967       PHINode *NewPN = PHINode::Create(SI->getType(), 2, "", SI);
1968       NewPN->addIncoming(SI->getTrueValue(), Term->getParent());
1969       NewPN->addIncoming(SI->getFalseValue(), BB);
1970       SI->replaceAllUsesWith(NewPN);
1971       SI->eraseFromParent();
1972       return true;
1973     }
1974   }
1975 
1976   return false;
1977 }
1978