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1 //===- JumpThreading.cpp - Thread control through conditional blocks ------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements the Jump Threading pass.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Transforms/Scalar/JumpThreading.h"
15 #include "llvm/Transforms/Scalar.h"
16 #include "llvm/ADT/DenseMap.h"
17 #include "llvm/ADT/DenseSet.h"
18 #include "llvm/ADT/STLExtras.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/GlobalsModRef.h"
21 #include "llvm/Analysis/CFG.h"
22 #include "llvm/Analysis/BlockFrequencyInfoImpl.h"
23 #include "llvm/Analysis/ConstantFolding.h"
24 #include "llvm/Analysis/InstructionSimplify.h"
25 #include "llvm/Analysis/Loads.h"
26 #include "llvm/Analysis/LoopInfo.h"
27 #include "llvm/Analysis/ValueTracking.h"
28 #include "llvm/IR/DataLayout.h"
29 #include "llvm/IR/IntrinsicInst.h"
30 #include "llvm/IR/LLVMContext.h"
31 #include "llvm/IR/MDBuilder.h"
32 #include "llvm/IR/Metadata.h"
33 #include "llvm/Pass.h"
34 #include "llvm/Support/CommandLine.h"
35 #include "llvm/Support/Debug.h"
36 #include "llvm/Support/raw_ostream.h"
37 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
38 #include "llvm/Transforms/Utils/Local.h"
39 #include "llvm/Transforms/Utils/SSAUpdater.h"
40 #include <algorithm>
41 #include <memory>
42 using namespace llvm;
43 using namespace jumpthreading;
44 
45 #define DEBUG_TYPE "jump-threading"
46 
47 STATISTIC(NumThreads, "Number of jumps threaded");
48 STATISTIC(NumFolds,   "Number of terminators folded");
49 STATISTIC(NumDupes,   "Number of branch blocks duplicated to eliminate phi");
50 
51 static cl::opt<unsigned>
52 BBDuplicateThreshold("jump-threading-threshold",
53           cl::desc("Max block size to duplicate for jump threading"),
54           cl::init(6), cl::Hidden);
55 
56 static cl::opt<unsigned>
57 ImplicationSearchThreshold(
58   "jump-threading-implication-search-threshold",
59   cl::desc("The number of predecessors to search for a stronger "
60            "condition to use to thread over a weaker condition"),
61   cl::init(3), cl::Hidden);
62 
63 namespace {
64   /// This pass performs 'jump threading', which looks at blocks that have
65   /// multiple predecessors and multiple successors.  If one or more of the
66   /// predecessors of the block can be proven to always jump to one of the
67   /// successors, we forward the edge from the predecessor to the successor by
68   /// duplicating the contents of this block.
69   ///
70   /// An example of when this can occur is code like this:
71   ///
72   ///   if () { ...
73   ///     X = 4;
74   ///   }
75   ///   if (X < 3) {
76   ///
77   /// In this case, the unconditional branch at the end of the first if can be
78   /// revectored to the false side of the second if.
79   ///
80   class JumpThreading : public FunctionPass {
81     JumpThreadingPass Impl;
82 
83   public:
84     static char ID; // Pass identification
JumpThreading(int T=-1)85     JumpThreading(int T = -1) : FunctionPass(ID), Impl(T) {
86       initializeJumpThreadingPass(*PassRegistry::getPassRegistry());
87     }
88 
89     bool runOnFunction(Function &F) override;
90 
getAnalysisUsage(AnalysisUsage & AU) const91     void getAnalysisUsage(AnalysisUsage &AU) const override {
92       AU.addRequired<LazyValueInfoWrapperPass>();
93       AU.addPreserved<LazyValueInfoWrapperPass>();
94       AU.addPreserved<GlobalsAAWrapperPass>();
95       AU.addRequired<TargetLibraryInfoWrapperPass>();
96     }
97 
releaseMemory()98     void releaseMemory() override { Impl.releaseMemory(); }
99   };
100 }
101 
102 char JumpThreading::ID = 0;
103 INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading",
104                 "Jump Threading", false, false)
INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass)105 INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass)
106 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
107 INITIALIZE_PASS_END(JumpThreading, "jump-threading",
108                 "Jump Threading", false, false)
109 
110 // Public interface to the Jump Threading pass
111 FunctionPass *llvm::createJumpThreadingPass(int Threshold) { return new JumpThreading(Threshold); }
112 
JumpThreadingPass(int T)113 JumpThreadingPass::JumpThreadingPass(int T) {
114   BBDupThreshold = (T == -1) ? BBDuplicateThreshold : unsigned(T);
115 }
116 
117 /// runOnFunction - Top level algorithm.
118 ///
runOnFunction(Function & F)119 bool JumpThreading::runOnFunction(Function &F) {
120   if (skipFunction(F))
121     return false;
122   auto TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
123   auto LVI = &getAnalysis<LazyValueInfoWrapperPass>().getLVI();
124   std::unique_ptr<BlockFrequencyInfo> BFI;
125   std::unique_ptr<BranchProbabilityInfo> BPI;
126   bool HasProfileData = F.getEntryCount().hasValue();
127   if (HasProfileData) {
128     LoopInfo LI{DominatorTree(F)};
129     BPI.reset(new BranchProbabilityInfo(F, LI));
130     BFI.reset(new BlockFrequencyInfo(F, *BPI, LI));
131   }
132   return Impl.runImpl(F, TLI, LVI, HasProfileData, std::move(BFI),
133                       std::move(BPI));
134 }
135 
run(Function & F,AnalysisManager<Function> & AM)136 PreservedAnalyses JumpThreadingPass::run(Function &F,
137                                          AnalysisManager<Function> &AM) {
138 
139   auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
140   auto &LVI = AM.getResult<LazyValueAnalysis>(F);
141   std::unique_ptr<BlockFrequencyInfo> BFI;
142   std::unique_ptr<BranchProbabilityInfo> BPI;
143   bool HasProfileData = F.getEntryCount().hasValue();
144   if (HasProfileData) {
145     LoopInfo LI{DominatorTree(F)};
146     BPI.reset(new BranchProbabilityInfo(F, LI));
147     BFI.reset(new BlockFrequencyInfo(F, *BPI, LI));
148   }
149   bool Changed =
150       runImpl(F, &TLI, &LVI, HasProfileData, std::move(BFI), std::move(BPI));
151 
152   // FIXME: We need to invalidate LVI to avoid PR28400. Is there a better
153   // solution?
154   AM.invalidate<LazyValueAnalysis>(F);
155 
156   if (!Changed)
157     return PreservedAnalyses::all();
158   PreservedAnalyses PA;
159   PA.preserve<GlobalsAA>();
160   return PA;
161 }
162 
runImpl(Function & F,TargetLibraryInfo * TLI_,LazyValueInfo * LVI_,bool HasProfileData_,std::unique_ptr<BlockFrequencyInfo> BFI_,std::unique_ptr<BranchProbabilityInfo> BPI_)163 bool JumpThreadingPass::runImpl(Function &F, TargetLibraryInfo *TLI_,
164                                 LazyValueInfo *LVI_, bool HasProfileData_,
165                                 std::unique_ptr<BlockFrequencyInfo> BFI_,
166                                 std::unique_ptr<BranchProbabilityInfo> BPI_) {
167 
168   DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
169   TLI = TLI_;
170   LVI = LVI_;
171   BFI.reset();
172   BPI.reset();
173   // When profile data is available, we need to update edge weights after
174   // successful jump threading, which requires both BPI and BFI being available.
175   HasProfileData = HasProfileData_;
176   if (HasProfileData) {
177     BPI = std::move(BPI_);
178     BFI = std::move(BFI_);
179   }
180 
181   // Remove unreachable blocks from function as they may result in infinite
182   // loop. We do threading if we found something profitable. Jump threading a
183   // branch can create other opportunities. If these opportunities form a cycle
184   // i.e. if any jump threading is undoing previous threading in the path, then
185   // we will loop forever. We take care of this issue by not jump threading for
186   // back edges. This works for normal cases but not for unreachable blocks as
187   // they may have cycle with no back edge.
188   bool EverChanged = false;
189   EverChanged |= removeUnreachableBlocks(F, LVI);
190 
191   FindLoopHeaders(F);
192 
193   bool Changed;
194   do {
195     Changed = false;
196     for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
197       BasicBlock *BB = &*I;
198       // Thread all of the branches we can over this block.
199       while (ProcessBlock(BB))
200         Changed = true;
201 
202       ++I;
203 
204       // If the block is trivially dead, zap it.  This eliminates the successor
205       // edges which simplifies the CFG.
206       if (pred_empty(BB) &&
207           BB != &BB->getParent()->getEntryBlock()) {
208         DEBUG(dbgs() << "  JT: Deleting dead block '" << BB->getName()
209               << "' with terminator: " << *BB->getTerminator() << '\n');
210         LoopHeaders.erase(BB);
211         LVI->eraseBlock(BB);
212         DeleteDeadBlock(BB);
213         Changed = true;
214         continue;
215       }
216 
217       BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
218 
219       // Can't thread an unconditional jump, but if the block is "almost
220       // empty", we can replace uses of it with uses of the successor and make
221       // this dead.
222       // We should not eliminate the loop header either, because eliminating
223       // a loop header might later prevent LoopSimplify from transforming nested
224       // loops into simplified form.
225       if (BI && BI->isUnconditional() &&
226           BB != &BB->getParent()->getEntryBlock() &&
227           // If the terminator is the only non-phi instruction, try to nuke it.
228           BB->getFirstNonPHIOrDbg()->isTerminator() && !LoopHeaders.count(BB)) {
229         // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
230         // block, we have to make sure it isn't in the LoopHeaders set.  We
231         // reinsert afterward if needed.
232         bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
233         BasicBlock *Succ = BI->getSuccessor(0);
234 
235         // FIXME: It is always conservatively correct to drop the info
236         // for a block even if it doesn't get erased.  This isn't totally
237         // awesome, but it allows us to use AssertingVH to prevent nasty
238         // dangling pointer issues within LazyValueInfo.
239         LVI->eraseBlock(BB);
240         if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
241           Changed = true;
242           // If we deleted BB and BB was the header of a loop, then the
243           // successor is now the header of the loop.
244           BB = Succ;
245         }
246 
247         if (ErasedFromLoopHeaders)
248           LoopHeaders.insert(BB);
249       }
250     }
251     EverChanged |= Changed;
252   } while (Changed);
253 
254   LoopHeaders.clear();
255   return EverChanged;
256 }
257 
258 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
259 /// thread across it. Stop scanning the block when passing the threshold.
getJumpThreadDuplicationCost(const BasicBlock * BB,unsigned Threshold)260 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB,
261                                              unsigned Threshold) {
262   /// Ignore PHI nodes, these will be flattened when duplication happens.
263   BasicBlock::const_iterator I(BB->getFirstNonPHI());
264 
265   // FIXME: THREADING will delete values that are just used to compute the
266   // branch, so they shouldn't count against the duplication cost.
267 
268   unsigned Bonus = 0;
269   const TerminatorInst *BBTerm = BB->getTerminator();
270   // Threading through a switch statement is particularly profitable.  If this
271   // block ends in a switch, decrease its cost to make it more likely to happen.
272   if (isa<SwitchInst>(BBTerm))
273     Bonus = 6;
274 
275   // The same holds for indirect branches, but slightly more so.
276   if (isa<IndirectBrInst>(BBTerm))
277     Bonus = 8;
278 
279   // Bump the threshold up so the early exit from the loop doesn't skip the
280   // terminator-based Size adjustment at the end.
281   Threshold += Bonus;
282 
283   // Sum up the cost of each instruction until we get to the terminator.  Don't
284   // include the terminator because the copy won't include it.
285   unsigned Size = 0;
286   for (; !isa<TerminatorInst>(I); ++I) {
287 
288     // Stop scanning the block if we've reached the threshold.
289     if (Size > Threshold)
290       return Size;
291 
292     // Debugger intrinsics don't incur code size.
293     if (isa<DbgInfoIntrinsic>(I)) continue;
294 
295     // If this is a pointer->pointer bitcast, it is free.
296     if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
297       continue;
298 
299     // Bail out if this instruction gives back a token type, it is not possible
300     // to duplicate it if it is used outside this BB.
301     if (I->getType()->isTokenTy() && I->isUsedOutsideOfBlock(BB))
302       return ~0U;
303 
304     // All other instructions count for at least one unit.
305     ++Size;
306 
307     // Calls are more expensive.  If they are non-intrinsic calls, we model them
308     // as having cost of 4.  If they are a non-vector intrinsic, we model them
309     // as having cost of 2 total, and if they are a vector intrinsic, we model
310     // them as having cost 1.
311     if (const CallInst *CI = dyn_cast<CallInst>(I)) {
312       if (CI->cannotDuplicate() || CI->isConvergent())
313         // Blocks with NoDuplicate are modelled as having infinite cost, so they
314         // are never duplicated.
315         return ~0U;
316       else if (!isa<IntrinsicInst>(CI))
317         Size += 3;
318       else if (!CI->getType()->isVectorTy())
319         Size += 1;
320     }
321   }
322 
323   return Size > Bonus ? Size - Bonus : 0;
324 }
325 
326 /// FindLoopHeaders - We do not want jump threading to turn proper loop
327 /// structures into irreducible loops.  Doing this breaks up the loop nesting
328 /// hierarchy and pessimizes later transformations.  To prevent this from
329 /// happening, we first have to find the loop headers.  Here we approximate this
330 /// by finding targets of backedges in the CFG.
331 ///
332 /// Note that there definitely are cases when we want to allow threading of
333 /// edges across a loop header.  For example, threading a jump from outside the
334 /// loop (the preheader) to an exit block of the loop is definitely profitable.
335 /// It is also almost always profitable to thread backedges from within the loop
336 /// to exit blocks, and is often profitable to thread backedges to other blocks
337 /// within the loop (forming a nested loop).  This simple analysis is not rich
338 /// enough to track all of these properties and keep it up-to-date as the CFG
339 /// mutates, so we don't allow any of these transformations.
340 ///
FindLoopHeaders(Function & F)341 void JumpThreadingPass::FindLoopHeaders(Function &F) {
342   SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
343   FindFunctionBackedges(F, Edges);
344 
345   for (const auto &Edge : Edges)
346     LoopHeaders.insert(Edge.second);
347 }
348 
349 /// getKnownConstant - Helper method to determine if we can thread over a
350 /// terminator with the given value as its condition, and if so what value to
351 /// use for that. What kind of value this is depends on whether we want an
352 /// integer or a block address, but an undef is always accepted.
353 /// Returns null if Val is null or not an appropriate constant.
getKnownConstant(Value * Val,ConstantPreference Preference)354 static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
355   if (!Val)
356     return nullptr;
357 
358   // Undef is "known" enough.
359   if (UndefValue *U = dyn_cast<UndefValue>(Val))
360     return U;
361 
362   if (Preference == WantBlockAddress)
363     return dyn_cast<BlockAddress>(Val->stripPointerCasts());
364 
365   return dyn_cast<ConstantInt>(Val);
366 }
367 
368 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
369 /// if we can infer that the value is a known ConstantInt/BlockAddress or undef
370 /// in any of our predecessors.  If so, return the known list of value and pred
371 /// BB in the result vector.
372 ///
373 /// This returns true if there were any known values.
374 ///
ComputeValueKnownInPredecessors(Value * V,BasicBlock * BB,PredValueInfo & Result,ConstantPreference Preference,Instruction * CxtI)375 bool JumpThreadingPass::ComputeValueKnownInPredecessors(
376     Value *V, BasicBlock *BB, PredValueInfo &Result,
377     ConstantPreference Preference, Instruction *CxtI) {
378   // This method walks up use-def chains recursively.  Because of this, we could
379   // get into an infinite loop going around loops in the use-def chain.  To
380   // prevent this, keep track of what (value, block) pairs we've already visited
381   // and terminate the search if we loop back to them
382   if (!RecursionSet.insert(std::make_pair(V, BB)).second)
383     return false;
384 
385   // An RAII help to remove this pair from the recursion set once the recursion
386   // stack pops back out again.
387   RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB));
388 
389   // If V is a constant, then it is known in all predecessors.
390   if (Constant *KC = getKnownConstant(V, Preference)) {
391     for (BasicBlock *Pred : predecessors(BB))
392       Result.push_back(std::make_pair(KC, Pred));
393 
394     return !Result.empty();
395   }
396 
397   // If V is a non-instruction value, or an instruction in a different block,
398   // then it can't be derived from a PHI.
399   Instruction *I = dyn_cast<Instruction>(V);
400   if (!I || I->getParent() != BB) {
401 
402     // Okay, if this is a live-in value, see if it has a known value at the end
403     // of any of our predecessors.
404     //
405     // FIXME: This should be an edge property, not a block end property.
406     /// TODO: Per PR2563, we could infer value range information about a
407     /// predecessor based on its terminator.
408     //
409     // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
410     // "I" is a non-local compare-with-a-constant instruction.  This would be
411     // able to handle value inequalities better, for example if the compare is
412     // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
413     // Perhaps getConstantOnEdge should be smart enough to do this?
414 
415     for (BasicBlock *P : predecessors(BB)) {
416       // If the value is known by LazyValueInfo to be a constant in a
417       // predecessor, use that information to try to thread this block.
418       Constant *PredCst = LVI->getConstantOnEdge(V, P, BB, CxtI);
419       if (Constant *KC = getKnownConstant(PredCst, Preference))
420         Result.push_back(std::make_pair(KC, P));
421     }
422 
423     return !Result.empty();
424   }
425 
426   /// If I is a PHI node, then we know the incoming values for any constants.
427   if (PHINode *PN = dyn_cast<PHINode>(I)) {
428     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
429       Value *InVal = PN->getIncomingValue(i);
430       if (Constant *KC = getKnownConstant(InVal, Preference)) {
431         Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
432       } else {
433         Constant *CI = LVI->getConstantOnEdge(InVal,
434                                               PN->getIncomingBlock(i),
435                                               BB, CxtI);
436         if (Constant *KC = getKnownConstant(CI, Preference))
437           Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
438       }
439     }
440 
441     return !Result.empty();
442   }
443 
444   // Handle Cast instructions.  Only see through Cast when the source operand is
445   // PHI or Cmp and the source type is i1 to save the compilation time.
446   if (CastInst *CI = dyn_cast<CastInst>(I)) {
447     Value *Source = CI->getOperand(0);
448     if (!Source->getType()->isIntegerTy(1))
449       return false;
450     if (!isa<PHINode>(Source) && !isa<CmpInst>(Source))
451       return false;
452     ComputeValueKnownInPredecessors(Source, BB, Result, Preference, CxtI);
453     if (Result.empty())
454       return false;
455 
456     // Convert the known values.
457     for (auto &R : Result)
458       R.first = ConstantExpr::getCast(CI->getOpcode(), R.first, CI->getType());
459 
460     return true;
461   }
462 
463   PredValueInfoTy LHSVals, RHSVals;
464 
465   // Handle some boolean conditions.
466   if (I->getType()->getPrimitiveSizeInBits() == 1) {
467     assert(Preference == WantInteger && "One-bit non-integer type?");
468     // X | true -> true
469     // X & false -> false
470     if (I->getOpcode() == Instruction::Or ||
471         I->getOpcode() == Instruction::And) {
472       ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
473                                       WantInteger, CxtI);
474       ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals,
475                                       WantInteger, CxtI);
476 
477       if (LHSVals.empty() && RHSVals.empty())
478         return false;
479 
480       ConstantInt *InterestingVal;
481       if (I->getOpcode() == Instruction::Or)
482         InterestingVal = ConstantInt::getTrue(I->getContext());
483       else
484         InterestingVal = ConstantInt::getFalse(I->getContext());
485 
486       SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
487 
488       // Scan for the sentinel.  If we find an undef, force it to the
489       // interesting value: x|undef -> true and x&undef -> false.
490       for (const auto &LHSVal : LHSVals)
491         if (LHSVal.first == InterestingVal || isa<UndefValue>(LHSVal.first)) {
492           Result.emplace_back(InterestingVal, LHSVal.second);
493           LHSKnownBBs.insert(LHSVal.second);
494         }
495       for (const auto &RHSVal : RHSVals)
496         if (RHSVal.first == InterestingVal || isa<UndefValue>(RHSVal.first)) {
497           // If we already inferred a value for this block on the LHS, don't
498           // re-add it.
499           if (!LHSKnownBBs.count(RHSVal.second))
500             Result.emplace_back(InterestingVal, RHSVal.second);
501         }
502 
503       return !Result.empty();
504     }
505 
506     // Handle the NOT form of XOR.
507     if (I->getOpcode() == Instruction::Xor &&
508         isa<ConstantInt>(I->getOperand(1)) &&
509         cast<ConstantInt>(I->getOperand(1))->isOne()) {
510       ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result,
511                                       WantInteger, CxtI);
512       if (Result.empty())
513         return false;
514 
515       // Invert the known values.
516       for (auto &R : Result)
517         R.first = ConstantExpr::getNot(R.first);
518 
519       return true;
520     }
521 
522   // Try to simplify some other binary operator values.
523   } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
524     assert(Preference != WantBlockAddress
525             && "A binary operator creating a block address?");
526     if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
527       PredValueInfoTy LHSVals;
528       ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals,
529                                       WantInteger, CxtI);
530 
531       // Try to use constant folding to simplify the binary operator.
532       for (const auto &LHSVal : LHSVals) {
533         Constant *V = LHSVal.first;
534         Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI);
535 
536         if (Constant *KC = getKnownConstant(Folded, WantInteger))
537           Result.push_back(std::make_pair(KC, LHSVal.second));
538       }
539     }
540 
541     return !Result.empty();
542   }
543 
544   // Handle compare with phi operand, where the PHI is defined in this block.
545   if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
546     assert(Preference == WantInteger && "Compares only produce integers");
547     PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
548     if (PN && PN->getParent() == BB) {
549       const DataLayout &DL = PN->getModule()->getDataLayout();
550       // We can do this simplification if any comparisons fold to true or false.
551       // See if any do.
552       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
553         BasicBlock *PredBB = PN->getIncomingBlock(i);
554         Value *LHS = PN->getIncomingValue(i);
555         Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
556 
557         Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, DL);
558         if (!Res) {
559           if (!isa<Constant>(RHS))
560             continue;
561 
562           LazyValueInfo::Tristate
563             ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
564                                            cast<Constant>(RHS), PredBB, BB,
565                                            CxtI ? CxtI : Cmp);
566           if (ResT == LazyValueInfo::Unknown)
567             continue;
568           Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
569         }
570 
571         if (Constant *KC = getKnownConstant(Res, WantInteger))
572           Result.push_back(std::make_pair(KC, PredBB));
573       }
574 
575       return !Result.empty();
576     }
577 
578     // If comparing a live-in value against a constant, see if we know the
579     // live-in value on any predecessors.
580     if (isa<Constant>(Cmp->getOperand(1)) && Cmp->getType()->isIntegerTy()) {
581       if (!isa<Instruction>(Cmp->getOperand(0)) ||
582           cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) {
583         Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
584 
585         for (BasicBlock *P : predecessors(BB)) {
586           // If the value is known by LazyValueInfo to be a constant in a
587           // predecessor, use that information to try to thread this block.
588           LazyValueInfo::Tristate Res =
589             LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
590                                     RHSCst, P, BB, CxtI ? CxtI : Cmp);
591           if (Res == LazyValueInfo::Unknown)
592             continue;
593 
594           Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
595           Result.push_back(std::make_pair(ResC, P));
596         }
597 
598         return !Result.empty();
599       }
600 
601       // Try to find a constant value for the LHS of a comparison,
602       // and evaluate it statically if we can.
603       if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) {
604         PredValueInfoTy LHSVals;
605         ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
606                                         WantInteger, CxtI);
607 
608         for (const auto &LHSVal : LHSVals) {
609           Constant *V = LHSVal.first;
610           Constant *Folded = ConstantExpr::getCompare(Cmp->getPredicate(),
611                                                       V, CmpConst);
612           if (Constant *KC = getKnownConstant(Folded, WantInteger))
613             Result.push_back(std::make_pair(KC, LHSVal.second));
614         }
615 
616         return !Result.empty();
617       }
618     }
619   }
620 
621   if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
622     // Handle select instructions where at least one operand is a known constant
623     // and we can figure out the condition value for any predecessor block.
624     Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
625     Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
626     PredValueInfoTy Conds;
627     if ((TrueVal || FalseVal) &&
628         ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
629                                         WantInteger, CxtI)) {
630       for (auto &C : Conds) {
631         Constant *Cond = C.first;
632 
633         // Figure out what value to use for the condition.
634         bool KnownCond;
635         if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
636           // A known boolean.
637           KnownCond = CI->isOne();
638         } else {
639           assert(isa<UndefValue>(Cond) && "Unexpected condition value");
640           // Either operand will do, so be sure to pick the one that's a known
641           // constant.
642           // FIXME: Do this more cleverly if both values are known constants?
643           KnownCond = (TrueVal != nullptr);
644         }
645 
646         // See if the select has a known constant value for this predecessor.
647         if (Constant *Val = KnownCond ? TrueVal : FalseVal)
648           Result.push_back(std::make_pair(Val, C.second));
649       }
650 
651       return !Result.empty();
652     }
653   }
654 
655   // If all else fails, see if LVI can figure out a constant value for us.
656   Constant *CI = LVI->getConstant(V, BB, CxtI);
657   if (Constant *KC = getKnownConstant(CI, Preference)) {
658     for (BasicBlock *Pred : predecessors(BB))
659       Result.push_back(std::make_pair(KC, Pred));
660   }
661 
662   return !Result.empty();
663 }
664 
665 
666 
667 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
668 /// in an undefined jump, decide which block is best to revector to.
669 ///
670 /// Since we can pick an arbitrary destination, we pick the successor with the
671 /// fewest predecessors.  This should reduce the in-degree of the others.
672 ///
GetBestDestForJumpOnUndef(BasicBlock * BB)673 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
674   TerminatorInst *BBTerm = BB->getTerminator();
675   unsigned MinSucc = 0;
676   BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
677   // Compute the successor with the minimum number of predecessors.
678   unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
679   for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
680     TestBB = BBTerm->getSuccessor(i);
681     unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
682     if (NumPreds < MinNumPreds) {
683       MinSucc = i;
684       MinNumPreds = NumPreds;
685     }
686   }
687 
688   return MinSucc;
689 }
690 
hasAddressTakenAndUsed(BasicBlock * BB)691 static bool hasAddressTakenAndUsed(BasicBlock *BB) {
692   if (!BB->hasAddressTaken()) return false;
693 
694   // If the block has its address taken, it may be a tree of dead constants
695   // hanging off of it.  These shouldn't keep the block alive.
696   BlockAddress *BA = BlockAddress::get(BB);
697   BA->removeDeadConstantUsers();
698   return !BA->use_empty();
699 }
700 
701 /// ProcessBlock - If there are any predecessors whose control can be threaded
702 /// through to a successor, transform them now.
ProcessBlock(BasicBlock * BB)703 bool JumpThreadingPass::ProcessBlock(BasicBlock *BB) {
704   // If the block is trivially dead, just return and let the caller nuke it.
705   // This simplifies other transformations.
706   if (pred_empty(BB) &&
707       BB != &BB->getParent()->getEntryBlock())
708     return false;
709 
710   // If this block has a single predecessor, and if that pred has a single
711   // successor, merge the blocks.  This encourages recursive jump threading
712   // because now the condition in this block can be threaded through
713   // predecessors of our predecessor block.
714   if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
715     const TerminatorInst *TI = SinglePred->getTerminator();
716     if (!TI->isExceptional() && TI->getNumSuccessors() == 1 &&
717         SinglePred != BB && !hasAddressTakenAndUsed(BB)) {
718       // If SinglePred was a loop header, BB becomes one.
719       if (LoopHeaders.erase(SinglePred))
720         LoopHeaders.insert(BB);
721 
722       LVI->eraseBlock(SinglePred);
723       MergeBasicBlockIntoOnlyPred(BB);
724 
725       return true;
726     }
727   }
728 
729   if (TryToUnfoldSelectInCurrBB(BB))
730     return true;
731 
732   // What kind of constant we're looking for.
733   ConstantPreference Preference = WantInteger;
734 
735   // Look to see if the terminator is a conditional branch, switch or indirect
736   // branch, if not we can't thread it.
737   Value *Condition;
738   Instruction *Terminator = BB->getTerminator();
739   if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) {
740     // Can't thread an unconditional jump.
741     if (BI->isUnconditional()) return false;
742     Condition = BI->getCondition();
743   } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) {
744     Condition = SI->getCondition();
745   } else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) {
746     // Can't thread indirect branch with no successors.
747     if (IB->getNumSuccessors() == 0) return false;
748     Condition = IB->getAddress()->stripPointerCasts();
749     Preference = WantBlockAddress;
750   } else {
751     return false; // Must be an invoke.
752   }
753 
754   // Run constant folding to see if we can reduce the condition to a simple
755   // constant.
756   if (Instruction *I = dyn_cast<Instruction>(Condition)) {
757     Value *SimpleVal =
758         ConstantFoldInstruction(I, BB->getModule()->getDataLayout(), TLI);
759     if (SimpleVal) {
760       I->replaceAllUsesWith(SimpleVal);
761       I->eraseFromParent();
762       Condition = SimpleVal;
763     }
764   }
765 
766   // If the terminator is branching on an undef, we can pick any of the
767   // successors to branch to.  Let GetBestDestForJumpOnUndef decide.
768   if (isa<UndefValue>(Condition)) {
769     unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
770 
771     // Fold the branch/switch.
772     TerminatorInst *BBTerm = BB->getTerminator();
773     for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
774       if (i == BestSucc) continue;
775       BBTerm->getSuccessor(i)->removePredecessor(BB, true);
776     }
777 
778     DEBUG(dbgs() << "  In block '" << BB->getName()
779           << "' folding undef terminator: " << *BBTerm << '\n');
780     BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
781     BBTerm->eraseFromParent();
782     return true;
783   }
784 
785   // If the terminator of this block is branching on a constant, simplify the
786   // terminator to an unconditional branch.  This can occur due to threading in
787   // other blocks.
788   if (getKnownConstant(Condition, Preference)) {
789     DEBUG(dbgs() << "  In block '" << BB->getName()
790           << "' folding terminator: " << *BB->getTerminator() << '\n');
791     ++NumFolds;
792     ConstantFoldTerminator(BB, true);
793     return true;
794   }
795 
796   Instruction *CondInst = dyn_cast<Instruction>(Condition);
797 
798   // All the rest of our checks depend on the condition being an instruction.
799   if (!CondInst) {
800     // FIXME: Unify this with code below.
801     if (ProcessThreadableEdges(Condition, BB, Preference, Terminator))
802       return true;
803     return false;
804   }
805 
806 
807   if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
808     // If we're branching on a conditional, LVI might be able to determine
809     // it's value at the branch instruction.  We only handle comparisons
810     // against a constant at this time.
811     // TODO: This should be extended to handle switches as well.
812     BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
813     Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
814     if (CondBr && CondConst && CondBr->isConditional()) {
815       LazyValueInfo::Tristate Ret =
816         LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0),
817                             CondConst, CondBr);
818       if (Ret != LazyValueInfo::Unknown) {
819         unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0;
820         unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1;
821         CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
822         BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
823         CondBr->eraseFromParent();
824         if (CondCmp->use_empty())
825           CondCmp->eraseFromParent();
826         else if (CondCmp->getParent() == BB) {
827           // If the fact we just learned is true for all uses of the
828           // condition, replace it with a constant value
829           auto *CI = Ret == LazyValueInfo::True ?
830             ConstantInt::getTrue(CondCmp->getType()) :
831             ConstantInt::getFalse(CondCmp->getType());
832           CondCmp->replaceAllUsesWith(CI);
833           CondCmp->eraseFromParent();
834         }
835         return true;
836       }
837     }
838 
839     if (CondBr && CondConst && TryToUnfoldSelect(CondCmp, BB))
840       return true;
841   }
842 
843   // Check for some cases that are worth simplifying.  Right now we want to look
844   // for loads that are used by a switch or by the condition for the branch.  If
845   // we see one, check to see if it's partially redundant.  If so, insert a PHI
846   // which can then be used to thread the values.
847   //
848   Value *SimplifyValue = CondInst;
849   if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
850     if (isa<Constant>(CondCmp->getOperand(1)))
851       SimplifyValue = CondCmp->getOperand(0);
852 
853   // TODO: There are other places where load PRE would be profitable, such as
854   // more complex comparisons.
855   if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
856     if (SimplifyPartiallyRedundantLoad(LI))
857       return true;
858 
859 
860   // Handle a variety of cases where we are branching on something derived from
861   // a PHI node in the current block.  If we can prove that any predecessors
862   // compute a predictable value based on a PHI node, thread those predecessors.
863   //
864   if (ProcessThreadableEdges(CondInst, BB, Preference, Terminator))
865     return true;
866 
867   // If this is an otherwise-unfoldable branch on a phi node in the current
868   // block, see if we can simplify.
869   if (PHINode *PN = dyn_cast<PHINode>(CondInst))
870     if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
871       return ProcessBranchOnPHI(PN);
872 
873 
874   // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
875   if (CondInst->getOpcode() == Instruction::Xor &&
876       CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
877     return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
878 
879   // Search for a stronger dominating condition that can be used to simplify a
880   // conditional branch leaving BB.
881   if (ProcessImpliedCondition(BB))
882     return true;
883 
884   return false;
885 }
886 
ProcessImpliedCondition(BasicBlock * BB)887 bool JumpThreadingPass::ProcessImpliedCondition(BasicBlock *BB) {
888   auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
889   if (!BI || !BI->isConditional())
890     return false;
891 
892   Value *Cond = BI->getCondition();
893   BasicBlock *CurrentBB = BB;
894   BasicBlock *CurrentPred = BB->getSinglePredecessor();
895   unsigned Iter = 0;
896 
897   auto &DL = BB->getModule()->getDataLayout();
898 
899   while (CurrentPred && Iter++ < ImplicationSearchThreshold) {
900     auto *PBI = dyn_cast<BranchInst>(CurrentPred->getTerminator());
901     if (!PBI || !PBI->isConditional())
902       return false;
903     if (PBI->getSuccessor(0) != CurrentBB && PBI->getSuccessor(1) != CurrentBB)
904       return false;
905 
906     bool FalseDest = PBI->getSuccessor(1) == CurrentBB;
907     Optional<bool> Implication =
908       isImpliedCondition(PBI->getCondition(), Cond, DL, FalseDest);
909     if (Implication) {
910       BI->getSuccessor(*Implication ? 1 : 0)->removePredecessor(BB);
911       BranchInst::Create(BI->getSuccessor(*Implication ? 0 : 1), BI);
912       BI->eraseFromParent();
913       return true;
914     }
915     CurrentBB = CurrentPred;
916     CurrentPred = CurrentBB->getSinglePredecessor();
917   }
918 
919   return false;
920 }
921 
922 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
923 /// load instruction, eliminate it by replacing it with a PHI node.  This is an
924 /// important optimization that encourages jump threading, and needs to be run
925 /// interlaced with other jump threading tasks.
SimplifyPartiallyRedundantLoad(LoadInst * LI)926 bool JumpThreadingPass::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
927   // Don't hack volatile and ordered loads.
928   if (!LI->isUnordered()) return false;
929 
930   // If the load is defined in a block with exactly one predecessor, it can't be
931   // partially redundant.
932   BasicBlock *LoadBB = LI->getParent();
933   if (LoadBB->getSinglePredecessor())
934     return false;
935 
936   // If the load is defined in an EH pad, it can't be partially redundant,
937   // because the edges between the invoke and the EH pad cannot have other
938   // instructions between them.
939   if (LoadBB->isEHPad())
940     return false;
941 
942   Value *LoadedPtr = LI->getOperand(0);
943 
944   // If the loaded operand is defined in the LoadBB, it can't be available.
945   // TODO: Could do simple PHI translation, that would be fun :)
946   if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
947     if (PtrOp->getParent() == LoadBB)
948       return false;
949 
950   // Scan a few instructions up from the load, to see if it is obviously live at
951   // the entry to its block.
952   BasicBlock::iterator BBIt(LI);
953 
954   if (Value *AvailableVal =
955         FindAvailableLoadedValue(LI, LoadBB, BBIt, DefMaxInstsToScan)) {
956     // If the value of the load is locally available within the block, just use
957     // it.  This frequently occurs for reg2mem'd allocas.
958 
959     // If the returned value is the load itself, replace with an undef. This can
960     // only happen in dead loops.
961     if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
962     if (AvailableVal->getType() != LI->getType())
963       AvailableVal =
964           CastInst::CreateBitOrPointerCast(AvailableVal, LI->getType(), "", LI);
965     LI->replaceAllUsesWith(AvailableVal);
966     LI->eraseFromParent();
967     return true;
968   }
969 
970   // Otherwise, if we scanned the whole block and got to the top of the block,
971   // we know the block is locally transparent to the load.  If not, something
972   // might clobber its value.
973   if (BBIt != LoadBB->begin())
974     return false;
975 
976   // If all of the loads and stores that feed the value have the same AA tags,
977   // then we can propagate them onto any newly inserted loads.
978   AAMDNodes AATags;
979   LI->getAAMetadata(AATags);
980 
981   SmallPtrSet<BasicBlock*, 8> PredsScanned;
982   typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
983   AvailablePredsTy AvailablePreds;
984   BasicBlock *OneUnavailablePred = nullptr;
985 
986   // If we got here, the loaded value is transparent through to the start of the
987   // block.  Check to see if it is available in any of the predecessor blocks.
988   for (BasicBlock *PredBB : predecessors(LoadBB)) {
989     // If we already scanned this predecessor, skip it.
990     if (!PredsScanned.insert(PredBB).second)
991       continue;
992 
993     // Scan the predecessor to see if the value is available in the pred.
994     BBIt = PredBB->end();
995     AAMDNodes ThisAATags;
996     Value *PredAvailable = FindAvailableLoadedValue(LI, PredBB, BBIt,
997                                                     DefMaxInstsToScan,
998                                                     nullptr, &ThisAATags);
999     if (!PredAvailable) {
1000       OneUnavailablePred = PredBB;
1001       continue;
1002     }
1003 
1004     // If AA tags disagree or are not present, forget about them.
1005     if (AATags != ThisAATags) AATags = AAMDNodes();
1006 
1007     // If so, this load is partially redundant.  Remember this info so that we
1008     // can create a PHI node.
1009     AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
1010   }
1011 
1012   // If the loaded value isn't available in any predecessor, it isn't partially
1013   // redundant.
1014   if (AvailablePreds.empty()) return false;
1015 
1016   // Okay, the loaded value is available in at least one (and maybe all!)
1017   // predecessors.  If the value is unavailable in more than one unique
1018   // predecessor, we want to insert a merge block for those common predecessors.
1019   // This ensures that we only have to insert one reload, thus not increasing
1020   // code size.
1021   BasicBlock *UnavailablePred = nullptr;
1022 
1023   // If there is exactly one predecessor where the value is unavailable, the
1024   // already computed 'OneUnavailablePred' block is it.  If it ends in an
1025   // unconditional branch, we know that it isn't a critical edge.
1026   if (PredsScanned.size() == AvailablePreds.size()+1 &&
1027       OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
1028     UnavailablePred = OneUnavailablePred;
1029   } else if (PredsScanned.size() != AvailablePreds.size()) {
1030     // Otherwise, we had multiple unavailable predecessors or we had a critical
1031     // edge from the one.
1032     SmallVector<BasicBlock*, 8> PredsToSplit;
1033     SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
1034 
1035     for (const auto &AvailablePred : AvailablePreds)
1036       AvailablePredSet.insert(AvailablePred.first);
1037 
1038     // Add all the unavailable predecessors to the PredsToSplit list.
1039     for (BasicBlock *P : predecessors(LoadBB)) {
1040       // If the predecessor is an indirect goto, we can't split the edge.
1041       if (isa<IndirectBrInst>(P->getTerminator()))
1042         return false;
1043 
1044       if (!AvailablePredSet.count(P))
1045         PredsToSplit.push_back(P);
1046     }
1047 
1048     // Split them out to their own block.
1049     UnavailablePred = SplitBlockPreds(LoadBB, PredsToSplit, "thread-pre-split");
1050   }
1051 
1052   // If the value isn't available in all predecessors, then there will be
1053   // exactly one where it isn't available.  Insert a load on that edge and add
1054   // it to the AvailablePreds list.
1055   if (UnavailablePred) {
1056     assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
1057            "Can't handle critical edge here!");
1058     LoadInst *NewVal =
1059         new LoadInst(LoadedPtr, LI->getName() + ".pr", false,
1060                      LI->getAlignment(), LI->getOrdering(), LI->getSynchScope(),
1061                      UnavailablePred->getTerminator());
1062     NewVal->setDebugLoc(LI->getDebugLoc());
1063     if (AATags)
1064       NewVal->setAAMetadata(AATags);
1065 
1066     AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
1067   }
1068 
1069   // Now we know that each predecessor of this block has a value in
1070   // AvailablePreds, sort them for efficient access as we're walking the preds.
1071   array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
1072 
1073   // Create a PHI node at the start of the block for the PRE'd load value.
1074   pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB);
1075   PHINode *PN = PHINode::Create(LI->getType(), std::distance(PB, PE), "",
1076                                 &LoadBB->front());
1077   PN->takeName(LI);
1078   PN->setDebugLoc(LI->getDebugLoc());
1079 
1080   // Insert new entries into the PHI for each predecessor.  A single block may
1081   // have multiple entries here.
1082   for (pred_iterator PI = PB; PI != PE; ++PI) {
1083     BasicBlock *P = *PI;
1084     AvailablePredsTy::iterator I =
1085       std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
1086                        std::make_pair(P, (Value*)nullptr));
1087 
1088     assert(I != AvailablePreds.end() && I->first == P &&
1089            "Didn't find entry for predecessor!");
1090 
1091     // If we have an available predecessor but it requires casting, insert the
1092     // cast in the predecessor and use the cast. Note that we have to update the
1093     // AvailablePreds vector as we go so that all of the PHI entries for this
1094     // predecessor use the same bitcast.
1095     Value *&PredV = I->second;
1096     if (PredV->getType() != LI->getType())
1097       PredV = CastInst::CreateBitOrPointerCast(PredV, LI->getType(), "",
1098                                                P->getTerminator());
1099 
1100     PN->addIncoming(PredV, I->first);
1101   }
1102 
1103   LI->replaceAllUsesWith(PN);
1104   LI->eraseFromParent();
1105 
1106   return true;
1107 }
1108 
1109 /// FindMostPopularDest - The specified list contains multiple possible
1110 /// threadable destinations.  Pick the one that occurs the most frequently in
1111 /// the list.
1112 static BasicBlock *
FindMostPopularDest(BasicBlock * BB,const SmallVectorImpl<std::pair<BasicBlock *,BasicBlock * >> & PredToDestList)1113 FindMostPopularDest(BasicBlock *BB,
1114                     const SmallVectorImpl<std::pair<BasicBlock*,
1115                                   BasicBlock*> > &PredToDestList) {
1116   assert(!PredToDestList.empty());
1117 
1118   // Determine popularity.  If there are multiple possible destinations, we
1119   // explicitly choose to ignore 'undef' destinations.  We prefer to thread
1120   // blocks with known and real destinations to threading undef.  We'll handle
1121   // them later if interesting.
1122   DenseMap<BasicBlock*, unsigned> DestPopularity;
1123   for (const auto &PredToDest : PredToDestList)
1124     if (PredToDest.second)
1125       DestPopularity[PredToDest.second]++;
1126 
1127   // Find the most popular dest.
1128   DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
1129   BasicBlock *MostPopularDest = DPI->first;
1130   unsigned Popularity = DPI->second;
1131   SmallVector<BasicBlock*, 4> SamePopularity;
1132 
1133   for (++DPI; DPI != DestPopularity.end(); ++DPI) {
1134     // If the popularity of this entry isn't higher than the popularity we've
1135     // seen so far, ignore it.
1136     if (DPI->second < Popularity)
1137       ; // ignore.
1138     else if (DPI->second == Popularity) {
1139       // If it is the same as what we've seen so far, keep track of it.
1140       SamePopularity.push_back(DPI->first);
1141     } else {
1142       // If it is more popular, remember it.
1143       SamePopularity.clear();
1144       MostPopularDest = DPI->first;
1145       Popularity = DPI->second;
1146     }
1147   }
1148 
1149   // Okay, now we know the most popular destination.  If there is more than one
1150   // destination, we need to determine one.  This is arbitrary, but we need
1151   // to make a deterministic decision.  Pick the first one that appears in the
1152   // successor list.
1153   if (!SamePopularity.empty()) {
1154     SamePopularity.push_back(MostPopularDest);
1155     TerminatorInst *TI = BB->getTerminator();
1156     for (unsigned i = 0; ; ++i) {
1157       assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
1158 
1159       if (std::find(SamePopularity.begin(), SamePopularity.end(),
1160                     TI->getSuccessor(i)) == SamePopularity.end())
1161         continue;
1162 
1163       MostPopularDest = TI->getSuccessor(i);
1164       break;
1165     }
1166   }
1167 
1168   // Okay, we have finally picked the most popular destination.
1169   return MostPopularDest;
1170 }
1171 
ProcessThreadableEdges(Value * Cond,BasicBlock * BB,ConstantPreference Preference,Instruction * CxtI)1172 bool JumpThreadingPass::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
1173                                                ConstantPreference Preference,
1174                                                Instruction *CxtI) {
1175   // If threading this would thread across a loop header, don't even try to
1176   // thread the edge.
1177   if (LoopHeaders.count(BB))
1178     return false;
1179 
1180   PredValueInfoTy PredValues;
1181   if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference, CxtI))
1182     return false;
1183 
1184   assert(!PredValues.empty() &&
1185          "ComputeValueKnownInPredecessors returned true with no values");
1186 
1187   DEBUG(dbgs() << "IN BB: " << *BB;
1188         for (const auto &PredValue : PredValues) {
1189           dbgs() << "  BB '" << BB->getName() << "': FOUND condition = "
1190             << *PredValue.first
1191             << " for pred '" << PredValue.second->getName() << "'.\n";
1192         });
1193 
1194   // Decide what we want to thread through.  Convert our list of known values to
1195   // a list of known destinations for each pred.  This also discards duplicate
1196   // predecessors and keeps track of the undefined inputs (which are represented
1197   // as a null dest in the PredToDestList).
1198   SmallPtrSet<BasicBlock*, 16> SeenPreds;
1199   SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
1200 
1201   BasicBlock *OnlyDest = nullptr;
1202   BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1203 
1204   for (const auto &PredValue : PredValues) {
1205     BasicBlock *Pred = PredValue.second;
1206     if (!SeenPreds.insert(Pred).second)
1207       continue;  // Duplicate predecessor entry.
1208 
1209     // If the predecessor ends with an indirect goto, we can't change its
1210     // destination.
1211     if (isa<IndirectBrInst>(Pred->getTerminator()))
1212       continue;
1213 
1214     Constant *Val = PredValue.first;
1215 
1216     BasicBlock *DestBB;
1217     if (isa<UndefValue>(Val))
1218       DestBB = nullptr;
1219     else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1220       DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero());
1221     else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
1222       DestBB = SI->findCaseValue(cast<ConstantInt>(Val)).getCaseSuccessor();
1223     } else {
1224       assert(isa<IndirectBrInst>(BB->getTerminator())
1225               && "Unexpected terminator");
1226       DestBB = cast<BlockAddress>(Val)->getBasicBlock();
1227     }
1228 
1229     // If we have exactly one destination, remember it for efficiency below.
1230     if (PredToDestList.empty())
1231       OnlyDest = DestBB;
1232     else if (OnlyDest != DestBB)
1233       OnlyDest = MultipleDestSentinel;
1234 
1235     PredToDestList.push_back(std::make_pair(Pred, DestBB));
1236   }
1237 
1238   // If all edges were unthreadable, we fail.
1239   if (PredToDestList.empty())
1240     return false;
1241 
1242   // Determine which is the most common successor.  If we have many inputs and
1243   // this block is a switch, we want to start by threading the batch that goes
1244   // to the most popular destination first.  If we only know about one
1245   // threadable destination (the common case) we can avoid this.
1246   BasicBlock *MostPopularDest = OnlyDest;
1247 
1248   if (MostPopularDest == MultipleDestSentinel)
1249     MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1250 
1251   // Now that we know what the most popular destination is, factor all
1252   // predecessors that will jump to it into a single predecessor.
1253   SmallVector<BasicBlock*, 16> PredsToFactor;
1254   for (const auto &PredToDest : PredToDestList)
1255     if (PredToDest.second == MostPopularDest) {
1256       BasicBlock *Pred = PredToDest.first;
1257 
1258       // This predecessor may be a switch or something else that has multiple
1259       // edges to the block.  Factor each of these edges by listing them
1260       // according to # occurrences in PredsToFactor.
1261       for (BasicBlock *Succ : successors(Pred))
1262         if (Succ == BB)
1263           PredsToFactor.push_back(Pred);
1264     }
1265 
1266   // If the threadable edges are branching on an undefined value, we get to pick
1267   // the destination that these predecessors should get to.
1268   if (!MostPopularDest)
1269     MostPopularDest = BB->getTerminator()->
1270                             getSuccessor(GetBestDestForJumpOnUndef(BB));
1271 
1272   // Ok, try to thread it!
1273   return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1274 }
1275 
1276 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1277 /// a PHI node in the current block.  See if there are any simplifications we
1278 /// can do based on inputs to the phi node.
1279 ///
ProcessBranchOnPHI(PHINode * PN)1280 bool JumpThreadingPass::ProcessBranchOnPHI(PHINode *PN) {
1281   BasicBlock *BB = PN->getParent();
1282 
1283   // TODO: We could make use of this to do it once for blocks with common PHI
1284   // values.
1285   SmallVector<BasicBlock*, 1> PredBBs;
1286   PredBBs.resize(1);
1287 
1288   // If any of the predecessor blocks end in an unconditional branch, we can
1289   // *duplicate* the conditional branch into that block in order to further
1290   // encourage jump threading and to eliminate cases where we have branch on a
1291   // phi of an icmp (branch on icmp is much better).
1292   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1293     BasicBlock *PredBB = PN->getIncomingBlock(i);
1294     if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1295       if (PredBr->isUnconditional()) {
1296         PredBBs[0] = PredBB;
1297         // Try to duplicate BB into PredBB.
1298         if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
1299           return true;
1300       }
1301   }
1302 
1303   return false;
1304 }
1305 
1306 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
1307 /// a xor instruction in the current block.  See if there are any
1308 /// simplifications we can do based on inputs to the xor.
1309 ///
ProcessBranchOnXOR(BinaryOperator * BO)1310 bool JumpThreadingPass::ProcessBranchOnXOR(BinaryOperator *BO) {
1311   BasicBlock *BB = BO->getParent();
1312 
1313   // If either the LHS or RHS of the xor is a constant, don't do this
1314   // optimization.
1315   if (isa<ConstantInt>(BO->getOperand(0)) ||
1316       isa<ConstantInt>(BO->getOperand(1)))
1317     return false;
1318 
1319   // If the first instruction in BB isn't a phi, we won't be able to infer
1320   // anything special about any particular predecessor.
1321   if (!isa<PHINode>(BB->front()))
1322     return false;
1323 
1324   // If we have a xor as the branch input to this block, and we know that the
1325   // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1326   // the condition into the predecessor and fix that value to true, saving some
1327   // logical ops on that path and encouraging other paths to simplify.
1328   //
1329   // This copies something like this:
1330   //
1331   //  BB:
1332   //    %X = phi i1 [1],  [%X']
1333   //    %Y = icmp eq i32 %A, %B
1334   //    %Z = xor i1 %X, %Y
1335   //    br i1 %Z, ...
1336   //
1337   // Into:
1338   //  BB':
1339   //    %Y = icmp ne i32 %A, %B
1340   //    br i1 %Y, ...
1341 
1342   PredValueInfoTy XorOpValues;
1343   bool isLHS = true;
1344   if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
1345                                        WantInteger, BO)) {
1346     assert(XorOpValues.empty());
1347     if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
1348                                          WantInteger, BO))
1349       return false;
1350     isLHS = false;
1351   }
1352 
1353   assert(!XorOpValues.empty() &&
1354          "ComputeValueKnownInPredecessors returned true with no values");
1355 
1356   // Scan the information to see which is most popular: true or false.  The
1357   // predecessors can be of the set true, false, or undef.
1358   unsigned NumTrue = 0, NumFalse = 0;
1359   for (const auto &XorOpValue : XorOpValues) {
1360     if (isa<UndefValue>(XorOpValue.first))
1361       // Ignore undefs for the count.
1362       continue;
1363     if (cast<ConstantInt>(XorOpValue.first)->isZero())
1364       ++NumFalse;
1365     else
1366       ++NumTrue;
1367   }
1368 
1369   // Determine which value to split on, true, false, or undef if neither.
1370   ConstantInt *SplitVal = nullptr;
1371   if (NumTrue > NumFalse)
1372     SplitVal = ConstantInt::getTrue(BB->getContext());
1373   else if (NumTrue != 0 || NumFalse != 0)
1374     SplitVal = ConstantInt::getFalse(BB->getContext());
1375 
1376   // Collect all of the blocks that this can be folded into so that we can
1377   // factor this once and clone it once.
1378   SmallVector<BasicBlock*, 8> BlocksToFoldInto;
1379   for (const auto &XorOpValue : XorOpValues) {
1380     if (XorOpValue.first != SplitVal && !isa<UndefValue>(XorOpValue.first))
1381       continue;
1382 
1383     BlocksToFoldInto.push_back(XorOpValue.second);
1384   }
1385 
1386   // If we inferred a value for all of the predecessors, then duplication won't
1387   // help us.  However, we can just replace the LHS or RHS with the constant.
1388   if (BlocksToFoldInto.size() ==
1389       cast<PHINode>(BB->front()).getNumIncomingValues()) {
1390     if (!SplitVal) {
1391       // If all preds provide undef, just nuke the xor, because it is undef too.
1392       BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
1393       BO->eraseFromParent();
1394     } else if (SplitVal->isZero()) {
1395       // If all preds provide 0, replace the xor with the other input.
1396       BO->replaceAllUsesWith(BO->getOperand(isLHS));
1397       BO->eraseFromParent();
1398     } else {
1399       // If all preds provide 1, set the computed value to 1.
1400       BO->setOperand(!isLHS, SplitVal);
1401     }
1402 
1403     return true;
1404   }
1405 
1406   // Try to duplicate BB into PredBB.
1407   return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
1408 }
1409 
1410 
1411 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1412 /// predecessor to the PHIBB block.  If it has PHI nodes, add entries for
1413 /// NewPred using the entries from OldPred (suitably mapped).
AddPHINodeEntriesForMappedBlock(BasicBlock * PHIBB,BasicBlock * OldPred,BasicBlock * NewPred,DenseMap<Instruction *,Value * > & ValueMap)1414 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1415                                             BasicBlock *OldPred,
1416                                             BasicBlock *NewPred,
1417                                      DenseMap<Instruction*, Value*> &ValueMap) {
1418   for (BasicBlock::iterator PNI = PHIBB->begin();
1419        PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1420     // Ok, we have a PHI node.  Figure out what the incoming value was for the
1421     // DestBlock.
1422     Value *IV = PN->getIncomingValueForBlock(OldPred);
1423 
1424     // Remap the value if necessary.
1425     if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1426       DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1427       if (I != ValueMap.end())
1428         IV = I->second;
1429     }
1430 
1431     PN->addIncoming(IV, NewPred);
1432   }
1433 }
1434 
1435 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1436 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1437 /// across BB.  Transform the IR to reflect this change.
ThreadEdge(BasicBlock * BB,const SmallVectorImpl<BasicBlock * > & PredBBs,BasicBlock * SuccBB)1438 bool JumpThreadingPass::ThreadEdge(BasicBlock *BB,
1439                                    const SmallVectorImpl<BasicBlock *> &PredBBs,
1440                                    BasicBlock *SuccBB) {
1441   // If threading to the same block as we come from, we would infinite loop.
1442   if (SuccBB == BB) {
1443     DEBUG(dbgs() << "  Not threading across BB '" << BB->getName()
1444           << "' - would thread to self!\n");
1445     return false;
1446   }
1447 
1448   // If threading this would thread across a loop header, don't thread the edge.
1449   // See the comments above FindLoopHeaders for justifications and caveats.
1450   if (LoopHeaders.count(BB)) {
1451     DEBUG(dbgs() << "  Not threading across loop header BB '" << BB->getName()
1452           << "' to dest BB '" << SuccBB->getName()
1453           << "' - it might create an irreducible loop!\n");
1454     return false;
1455   }
1456 
1457   unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB, BBDupThreshold);
1458   if (JumpThreadCost > BBDupThreshold) {
1459     DEBUG(dbgs() << "  Not threading BB '" << BB->getName()
1460           << "' - Cost is too high: " << JumpThreadCost << "\n");
1461     return false;
1462   }
1463 
1464   // And finally, do it!  Start by factoring the predecessors if needed.
1465   BasicBlock *PredBB;
1466   if (PredBBs.size() == 1)
1467     PredBB = PredBBs[0];
1468   else {
1469     DEBUG(dbgs() << "  Factoring out " << PredBBs.size()
1470           << " common predecessors.\n");
1471     PredBB = SplitBlockPreds(BB, PredBBs, ".thr_comm");
1472   }
1473 
1474   // And finally, do it!
1475   DEBUG(dbgs() << "  Threading edge from '" << PredBB->getName() << "' to '"
1476         << SuccBB->getName() << "' with cost: " << JumpThreadCost
1477         << ", across block:\n    "
1478         << *BB << "\n");
1479 
1480   LVI->threadEdge(PredBB, BB, SuccBB);
1481 
1482   // We are going to have to map operands from the original BB block to the new
1483   // copy of the block 'NewBB'.  If there are PHI nodes in BB, evaluate them to
1484   // account for entry from PredBB.
1485   DenseMap<Instruction*, Value*> ValueMapping;
1486 
1487   BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1488                                          BB->getName()+".thread",
1489                                          BB->getParent(), BB);
1490   NewBB->moveAfter(PredBB);
1491 
1492   // Set the block frequency of NewBB.
1493   if (HasProfileData) {
1494     auto NewBBFreq =
1495         BFI->getBlockFreq(PredBB) * BPI->getEdgeProbability(PredBB, BB);
1496     BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
1497   }
1498 
1499   BasicBlock::iterator BI = BB->begin();
1500   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1501     ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1502 
1503   // Clone the non-phi instructions of BB into NewBB, keeping track of the
1504   // mapping and using it to remap operands in the cloned instructions.
1505   for (; !isa<TerminatorInst>(BI); ++BI) {
1506     Instruction *New = BI->clone();
1507     New->setName(BI->getName());
1508     NewBB->getInstList().push_back(New);
1509     ValueMapping[&*BI] = New;
1510 
1511     // Remap operands to patch up intra-block references.
1512     for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1513       if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1514         DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1515         if (I != ValueMapping.end())
1516           New->setOperand(i, I->second);
1517       }
1518   }
1519 
1520   // We didn't copy the terminator from BB over to NewBB, because there is now
1521   // an unconditional jump to SuccBB.  Insert the unconditional jump.
1522   BranchInst *NewBI = BranchInst::Create(SuccBB, NewBB);
1523   NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
1524 
1525   // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1526   // PHI nodes for NewBB now.
1527   AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1528 
1529   // If there were values defined in BB that are used outside the block, then we
1530   // now have to update all uses of the value to use either the original value,
1531   // the cloned value, or some PHI derived value.  This can require arbitrary
1532   // PHI insertion, of which we are prepared to do, clean these up now.
1533   SSAUpdater SSAUpdate;
1534   SmallVector<Use*, 16> UsesToRename;
1535   for (Instruction &I : *BB) {
1536     // Scan all uses of this instruction to see if it is used outside of its
1537     // block, and if so, record them in UsesToRename.
1538     for (Use &U : I.uses()) {
1539       Instruction *User = cast<Instruction>(U.getUser());
1540       if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1541         if (UserPN->getIncomingBlock(U) == BB)
1542           continue;
1543       } else if (User->getParent() == BB)
1544         continue;
1545 
1546       UsesToRename.push_back(&U);
1547     }
1548 
1549     // If there are no uses outside the block, we're done with this instruction.
1550     if (UsesToRename.empty())
1551       continue;
1552 
1553     DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I << "\n");
1554 
1555     // We found a use of I outside of BB.  Rename all uses of I that are outside
1556     // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
1557     // with the two values we know.
1558     SSAUpdate.Initialize(I.getType(), I.getName());
1559     SSAUpdate.AddAvailableValue(BB, &I);
1560     SSAUpdate.AddAvailableValue(NewBB, ValueMapping[&I]);
1561 
1562     while (!UsesToRename.empty())
1563       SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1564     DEBUG(dbgs() << "\n");
1565   }
1566 
1567 
1568   // Ok, NewBB is good to go.  Update the terminator of PredBB to jump to
1569   // NewBB instead of BB.  This eliminates predecessors from BB, which requires
1570   // us to simplify any PHI nodes in BB.
1571   TerminatorInst *PredTerm = PredBB->getTerminator();
1572   for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1573     if (PredTerm->getSuccessor(i) == BB) {
1574       BB->removePredecessor(PredBB, true);
1575       PredTerm->setSuccessor(i, NewBB);
1576     }
1577 
1578   // At this point, the IR is fully up to date and consistent.  Do a quick scan
1579   // over the new instructions and zap any that are constants or dead.  This
1580   // frequently happens because of phi translation.
1581   SimplifyInstructionsInBlock(NewBB, TLI);
1582 
1583   // Update the edge weight from BB to SuccBB, which should be less than before.
1584   UpdateBlockFreqAndEdgeWeight(PredBB, BB, NewBB, SuccBB);
1585 
1586   // Threaded an edge!
1587   ++NumThreads;
1588   return true;
1589 }
1590 
1591 /// Create a new basic block that will be the predecessor of BB and successor of
1592 /// all blocks in Preds. When profile data is availble, update the frequency of
1593 /// this new block.
SplitBlockPreds(BasicBlock * BB,ArrayRef<BasicBlock * > Preds,const char * Suffix)1594 BasicBlock *JumpThreadingPass::SplitBlockPreds(BasicBlock *BB,
1595                                                ArrayRef<BasicBlock *> Preds,
1596                                                const char *Suffix) {
1597   // Collect the frequencies of all predecessors of BB, which will be used to
1598   // update the edge weight on BB->SuccBB.
1599   BlockFrequency PredBBFreq(0);
1600   if (HasProfileData)
1601     for (auto Pred : Preds)
1602       PredBBFreq += BFI->getBlockFreq(Pred) * BPI->getEdgeProbability(Pred, BB);
1603 
1604   BasicBlock *PredBB = SplitBlockPredecessors(BB, Preds, Suffix);
1605 
1606   // Set the block frequency of the newly created PredBB, which is the sum of
1607   // frequencies of Preds.
1608   if (HasProfileData)
1609     BFI->setBlockFreq(PredBB, PredBBFreq.getFrequency());
1610   return PredBB;
1611 }
1612 
1613 /// Update the block frequency of BB and branch weight and the metadata on the
1614 /// edge BB->SuccBB. This is done by scaling the weight of BB->SuccBB by 1 -
1615 /// Freq(PredBB->BB) / Freq(BB->SuccBB).
UpdateBlockFreqAndEdgeWeight(BasicBlock * PredBB,BasicBlock * BB,BasicBlock * NewBB,BasicBlock * SuccBB)1616 void JumpThreadingPass::UpdateBlockFreqAndEdgeWeight(BasicBlock *PredBB,
1617                                                      BasicBlock *BB,
1618                                                      BasicBlock *NewBB,
1619                                                      BasicBlock *SuccBB) {
1620   if (!HasProfileData)
1621     return;
1622 
1623   assert(BFI && BPI && "BFI & BPI should have been created here");
1624 
1625   // As the edge from PredBB to BB is deleted, we have to update the block
1626   // frequency of BB.
1627   auto BBOrigFreq = BFI->getBlockFreq(BB);
1628   auto NewBBFreq = BFI->getBlockFreq(NewBB);
1629   auto BB2SuccBBFreq = BBOrigFreq * BPI->getEdgeProbability(BB, SuccBB);
1630   auto BBNewFreq = BBOrigFreq - NewBBFreq;
1631   BFI->setBlockFreq(BB, BBNewFreq.getFrequency());
1632 
1633   // Collect updated outgoing edges' frequencies from BB and use them to update
1634   // edge probabilities.
1635   SmallVector<uint64_t, 4> BBSuccFreq;
1636   for (BasicBlock *Succ : successors(BB)) {
1637     auto SuccFreq = (Succ == SuccBB)
1638                         ? BB2SuccBBFreq - NewBBFreq
1639                         : BBOrigFreq * BPI->getEdgeProbability(BB, Succ);
1640     BBSuccFreq.push_back(SuccFreq.getFrequency());
1641   }
1642 
1643   uint64_t MaxBBSuccFreq =
1644       *std::max_element(BBSuccFreq.begin(), BBSuccFreq.end());
1645 
1646   SmallVector<BranchProbability, 4> BBSuccProbs;
1647   if (MaxBBSuccFreq == 0)
1648     BBSuccProbs.assign(BBSuccFreq.size(),
1649                        {1, static_cast<unsigned>(BBSuccFreq.size())});
1650   else {
1651     for (uint64_t Freq : BBSuccFreq)
1652       BBSuccProbs.push_back(
1653           BranchProbability::getBranchProbability(Freq, MaxBBSuccFreq));
1654     // Normalize edge probabilities so that they sum up to one.
1655     BranchProbability::normalizeProbabilities(BBSuccProbs.begin(),
1656                                               BBSuccProbs.end());
1657   }
1658 
1659   // Update edge probabilities in BPI.
1660   for (int I = 0, E = BBSuccProbs.size(); I < E; I++)
1661     BPI->setEdgeProbability(BB, I, BBSuccProbs[I]);
1662 
1663   if (BBSuccProbs.size() >= 2) {
1664     SmallVector<uint32_t, 4> Weights;
1665     for (auto Prob : BBSuccProbs)
1666       Weights.push_back(Prob.getNumerator());
1667 
1668     auto TI = BB->getTerminator();
1669     TI->setMetadata(
1670         LLVMContext::MD_prof,
1671         MDBuilder(TI->getParent()->getContext()).createBranchWeights(Weights));
1672   }
1673 }
1674 
1675 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1676 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1677 /// If we can duplicate the contents of BB up into PredBB do so now, this
1678 /// improves the odds that the branch will be on an analyzable instruction like
1679 /// a compare.
DuplicateCondBranchOnPHIIntoPred(BasicBlock * BB,const SmallVectorImpl<BasicBlock * > & PredBBs)1680 bool JumpThreadingPass::DuplicateCondBranchOnPHIIntoPred(
1681     BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs) {
1682   assert(!PredBBs.empty() && "Can't handle an empty set");
1683 
1684   // If BB is a loop header, then duplicating this block outside the loop would
1685   // cause us to transform this into an irreducible loop, don't do this.
1686   // See the comments above FindLoopHeaders for justifications and caveats.
1687   if (LoopHeaders.count(BB)) {
1688     DEBUG(dbgs() << "  Not duplicating loop header '" << BB->getName()
1689           << "' into predecessor block '" << PredBBs[0]->getName()
1690           << "' - it might create an irreducible loop!\n");
1691     return false;
1692   }
1693 
1694   unsigned DuplicationCost = getJumpThreadDuplicationCost(BB, BBDupThreshold);
1695   if (DuplicationCost > BBDupThreshold) {
1696     DEBUG(dbgs() << "  Not duplicating BB '" << BB->getName()
1697           << "' - Cost is too high: " << DuplicationCost << "\n");
1698     return false;
1699   }
1700 
1701   // And finally, do it!  Start by factoring the predecessors if needed.
1702   BasicBlock *PredBB;
1703   if (PredBBs.size() == 1)
1704     PredBB = PredBBs[0];
1705   else {
1706     DEBUG(dbgs() << "  Factoring out " << PredBBs.size()
1707           << " common predecessors.\n");
1708     PredBB = SplitBlockPreds(BB, PredBBs, ".thr_comm");
1709   }
1710 
1711   // Okay, we decided to do this!  Clone all the instructions in BB onto the end
1712   // of PredBB.
1713   DEBUG(dbgs() << "  Duplicating block '" << BB->getName() << "' into end of '"
1714         << PredBB->getName() << "' to eliminate branch on phi.  Cost: "
1715         << DuplicationCost << " block is:" << *BB << "\n");
1716 
1717   // Unless PredBB ends with an unconditional branch, split the edge so that we
1718   // can just clone the bits from BB into the end of the new PredBB.
1719   BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
1720 
1721   if (!OldPredBranch || !OldPredBranch->isUnconditional()) {
1722     PredBB = SplitEdge(PredBB, BB);
1723     OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1724   }
1725 
1726   // We are going to have to map operands from the original BB block into the
1727   // PredBB block.  Evaluate PHI nodes in BB.
1728   DenseMap<Instruction*, Value*> ValueMapping;
1729 
1730   BasicBlock::iterator BI = BB->begin();
1731   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1732     ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1733   // Clone the non-phi instructions of BB into PredBB, keeping track of the
1734   // mapping and using it to remap operands in the cloned instructions.
1735   for (; BI != BB->end(); ++BI) {
1736     Instruction *New = BI->clone();
1737 
1738     // Remap operands to patch up intra-block references.
1739     for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1740       if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1741         DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1742         if (I != ValueMapping.end())
1743           New->setOperand(i, I->second);
1744       }
1745 
1746     // If this instruction can be simplified after the operands are updated,
1747     // just use the simplified value instead.  This frequently happens due to
1748     // phi translation.
1749     if (Value *IV =
1750             SimplifyInstruction(New, BB->getModule()->getDataLayout())) {
1751       ValueMapping[&*BI] = IV;
1752       if (!New->mayHaveSideEffects()) {
1753         delete New;
1754         New = nullptr;
1755       }
1756     } else {
1757       ValueMapping[&*BI] = New;
1758     }
1759     if (New) {
1760       // Otherwise, insert the new instruction into the block.
1761       New->setName(BI->getName());
1762       PredBB->getInstList().insert(OldPredBranch->getIterator(), New);
1763     }
1764   }
1765 
1766   // Check to see if the targets of the branch had PHI nodes. If so, we need to
1767   // add entries to the PHI nodes for branch from PredBB now.
1768   BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1769   AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1770                                   ValueMapping);
1771   AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1772                                   ValueMapping);
1773 
1774   // If there were values defined in BB that are used outside the block, then we
1775   // now have to update all uses of the value to use either the original value,
1776   // the cloned value, or some PHI derived value.  This can require arbitrary
1777   // PHI insertion, of which we are prepared to do, clean these up now.
1778   SSAUpdater SSAUpdate;
1779   SmallVector<Use*, 16> UsesToRename;
1780   for (Instruction &I : *BB) {
1781     // Scan all uses of this instruction to see if it is used outside of its
1782     // block, and if so, record them in UsesToRename.
1783     for (Use &U : I.uses()) {
1784       Instruction *User = cast<Instruction>(U.getUser());
1785       if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1786         if (UserPN->getIncomingBlock(U) == BB)
1787           continue;
1788       } else if (User->getParent() == BB)
1789         continue;
1790 
1791       UsesToRename.push_back(&U);
1792     }
1793 
1794     // If there are no uses outside the block, we're done with this instruction.
1795     if (UsesToRename.empty())
1796       continue;
1797 
1798     DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I << "\n");
1799 
1800     // We found a use of I outside of BB.  Rename all uses of I that are outside
1801     // its block to be uses of the appropriate PHI node etc.  See ValuesInBlocks
1802     // with the two values we know.
1803     SSAUpdate.Initialize(I.getType(), I.getName());
1804     SSAUpdate.AddAvailableValue(BB, &I);
1805     SSAUpdate.AddAvailableValue(PredBB, ValueMapping[&I]);
1806 
1807     while (!UsesToRename.empty())
1808       SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1809     DEBUG(dbgs() << "\n");
1810   }
1811 
1812   // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1813   // that we nuked.
1814   BB->removePredecessor(PredBB, true);
1815 
1816   // Remove the unconditional branch at the end of the PredBB block.
1817   OldPredBranch->eraseFromParent();
1818 
1819   ++NumDupes;
1820   return true;
1821 }
1822 
1823 /// TryToUnfoldSelect - Look for blocks of the form
1824 /// bb1:
1825 ///   %a = select
1826 ///   br bb
1827 ///
1828 /// bb2:
1829 ///   %p = phi [%a, %bb] ...
1830 ///   %c = icmp %p
1831 ///   br i1 %c
1832 ///
1833 /// And expand the select into a branch structure if one of its arms allows %c
1834 /// to be folded. This later enables threading from bb1 over bb2.
TryToUnfoldSelect(CmpInst * CondCmp,BasicBlock * BB)1835 bool JumpThreadingPass::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
1836   BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
1837   PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0));
1838   Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1));
1839 
1840   if (!CondBr || !CondBr->isConditional() || !CondLHS ||
1841       CondLHS->getParent() != BB)
1842     return false;
1843 
1844   for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) {
1845     BasicBlock *Pred = CondLHS->getIncomingBlock(I);
1846     SelectInst *SI = dyn_cast<SelectInst>(CondLHS->getIncomingValue(I));
1847 
1848     // Look if one of the incoming values is a select in the corresponding
1849     // predecessor.
1850     if (!SI || SI->getParent() != Pred || !SI->hasOneUse())
1851       continue;
1852 
1853     BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
1854     if (!PredTerm || !PredTerm->isUnconditional())
1855       continue;
1856 
1857     // Now check if one of the select values would allow us to constant fold the
1858     // terminator in BB. We don't do the transform if both sides fold, those
1859     // cases will be threaded in any case.
1860     LazyValueInfo::Tristate LHSFolds =
1861         LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(1),
1862                                 CondRHS, Pred, BB, CondCmp);
1863     LazyValueInfo::Tristate RHSFolds =
1864         LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(2),
1865                                 CondRHS, Pred, BB, CondCmp);
1866     if ((LHSFolds != LazyValueInfo::Unknown ||
1867          RHSFolds != LazyValueInfo::Unknown) &&
1868         LHSFolds != RHSFolds) {
1869       // Expand the select.
1870       //
1871       // Pred --
1872       //  |    v
1873       //  |  NewBB
1874       //  |    |
1875       //  |-----
1876       //  v
1877       // BB
1878       BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold",
1879                                              BB->getParent(), BB);
1880       // Move the unconditional branch to NewBB.
1881       PredTerm->removeFromParent();
1882       NewBB->getInstList().insert(NewBB->end(), PredTerm);
1883       // Create a conditional branch and update PHI nodes.
1884       BranchInst::Create(NewBB, BB, SI->getCondition(), Pred);
1885       CondLHS->setIncomingValue(I, SI->getFalseValue());
1886       CondLHS->addIncoming(SI->getTrueValue(), NewBB);
1887       // The select is now dead.
1888       SI->eraseFromParent();
1889 
1890       // Update any other PHI nodes in BB.
1891       for (BasicBlock::iterator BI = BB->begin();
1892            PHINode *Phi = dyn_cast<PHINode>(BI); ++BI)
1893         if (Phi != CondLHS)
1894           Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB);
1895       return true;
1896     }
1897   }
1898   return false;
1899 }
1900 
1901 /// TryToUnfoldSelectInCurrBB - Look for PHI/Select in the same BB of the form
1902 /// bb:
1903 ///   %p = phi [false, %bb1], [true, %bb2], [false, %bb3], [true, %bb4], ...
1904 ///   %s = select p, trueval, falseval
1905 ///
1906 /// And expand the select into a branch structure. This later enables
1907 /// jump-threading over bb in this pass.
1908 ///
1909 /// Using the similar approach of SimplifyCFG::FoldCondBranchOnPHI(), unfold
1910 /// select if the associated PHI has at least one constant.  If the unfolded
1911 /// select is not jump-threaded, it will be folded again in the later
1912 /// optimizations.
TryToUnfoldSelectInCurrBB(BasicBlock * BB)1913 bool JumpThreadingPass::TryToUnfoldSelectInCurrBB(BasicBlock *BB) {
1914   // If threading this would thread across a loop header, don't thread the edge.
1915   // See the comments above FindLoopHeaders for justifications and caveats.
1916   if (LoopHeaders.count(BB))
1917     return false;
1918 
1919   // Look for a Phi/Select pair in the same basic block.  The Phi feeds the
1920   // condition of the Select and at least one of the incoming values is a
1921   // constant.
1922   for (BasicBlock::iterator BI = BB->begin();
1923        PHINode *PN = dyn_cast<PHINode>(BI); ++BI) {
1924     unsigned NumPHIValues = PN->getNumIncomingValues();
1925     if (NumPHIValues == 0 || !PN->hasOneUse())
1926       continue;
1927 
1928     SelectInst *SI = dyn_cast<SelectInst>(PN->user_back());
1929     if (!SI || SI->getParent() != BB)
1930       continue;
1931 
1932     Value *Cond = SI->getCondition();
1933     if (!Cond || Cond != PN || !Cond->getType()->isIntegerTy(1))
1934       continue;
1935 
1936     bool HasConst = false;
1937     for (unsigned i = 0; i != NumPHIValues; ++i) {
1938       if (PN->getIncomingBlock(i) == BB)
1939         return false;
1940       if (isa<ConstantInt>(PN->getIncomingValue(i)))
1941         HasConst = true;
1942     }
1943 
1944     if (HasConst) {
1945       // Expand the select.
1946       TerminatorInst *Term =
1947           SplitBlockAndInsertIfThen(SI->getCondition(), SI, false);
1948       PHINode *NewPN = PHINode::Create(SI->getType(), 2, "", SI);
1949       NewPN->addIncoming(SI->getTrueValue(), Term->getParent());
1950       NewPN->addIncoming(SI->getFalseValue(), BB);
1951       SI->replaceAllUsesWith(NewPN);
1952       SI->eraseFromParent();
1953       return true;
1954     }
1955   }
1956 
1957   return false;
1958 }
1959