LLVM  3.7.0
JumpThreading.cpp
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1 //===- JumpThreading.cpp - Thread control through conditional blocks ------===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements the Jump Threading pass.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Transforms/Scalar.h"
15 #include "llvm/ADT/DenseMap.h"
16 #include "llvm/ADT/DenseSet.h"
17 #include "llvm/ADT/STLExtras.h"
18 #include "llvm/ADT/SmallPtrSet.h"
19 #include "llvm/ADT/SmallSet.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/Analysis/CFG.h"
25 #include "llvm/Analysis/Loads.h"
27 #include "llvm/IR/DataLayout.h"
28 #include "llvm/IR/IntrinsicInst.h"
29 #include "llvm/IR/LLVMContext.h"
30 #include "llvm/IR/Metadata.h"
31 #include "llvm/IR/ValueHandle.h"
32 #include "llvm/Pass.h"
34 #include "llvm/Support/Debug.h"
39 using namespace llvm;
40 
41 #define DEBUG_TYPE "jump-threading"
42 
43 STATISTIC(NumThreads, "Number of jumps threaded");
44 STATISTIC(NumFolds, "Number of terminators folded");
45 STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
46 
47 static cl::opt<unsigned>
48 BBDuplicateThreshold("jump-threading-threshold",
49  cl::desc("Max block size to duplicate for jump threading"),
50  cl::init(6), cl::Hidden);
51 
52 namespace {
53  // These are at global scope so static functions can use them too.
55  typedef SmallVector<std::pair<Constant*, BasicBlock*>, 8> PredValueInfoTy;
56 
57  // This is used to keep track of what kind of constant we're currently hoping
58  // to find.
60  WantInteger,
61  WantBlockAddress
62  };
63 
64  /// This pass performs 'jump threading', which looks at blocks that have
65  /// multiple predecessors and multiple successors. If one or more of the
66  /// predecessors of the block can be proven to always jump to one of the
67  /// successors, we forward the edge from the predecessor to the successor by
68  /// duplicating the contents of this block.
69  ///
70  /// An example of when this can occur is code like this:
71  ///
72  /// if () { ...
73  /// X = 4;
74  /// }
75  /// if (X < 3) {
76  ///
77  /// In this case, the unconditional branch at the end of the first if can be
78  /// revectored to the false side of the second if.
79  ///
80  class JumpThreading : public FunctionPass {
81  TargetLibraryInfo *TLI;
82  LazyValueInfo *LVI;
83 #ifdef NDEBUG
84  SmallPtrSet<BasicBlock*, 16> LoopHeaders;
85 #else
86  SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
87 #endif
89 
90  unsigned BBDupThreshold;
91 
92  // RAII helper for updating the recursion stack.
93  struct RecursionSetRemover {
95  std::pair<Value*, BasicBlock*> ThePair;
96 
97  RecursionSetRemover(DenseSet<std::pair<Value*, BasicBlock*> > &S,
98  std::pair<Value*, BasicBlock*> P)
99  : TheSet(S), ThePair(P) { }
100 
101  ~RecursionSetRemover() {
102  TheSet.erase(ThePair);
103  }
104  };
105  public:
106  static char ID; // Pass identification
107  JumpThreading(int T = -1) : FunctionPass(ID) {
108  BBDupThreshold = (T == -1) ? BBDuplicateThreshold : unsigned(T);
110  }
111 
112  bool runOnFunction(Function &F) override;
113 
114  void getAnalysisUsage(AnalysisUsage &AU) const override {
118  }
119 
120  void FindLoopHeaders(Function &F);
121  bool ProcessBlock(BasicBlock *BB);
122  bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
123  BasicBlock *SuccBB);
124  bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
125  const SmallVectorImpl<BasicBlock *> &PredBBs);
126 
127  bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
128  PredValueInfo &Result,
130  Instruction *CxtI = nullptr);
131  bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
133  Instruction *CxtI = nullptr);
134 
135  bool ProcessBranchOnPHI(PHINode *PN);
136  bool ProcessBranchOnXOR(BinaryOperator *BO);
137 
138  bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
139  bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
140  };
141 }
142 
143 char JumpThreading::ID = 0;
144 INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading",
145  "Jump Threading", false, false)
148 INITIALIZE_PASS_END(JumpThreading, "jump-threading",
149  "Jump Threading", false, false)
150 
151 // Public interface to the Jump Threading pass
152 FunctionPass *llvm::createJumpThreadingPass(int Threshold) { return new JumpThreading(Threshold); }
153 
154 /// runOnFunction - Top level algorithm.
155 ///
156 bool JumpThreading::runOnFunction(Function &F) {
157  if (skipOptnoneFunction(F))
158  return false;
159 
160  DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
161  TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
162  LVI = &getAnalysis<LazyValueInfo>();
163 
164  // Remove unreachable blocks from function as they may result in infinite
165  // loop. We do threading if we found something profitable. Jump threading a
166  // branch can create other opportunities. If these opportunities form a cycle
167  // i.e. if any jump treading is undoing previous threading in the path, then
168  // we will loop forever. We take care of this issue by not jump threading for
169  // back edges. This works for normal cases but not for unreachable blocks as
170  // they may have cycle with no back edge.
172 
173  FindLoopHeaders(F);
174 
175  bool Changed, EverChanged = false;
176  do {
177  Changed = false;
178  for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
179  BasicBlock *BB = I;
180  // Thread all of the branches we can over this block.
181  while (ProcessBlock(BB))
182  Changed = true;
183 
184  ++I;
185 
186  // If the block is trivially dead, zap it. This eliminates the successor
187  // edges which simplifies the CFG.
188  if (pred_empty(BB) &&
189  BB != &BB->getParent()->getEntryBlock()) {
190  DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName()
191  << "' with terminator: " << *BB->getTerminator() << '\n');
192  LoopHeaders.erase(BB);
193  LVI->eraseBlock(BB);
194  DeleteDeadBlock(BB);
195  Changed = true;
196  continue;
197  }
198 
200 
201  // Can't thread an unconditional jump, but if the block is "almost
202  // empty", we can replace uses of it with uses of the successor and make
203  // this dead.
204  if (BI && BI->isUnconditional() &&
205  BB != &BB->getParent()->getEntryBlock() &&
206  // If the terminator is the only non-phi instruction, try to nuke it.
208  // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
209  // block, we have to make sure it isn't in the LoopHeaders set. We
210  // reinsert afterward if needed.
211  bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
212  BasicBlock *Succ = BI->getSuccessor(0);
213 
214  // FIXME: It is always conservatively correct to drop the info
215  // for a block even if it doesn't get erased. This isn't totally
216  // awesome, but it allows us to use AssertingVH to prevent nasty
217  // dangling pointer issues within LazyValueInfo.
218  LVI->eraseBlock(BB);
220  Changed = true;
221  // If we deleted BB and BB was the header of a loop, then the
222  // successor is now the header of the loop.
223  BB = Succ;
224  }
225 
226  if (ErasedFromLoopHeaders)
227  LoopHeaders.insert(BB);
228  }
229  }
230  EverChanged |= Changed;
231  } while (Changed);
232 
233  LoopHeaders.clear();
234  return EverChanged;
235 }
236 
237 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
238 /// thread across it. Stop scanning the block when passing the threshold.
239 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB,
240  unsigned Threshold) {
241  /// Ignore PHI nodes, these will be flattened when duplication happens.
243 
244  // FIXME: THREADING will delete values that are just used to compute the
245  // branch, so they shouldn't count against the duplication cost.
246 
247  // Sum up the cost of each instruction until we get to the terminator. Don't
248  // include the terminator because the copy won't include it.
249  unsigned Size = 0;
250  for (; !isa<TerminatorInst>(I); ++I) {
251 
252  // Stop scanning the block if we've reached the threshold.
253  if (Size > Threshold)
254  return Size;
255 
256  // Debugger intrinsics don't incur code size.
257  if (isa<DbgInfoIntrinsic>(I)) continue;
258 
259  // If this is a pointer->pointer bitcast, it is free.
260  if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
261  continue;
262 
263  // All other instructions count for at least one unit.
264  ++Size;
265 
266  // Calls are more expensive. If they are non-intrinsic calls, we model them
267  // as having cost of 4. If they are a non-vector intrinsic, we model them
268  // as having cost of 2 total, and if they are a vector intrinsic, we model
269  // them as having cost 1.
270  if (const CallInst *CI = dyn_cast<CallInst>(I)) {
271  if (CI->cannotDuplicate())
272  // Blocks with NoDuplicate are modelled as having infinite cost, so they
273  // are never duplicated.
274  return ~0U;
275  else if (!isa<IntrinsicInst>(CI))
276  Size += 3;
277  else if (!CI->getType()->isVectorTy())
278  Size += 1;
279  }
280  }
281 
282  // Threading through a switch statement is particularly profitable. If this
283  // block ends in a switch, decrease its cost to make it more likely to happen.
284  if (isa<SwitchInst>(I))
285  Size = Size > 6 ? Size-6 : 0;
286 
287  // The same holds for indirect branches, but slightly more so.
288  if (isa<IndirectBrInst>(I))
289  Size = Size > 8 ? Size-8 : 0;
290 
291  return Size;
292 }
293 
294 /// FindLoopHeaders - We do not want jump threading to turn proper loop
295 /// structures into irreducible loops. Doing this breaks up the loop nesting
296 /// hierarchy and pessimizes later transformations. To prevent this from
297 /// happening, we first have to find the loop headers. Here we approximate this
298 /// by finding targets of backedges in the CFG.
299 ///
300 /// Note that there definitely are cases when we want to allow threading of
301 /// edges across a loop header. For example, threading a jump from outside the
302 /// loop (the preheader) to an exit block of the loop is definitely profitable.
303 /// It is also almost always profitable to thread backedges from within the loop
304 /// to exit blocks, and is often profitable to thread backedges to other blocks
305 /// within the loop (forming a nested loop). This simple analysis is not rich
306 /// enough to track all of these properties and keep it up-to-date as the CFG
307 /// mutates, so we don't allow any of these transformations.
308 ///
309 void JumpThreading::FindLoopHeaders(Function &F) {
311  FindFunctionBackedges(F, Edges);
312 
313  for (unsigned i = 0, e = Edges.size(); i != e; ++i)
314  LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
315 }
316 
317 /// getKnownConstant - Helper method to determine if we can thread over a
318 /// terminator with the given value as its condition, and if so what value to
319 /// use for that. What kind of value this is depends on whether we want an
320 /// integer or a block address, but an undef is always accepted.
321 /// Returns null if Val is null or not an appropriate constant.
323  if (!Val)
324  return nullptr;
325 
326  // Undef is "known" enough.
327  if (UndefValue *U = dyn_cast<UndefValue>(Val))
328  return U;
329 
330  if (Preference == WantBlockAddress)
331  return dyn_cast<BlockAddress>(Val->stripPointerCasts());
332 
333  return dyn_cast<ConstantInt>(Val);
334 }
335 
336 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
337 /// if we can infer that the value is a known ConstantInt/BlockAddress or undef
338 /// in any of our predecessors. If so, return the known list of value and pred
339 /// BB in the result vector.
340 ///
341 /// This returns true if there were any known values.
342 ///
343 bool JumpThreading::
344 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
346  Instruction *CxtI) {
347  // This method walks up use-def chains recursively. Because of this, we could
348  // get into an infinite loop going around loops in the use-def chain. To
349  // prevent this, keep track of what (value, block) pairs we've already visited
350  // and terminate the search if we loop back to them
351  if (!RecursionSet.insert(std::make_pair(V, BB)).second)
352  return false;
353 
354  // An RAII help to remove this pair from the recursion set once the recursion
355  // stack pops back out again.
356  RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB));
357 
358  // If V is a constant, then it is known in all predecessors.
359  if (Constant *KC = getKnownConstant(V, Preference)) {
360  for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
361  Result.push_back(std::make_pair(KC, *PI));
362 
363  return true;
364  }
365 
366  // If V is a non-instruction value, or an instruction in a different block,
367  // then it can't be derived from a PHI.
369  if (!I || I->getParent() != BB) {
370 
371  // Okay, if this is a live-in value, see if it has a known value at the end
372  // of any of our predecessors.
373  //
374  // FIXME: This should be an edge property, not a block end property.
375  /// TODO: Per PR2563, we could infer value range information about a
376  /// predecessor based on its terminator.
377  //
378  // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
379  // "I" is a non-local compare-with-a-constant instruction. This would be
380  // able to handle value inequalities better, for example if the compare is
381  // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
382  // Perhaps getConstantOnEdge should be smart enough to do this?
383 
384  for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
385  BasicBlock *P = *PI;
386  // If the value is known by LazyValueInfo to be a constant in a
387  // predecessor, use that information to try to thread this block.
388  Constant *PredCst = LVI->getConstantOnEdge(V, P, BB, CxtI);
389  if (Constant *KC = getKnownConstant(PredCst, Preference))
390  Result.push_back(std::make_pair(KC, P));
391  }
392 
393  return !Result.empty();
394  }
395 
396  /// If I is a PHI node, then we know the incoming values for any constants.
397  if (PHINode *PN = dyn_cast<PHINode>(I)) {
398  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
399  Value *InVal = PN->getIncomingValue(i);
400  if (Constant *KC = getKnownConstant(InVal, Preference)) {
401  Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
402  } else {
403  Constant *CI = LVI->getConstantOnEdge(InVal,
404  PN->getIncomingBlock(i),
405  BB, CxtI);
406  if (Constant *KC = getKnownConstant(CI, Preference))
407  Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
408  }
409  }
410 
411  return !Result.empty();
412  }
413 
414  PredValueInfoTy LHSVals, RHSVals;
415 
416  // Handle some boolean conditions.
417  if (I->getType()->getPrimitiveSizeInBits() == 1) {
418  assert(Preference == WantInteger && "One-bit non-integer type?");
419  // X | true -> true
420  // X & false -> false
421  if (I->getOpcode() == Instruction::Or ||
422  I->getOpcode() == Instruction::And) {
423  ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
424  WantInteger, CxtI);
425  ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals,
426  WantInteger, CxtI);
427 
428  if (LHSVals.empty() && RHSVals.empty())
429  return false;
430 
431  ConstantInt *InterestingVal;
432  if (I->getOpcode() == Instruction::Or)
433  InterestingVal = ConstantInt::getTrue(I->getContext());
434  else
435  InterestingVal = ConstantInt::getFalse(I->getContext());
436 
437  SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
438 
439  // Scan for the sentinel. If we find an undef, force it to the
440  // interesting value: x|undef -> true and x&undef -> false.
441  for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
442  if (LHSVals[i].first == InterestingVal ||
443  isa<UndefValue>(LHSVals[i].first)) {
444  Result.push_back(LHSVals[i]);
445  Result.back().first = InterestingVal;
446  LHSKnownBBs.insert(LHSVals[i].second);
447  }
448  for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
449  if (RHSVals[i].first == InterestingVal ||
450  isa<UndefValue>(RHSVals[i].first)) {
451  // If we already inferred a value for this block on the LHS, don't
452  // re-add it.
453  if (!LHSKnownBBs.count(RHSVals[i].second)) {
454  Result.push_back(RHSVals[i]);
455  Result.back().first = InterestingVal;
456  }
457  }
458 
459  return !Result.empty();
460  }
461 
462  // Handle the NOT form of XOR.
463  if (I->getOpcode() == Instruction::Xor &&
464  isa<ConstantInt>(I->getOperand(1)) &&
465  cast<ConstantInt>(I->getOperand(1))->isOne()) {
466  ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result,
467  WantInteger, CxtI);
468  if (Result.empty())
469  return false;
470 
471  // Invert the known values.
472  for (unsigned i = 0, e = Result.size(); i != e; ++i)
473  Result[i].first = ConstantExpr::getNot(Result[i].first);
474 
475  return true;
476  }
477 
478  // Try to simplify some other binary operator values.
479  } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
480  assert(Preference != WantBlockAddress
481  && "A binary operator creating a block address?");
482  if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
483  PredValueInfoTy LHSVals;
484  ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals,
485  WantInteger, CxtI);
486 
487  // Try to use constant folding to simplify the binary operator.
488  for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
489  Constant *V = LHSVals[i].first;
490  Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI);
491 
492  if (Constant *KC = getKnownConstant(Folded, WantInteger))
493  Result.push_back(std::make_pair(KC, LHSVals[i].second));
494  }
495  }
496 
497  return !Result.empty();
498  }
499 
500  // Handle compare with phi operand, where the PHI is defined in this block.
501  if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
502  assert(Preference == WantInteger && "Compares only produce integers");
503  PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
504  if (PN && PN->getParent() == BB) {
505  const DataLayout &DL = PN->getModule()->getDataLayout();
506  // We can do this simplification if any comparisons fold to true or false.
507  // See if any do.
508  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
509  BasicBlock *PredBB = PN->getIncomingBlock(i);
510  Value *LHS = PN->getIncomingValue(i);
511  Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
512 
513  Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, DL);
514  if (!Res) {
515  if (!isa<Constant>(RHS))
516  continue;
517 
519  ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
520  cast<Constant>(RHS), PredBB, BB,
521  CxtI ? CxtI : Cmp);
522  if (ResT == LazyValueInfo::Unknown)
523  continue;
524  Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
525  }
526 
527  if (Constant *KC = getKnownConstant(Res, WantInteger))
528  Result.push_back(std::make_pair(KC, PredBB));
529  }
530 
531  return !Result.empty();
532  }
533 
534  // If comparing a live-in value against a constant, see if we know the
535  // live-in value on any predecessors.
536  if (isa<Constant>(Cmp->getOperand(1)) && Cmp->getType()->isIntegerTy()) {
537  if (!isa<Instruction>(Cmp->getOperand(0)) ||
538  cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) {
539  Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
540 
541  for (pred_iterator PI = pred_begin(BB), E = pred_end(BB);PI != E; ++PI){
542  BasicBlock *P = *PI;
543  // If the value is known by LazyValueInfo to be a constant in a
544  // predecessor, use that information to try to thread this block.
546  LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
547  RHSCst, P, BB, CxtI ? CxtI : Cmp);
548  if (Res == LazyValueInfo::Unknown)
549  continue;
550 
551  Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
552  Result.push_back(std::make_pair(ResC, P));
553  }
554 
555  return !Result.empty();
556  }
557 
558  // Try to find a constant value for the LHS of a comparison,
559  // and evaluate it statically if we can.
560  if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) {
561  PredValueInfoTy LHSVals;
562  ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
563  WantInteger, CxtI);
564 
565  for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
566  Constant *V = LHSVals[i].first;
567  Constant *Folded = ConstantExpr::getCompare(Cmp->getPredicate(),
568  V, CmpConst);
569  if (Constant *KC = getKnownConstant(Folded, WantInteger))
570  Result.push_back(std::make_pair(KC, LHSVals[i].second));
571  }
572 
573  return !Result.empty();
574  }
575  }
576  }
577 
578  if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
579  // Handle select instructions where at least one operand is a known constant
580  // and we can figure out the condition value for any predecessor block.
581  Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
582  Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
583  PredValueInfoTy Conds;
584  if ((TrueVal || FalseVal) &&
585  ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
586  WantInteger, CxtI)) {
587  for (unsigned i = 0, e = Conds.size(); i != e; ++i) {
588  Constant *Cond = Conds[i].first;
589 
590  // Figure out what value to use for the condition.
591  bool KnownCond;
592  if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
593  // A known boolean.
594  KnownCond = CI->isOne();
595  } else {
596  assert(isa<UndefValue>(Cond) && "Unexpected condition value");
597  // Either operand will do, so be sure to pick the one that's a known
598  // constant.
599  // FIXME: Do this more cleverly if both values are known constants?
600  KnownCond = (TrueVal != nullptr);
601  }
602 
603  // See if the select has a known constant value for this predecessor.
604  if (Constant *Val = KnownCond ? TrueVal : FalseVal)
605  Result.push_back(std::make_pair(Val, Conds[i].second));
606  }
607 
608  return !Result.empty();
609  }
610  }
611 
612  // If all else fails, see if LVI can figure out a constant value for us.
613  Constant *CI = LVI->getConstant(V, BB, CxtI);
614  if (Constant *KC = getKnownConstant(CI, Preference)) {
615  for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
616  Result.push_back(std::make_pair(KC, *PI));
617  }
618 
619  return !Result.empty();
620 }
621 
622 
623 
624 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
625 /// in an undefined jump, decide which block is best to revector to.
626 ///
627 /// Since we can pick an arbitrary destination, we pick the successor with the
628 /// fewest predecessors. This should reduce the in-degree of the others.
629 ///
630 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
631  TerminatorInst *BBTerm = BB->getTerminator();
632  unsigned MinSucc = 0;
633  BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
634  // Compute the successor with the minimum number of predecessors.
635  unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
636  for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
637  TestBB = BBTerm->getSuccessor(i);
638  unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
639  if (NumPreds < MinNumPreds) {
640  MinSucc = i;
641  MinNumPreds = NumPreds;
642  }
643  }
644 
645  return MinSucc;
646 }
647 
649  if (!BB->hasAddressTaken()) return false;
650 
651  // If the block has its address taken, it may be a tree of dead constants
652  // hanging off of it. These shouldn't keep the block alive.
655  return !BA->use_empty();
656 }
657 
658 /// ProcessBlock - If there are any predecessors whose control can be threaded
659 /// through to a successor, transform them now.
660 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
661  // If the block is trivially dead, just return and let the caller nuke it.
662  // This simplifies other transformations.
663  if (pred_empty(BB) &&
664  BB != &BB->getParent()->getEntryBlock())
665  return false;
666 
667  // If this block has a single predecessor, and if that pred has a single
668  // successor, merge the blocks. This encourages recursive jump threading
669  // because now the condition in this block can be threaded through
670  // predecessors of our predecessor block.
671  if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
672  if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
673  SinglePred != BB && !hasAddressTakenAndUsed(BB)) {
674  // If SinglePred was a loop header, BB becomes one.
675  if (LoopHeaders.erase(SinglePred))
676  LoopHeaders.insert(BB);
677 
678  LVI->eraseBlock(SinglePred);
680 
681  return true;
682  }
683  }
684 
685  // What kind of constant we're looking for.
686  ConstantPreference Preference = WantInteger;
687 
688  // Look to see if the terminator is a conditional branch, switch or indirect
689  // branch, if not we can't thread it.
690  Value *Condition;
692  if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) {
693  // Can't thread an unconditional jump.
694  if (BI->isUnconditional()) return false;
695  Condition = BI->getCondition();
696  } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) {
697  Condition = SI->getCondition();
698  } else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) {
699  // Can't thread indirect branch with no successors.
700  if (IB->getNumSuccessors() == 0) return false;
701  Condition = IB->getAddress()->stripPointerCasts();
702  Preference = WantBlockAddress;
703  } else {
704  return false; // Must be an invoke.
705  }
706 
707  // Run constant folding to see if we can reduce the condition to a simple
708  // constant.
709  if (Instruction *I = dyn_cast<Instruction>(Condition)) {
710  Value *SimpleVal =
712  if (SimpleVal) {
713  I->replaceAllUsesWith(SimpleVal);
714  I->eraseFromParent();
715  Condition = SimpleVal;
716  }
717  }
718 
719  // If the terminator is branching on an undef, we can pick any of the
720  // successors to branch to. Let GetBestDestForJumpOnUndef decide.
721  if (isa<UndefValue>(Condition)) {
722  unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
723 
724  // Fold the branch/switch.
725  TerminatorInst *BBTerm = BB->getTerminator();
726  for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
727  if (i == BestSucc) continue;
728  BBTerm->getSuccessor(i)->removePredecessor(BB, true);
729  }
730 
731  DEBUG(dbgs() << " In block '" << BB->getName()
732  << "' folding undef terminator: " << *BBTerm << '\n');
733  BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
734  BBTerm->eraseFromParent();
735  return true;
736  }
737 
738  // If the terminator of this block is branching on a constant, simplify the
739  // terminator to an unconditional branch. This can occur due to threading in
740  // other blocks.
741  if (getKnownConstant(Condition, Preference)) {
742  DEBUG(dbgs() << " In block '" << BB->getName()
743  << "' folding terminator: " << *BB->getTerminator() << '\n');
744  ++NumFolds;
745  ConstantFoldTerminator(BB, true);
746  return true;
747  }
748 
749  Instruction *CondInst = dyn_cast<Instruction>(Condition);
750 
751  // All the rest of our checks depend on the condition being an instruction.
752  if (!CondInst) {
753  // FIXME: Unify this with code below.
754  if (ProcessThreadableEdges(Condition, BB, Preference, Terminator))
755  return true;
756  return false;
757  }
758 
759 
760  if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
761  // If we're branching on a conditional, LVI might be able to determine
762  // it's value at the branch instruction. We only handle comparisons
763  // against a constant at this time.
764  // TODO: This should be extended to handle switches as well.
765  BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
766  Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
767  if (CondBr && CondConst && CondBr->isConditional()) {
769  LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0),
770  CondConst, CondBr);
771  if (Ret != LazyValueInfo::Unknown) {
772  unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0;
773  unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1;
774  CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
775  BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
776  CondBr->eraseFromParent();
777  if (CondCmp->use_empty())
778  CondCmp->eraseFromParent();
779  else if (CondCmp->getParent() == BB) {
780  // If the fact we just learned is true for all uses of the
781  // condition, replace it with a constant value
782  auto *CI = Ret == LazyValueInfo::True ?
783  ConstantInt::getTrue(CondCmp->getType()) :
784  ConstantInt::getFalse(CondCmp->getType());
785  CondCmp->replaceAllUsesWith(CI);
786  CondCmp->eraseFromParent();
787  }
788  return true;
789  }
790  }
791 
792  if (CondBr && CondConst && TryToUnfoldSelect(CondCmp, BB))
793  return true;
794  }
795 
796  // Check for some cases that are worth simplifying. Right now we want to look
797  // for loads that are used by a switch or by the condition for the branch. If
798  // we see one, check to see if it's partially redundant. If so, insert a PHI
799  // which can then be used to thread the values.
800  //
801  Value *SimplifyValue = CondInst;
802  if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
803  if (isa<Constant>(CondCmp->getOperand(1)))
804  SimplifyValue = CondCmp->getOperand(0);
805 
806  // TODO: There are other places where load PRE would be profitable, such as
807  // more complex comparisons.
808  if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
809  if (SimplifyPartiallyRedundantLoad(LI))
810  return true;
811 
812 
813  // Handle a variety of cases where we are branching on something derived from
814  // a PHI node in the current block. If we can prove that any predecessors
815  // compute a predictable value based on a PHI node, thread those predecessors.
816  //
817  if (ProcessThreadableEdges(CondInst, BB, Preference, Terminator))
818  return true;
819 
820  // If this is an otherwise-unfoldable branch on a phi node in the current
821  // block, see if we can simplify.
822  if (PHINode *PN = dyn_cast<PHINode>(CondInst))
823  if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
824  return ProcessBranchOnPHI(PN);
825 
826 
827  // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
828  if (CondInst->getOpcode() == Instruction::Xor &&
829  CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
830  return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
831 
832 
833  // TODO: If we have: "br (X > 0)" and we have a predecessor where we know
834  // "(X == 4)", thread through this block.
835 
836  return false;
837 }
838 
839 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
840 /// load instruction, eliminate it by replacing it with a PHI node. This is an
841 /// important optimization that encourages jump threading, and needs to be run
842 /// interlaced with other jump threading tasks.
843 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
844  // Don't hack volatile/atomic loads.
845  if (!LI->isSimple()) return false;
846 
847  // If the load is defined in a block with exactly one predecessor, it can't be
848  // partially redundant.
849  BasicBlock *LoadBB = LI->getParent();
850  if (LoadBB->getSinglePredecessor())
851  return false;
852 
853  // If the load is defined in a landing pad, it can't be partially redundant,
854  // because the edges between the invoke and the landing pad cannot have other
855  // instructions between them.
856  if (LoadBB->isLandingPad())
857  return false;
858 
859  Value *LoadedPtr = LI->getOperand(0);
860 
861  // If the loaded operand is defined in the LoadBB, it can't be available.
862  // TODO: Could do simple PHI translation, that would be fun :)
863  if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
864  if (PtrOp->getParent() == LoadBB)
865  return false;
866 
867  // Scan a few instructions up from the load, to see if it is obviously live at
868  // the entry to its block.
869  BasicBlock::iterator BBIt = LI;
870 
871  if (Value *AvailableVal =
872  FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
873  // If the value if the load is locally available within the block, just use
874  // it. This frequently occurs for reg2mem'd allocas.
875  //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
876 
877  // If the returned value is the load itself, replace with an undef. This can
878  // only happen in dead loops.
879  if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
880  if (AvailableVal->getType() != LI->getType())
881  AvailableVal =
882  CastInst::CreateBitOrPointerCast(AvailableVal, LI->getType(), "", LI);
883  LI->replaceAllUsesWith(AvailableVal);
884  LI->eraseFromParent();
885  return true;
886  }
887 
888  // Otherwise, if we scanned the whole block and got to the top of the block,
889  // we know the block is locally transparent to the load. If not, something
890  // might clobber its value.
891  if (BBIt != LoadBB->begin())
892  return false;
893 
894  // If all of the loads and stores that feed the value have the same AA tags,
895  // then we can propagate them onto any newly inserted loads.
896  AAMDNodes AATags;
897  LI->getAAMetadata(AATags);
898 
899  SmallPtrSet<BasicBlock*, 8> PredsScanned;
900  typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
901  AvailablePredsTy AvailablePreds;
902  BasicBlock *OneUnavailablePred = nullptr;
903 
904  // If we got here, the loaded value is transparent through to the start of the
905  // block. Check to see if it is available in any of the predecessor blocks.
906  for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
907  PI != PE; ++PI) {
908  BasicBlock *PredBB = *PI;
909 
910  // If we already scanned this predecessor, skip it.
911  if (!PredsScanned.insert(PredBB).second)
912  continue;
913 
914  // Scan the predecessor to see if the value is available in the pred.
915  BBIt = PredBB->end();
916  AAMDNodes ThisAATags;
917  Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6,
918  nullptr, &ThisAATags);
919  if (!PredAvailable) {
920  OneUnavailablePred = PredBB;
921  continue;
922  }
923 
924  // If AA tags disagree or are not present, forget about them.
925  if (AATags != ThisAATags) AATags = AAMDNodes();
926 
927  // If so, this load is partially redundant. Remember this info so that we
928  // can create a PHI node.
929  AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
930  }
931 
932  // If the loaded value isn't available in any predecessor, it isn't partially
933  // redundant.
934  if (AvailablePreds.empty()) return false;
935 
936  // Okay, the loaded value is available in at least one (and maybe all!)
937  // predecessors. If the value is unavailable in more than one unique
938  // predecessor, we want to insert a merge block for those common predecessors.
939  // This ensures that we only have to insert one reload, thus not increasing
940  // code size.
941  BasicBlock *UnavailablePred = nullptr;
942 
943  // If there is exactly one predecessor where the value is unavailable, the
944  // already computed 'OneUnavailablePred' block is it. If it ends in an
945  // unconditional branch, we know that it isn't a critical edge.
946  if (PredsScanned.size() == AvailablePreds.size()+1 &&
947  OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
948  UnavailablePred = OneUnavailablePred;
949  } else if (PredsScanned.size() != AvailablePreds.size()) {
950  // Otherwise, we had multiple unavailable predecessors or we had a critical
951  // edge from the one.
952  SmallVector<BasicBlock*, 8> PredsToSplit;
953  SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
954 
955  for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
956  AvailablePredSet.insert(AvailablePreds[i].first);
957 
958  // Add all the unavailable predecessors to the PredsToSplit list.
959  for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
960  PI != PE; ++PI) {
961  BasicBlock *P = *PI;
962  // If the predecessor is an indirect goto, we can't split the edge.
963  if (isa<IndirectBrInst>(P->getTerminator()))
964  return false;
965 
966  if (!AvailablePredSet.count(P))
967  PredsToSplit.push_back(P);
968  }
969 
970  // Split them out to their own block.
971  UnavailablePred =
972  SplitBlockPredecessors(LoadBB, PredsToSplit, "thread-pre-split");
973  }
974 
975  // If the value isn't available in all predecessors, then there will be
976  // exactly one where it isn't available. Insert a load on that edge and add
977  // it to the AvailablePreds list.
978  if (UnavailablePred) {
979  assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
980  "Can't handle critical edge here!");
981  LoadInst *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
982  LI->getAlignment(),
983  UnavailablePred->getTerminator());
984  NewVal->setDebugLoc(LI->getDebugLoc());
985  if (AATags)
986  NewVal->setAAMetadata(AATags);
987 
988  AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
989  }
990 
991  // Now we know that each predecessor of this block has a value in
992  // AvailablePreds, sort them for efficient access as we're walking the preds.
993  array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
994 
995  // Create a PHI node at the start of the block for the PRE'd load value.
996  pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB);
997  PHINode *PN = PHINode::Create(LI->getType(), std::distance(PB, PE), "",
998  LoadBB->begin());
999  PN->takeName(LI);
1000  PN->setDebugLoc(LI->getDebugLoc());
1001 
1002  // Insert new entries into the PHI for each predecessor. A single block may
1003  // have multiple entries here.
1004  for (pred_iterator PI = PB; PI != PE; ++PI) {
1005  BasicBlock *P = *PI;
1006  AvailablePredsTy::iterator I =
1007  std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
1008  std::make_pair(P, (Value*)nullptr));
1009 
1010  assert(I != AvailablePreds.end() && I->first == P &&
1011  "Didn't find entry for predecessor!");
1012 
1013  // If we have an available predecessor but it requires casting, insert the
1014  // cast in the predecessor and use the cast. Note that we have to update the
1015  // AvailablePreds vector as we go so that all of the PHI entries for this
1016  // predecessor use the same bitcast.
1017  Value *&PredV = I->second;
1018  if (PredV->getType() != LI->getType())
1019  PredV = CastInst::CreateBitOrPointerCast(PredV, LI->getType(), "",
1020  P->getTerminator());
1021 
1022  PN->addIncoming(PredV, I->first);
1023  }
1024 
1025  //cerr << "PRE: " << *LI << *PN << "\n";
1026 
1027  LI->replaceAllUsesWith(PN);
1028  LI->eraseFromParent();
1029 
1030  return true;
1031 }
1032 
1033 /// FindMostPopularDest - The specified list contains multiple possible
1034 /// threadable destinations. Pick the one that occurs the most frequently in
1035 /// the list.
1036 static BasicBlock *
1038  const SmallVectorImpl<std::pair<BasicBlock*,
1039  BasicBlock*> > &PredToDestList) {
1040  assert(!PredToDestList.empty());
1041 
1042  // Determine popularity. If there are multiple possible destinations, we
1043  // explicitly choose to ignore 'undef' destinations. We prefer to thread
1044  // blocks with known and real destinations to threading undef. We'll handle
1045  // them later if interesting.
1046  DenseMap<BasicBlock*, unsigned> DestPopularity;
1047  for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1048  if (PredToDestList[i].second)
1049  DestPopularity[PredToDestList[i].second]++;
1050 
1051  // Find the most popular dest.
1052  DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
1053  BasicBlock *MostPopularDest = DPI->first;
1054  unsigned Popularity = DPI->second;
1055  SmallVector<BasicBlock*, 4> SamePopularity;
1056 
1057  for (++DPI; DPI != DestPopularity.end(); ++DPI) {
1058  // If the popularity of this entry isn't higher than the popularity we've
1059  // seen so far, ignore it.
1060  if (DPI->second < Popularity)
1061  ; // ignore.
1062  else if (DPI->second == Popularity) {
1063  // If it is the same as what we've seen so far, keep track of it.
1064  SamePopularity.push_back(DPI->first);
1065  } else {
1066  // If it is more popular, remember it.
1067  SamePopularity.clear();
1068  MostPopularDest = DPI->first;
1069  Popularity = DPI->second;
1070  }
1071  }
1072 
1073  // Okay, now we know the most popular destination. If there is more than one
1074  // destination, we need to determine one. This is arbitrary, but we need
1075  // to make a deterministic decision. Pick the first one that appears in the
1076  // successor list.
1077  if (!SamePopularity.empty()) {
1078  SamePopularity.push_back(MostPopularDest);
1079  TerminatorInst *TI = BB->getTerminator();
1080  for (unsigned i = 0; ; ++i) {
1081  assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
1082 
1083  if (std::find(SamePopularity.begin(), SamePopularity.end(),
1084  TI->getSuccessor(i)) == SamePopularity.end())
1085  continue;
1086 
1087  MostPopularDest = TI->getSuccessor(i);
1088  break;
1089  }
1090  }
1091 
1092  // Okay, we have finally picked the most popular destination.
1093  return MostPopularDest;
1094 }
1095 
1096 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
1097  ConstantPreference Preference,
1098  Instruction *CxtI) {
1099  // If threading this would thread across a loop header, don't even try to
1100  // thread the edge.
1101  if (LoopHeaders.count(BB))
1102  return false;
1103 
1104  PredValueInfoTy PredValues;
1105  if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference, CxtI))
1106  return false;
1107 
1108  assert(!PredValues.empty() &&
1109  "ComputeValueKnownInPredecessors returned true with no values");
1110 
1111  DEBUG(dbgs() << "IN BB: " << *BB;
1112  for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1113  dbgs() << " BB '" << BB->getName() << "': FOUND condition = "
1114  << *PredValues[i].first
1115  << " for pred '" << PredValues[i].second->getName() << "'.\n";
1116  });
1117 
1118  // Decide what we want to thread through. Convert our list of known values to
1119  // a list of known destinations for each pred. This also discards duplicate
1120  // predecessors and keeps track of the undefined inputs (which are represented
1121  // as a null dest in the PredToDestList).
1122  SmallPtrSet<BasicBlock*, 16> SeenPreds;
1124 
1125  BasicBlock *OnlyDest = nullptr;
1126  BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1127 
1128  for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1129  BasicBlock *Pred = PredValues[i].second;
1130  if (!SeenPreds.insert(Pred).second)
1131  continue; // Duplicate predecessor entry.
1132 
1133  // If the predecessor ends with an indirect goto, we can't change its
1134  // destination.
1135  if (isa<IndirectBrInst>(Pred->getTerminator()))
1136  continue;
1137 
1138  Constant *Val = PredValues[i].first;
1139 
1140  BasicBlock *DestBB;
1141  if (isa<UndefValue>(Val))
1142  DestBB = nullptr;
1143  else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1144  DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero());
1145  else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
1146  DestBB = SI->findCaseValue(cast<ConstantInt>(Val)).getCaseSuccessor();
1147  } else {
1148  assert(isa<IndirectBrInst>(BB->getTerminator())
1149  && "Unexpected terminator");
1150  DestBB = cast<BlockAddress>(Val)->getBasicBlock();
1151  }
1152 
1153  // If we have exactly one destination, remember it for efficiency below.
1154  if (PredToDestList.empty())
1155  OnlyDest = DestBB;
1156  else if (OnlyDest != DestBB)
1157  OnlyDest = MultipleDestSentinel;
1158 
1159  PredToDestList.push_back(std::make_pair(Pred, DestBB));
1160  }
1161 
1162  // If all edges were unthreadable, we fail.
1163  if (PredToDestList.empty())
1164  return false;
1165 
1166  // Determine which is the most common successor. If we have many inputs and
1167  // this block is a switch, we want to start by threading the batch that goes
1168  // to the most popular destination first. If we only know about one
1169  // threadable destination (the common case) we can avoid this.
1170  BasicBlock *MostPopularDest = OnlyDest;
1171 
1172  if (MostPopularDest == MultipleDestSentinel)
1173  MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1174 
1175  // Now that we know what the most popular destination is, factor all
1176  // predecessors that will jump to it into a single predecessor.
1177  SmallVector<BasicBlock*, 16> PredsToFactor;
1178  for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1179  if (PredToDestList[i].second == MostPopularDest) {
1180  BasicBlock *Pred = PredToDestList[i].first;
1181 
1182  // This predecessor may be a switch or something else that has multiple
1183  // edges to the block. Factor each of these edges by listing them
1184  // according to # occurrences in PredsToFactor.
1185  TerminatorInst *PredTI = Pred->getTerminator();
1186  for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
1187  if (PredTI->getSuccessor(i) == BB)
1188  PredsToFactor.push_back(Pred);
1189  }
1190 
1191  // If the threadable edges are branching on an undefined value, we get to pick
1192  // the destination that these predecessors should get to.
1193  if (!MostPopularDest)
1194  MostPopularDest = BB->getTerminator()->
1195  getSuccessor(GetBestDestForJumpOnUndef(BB));
1196 
1197  // Ok, try to thread it!
1198  return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1199 }
1200 
1201 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1202 /// a PHI node in the current block. See if there are any simplifications we
1203 /// can do based on inputs to the phi node.
1204 ///
1205 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
1206  BasicBlock *BB = PN->getParent();
1207 
1208  // TODO: We could make use of this to do it once for blocks with common PHI
1209  // values.
1211  PredBBs.resize(1);
1212 
1213  // If any of the predecessor blocks end in an unconditional branch, we can
1214  // *duplicate* the conditional branch into that block in order to further
1215  // encourage jump threading and to eliminate cases where we have branch on a
1216  // phi of an icmp (branch on icmp is much better).
1217  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1218  BasicBlock *PredBB = PN->getIncomingBlock(i);
1219  if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1220  if (PredBr->isUnconditional()) {
1221  PredBBs[0] = PredBB;
1222  // Try to duplicate BB into PredBB.
1223  if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
1224  return true;
1225  }
1226  }
1227 
1228  return false;
1229 }
1230 
1231 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
1232 /// a xor instruction in the current block. See if there are any
1233 /// simplifications we can do based on inputs to the xor.
1234 ///
1235 bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
1236  BasicBlock *BB = BO->getParent();
1237 
1238  // If either the LHS or RHS of the xor is a constant, don't do this
1239  // optimization.
1240  if (isa<ConstantInt>(BO->getOperand(0)) ||
1241  isa<ConstantInt>(BO->getOperand(1)))
1242  return false;
1243 
1244  // If the first instruction in BB isn't a phi, we won't be able to infer
1245  // anything special about any particular predecessor.
1246  if (!isa<PHINode>(BB->front()))
1247  return false;
1248 
1249  // If we have a xor as the branch input to this block, and we know that the
1250  // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1251  // the condition into the predecessor and fix that value to true, saving some
1252  // logical ops on that path and encouraging other paths to simplify.
1253  //
1254  // This copies something like this:
1255  //
1256  // BB:
1257  // %X = phi i1 [1], [%X']
1258  // %Y = icmp eq i32 %A, %B
1259  // %Z = xor i1 %X, %Y
1260  // br i1 %Z, ...
1261  //
1262  // Into:
1263  // BB':
1264  // %Y = icmp ne i32 %A, %B
1265  // br i1 %Z, ...
1266 
1267  PredValueInfoTy XorOpValues;
1268  bool isLHS = true;
1269  if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
1270  WantInteger, BO)) {
1271  assert(XorOpValues.empty());
1272  if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
1273  WantInteger, BO))
1274  return false;
1275  isLHS = false;
1276  }
1277 
1278  assert(!XorOpValues.empty() &&
1279  "ComputeValueKnownInPredecessors returned true with no values");
1280 
1281  // Scan the information to see which is most popular: true or false. The
1282  // predecessors can be of the set true, false, or undef.
1283  unsigned NumTrue = 0, NumFalse = 0;
1284  for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1285  if (isa<UndefValue>(XorOpValues[i].first))
1286  // Ignore undefs for the count.
1287  continue;
1288  if (cast<ConstantInt>(XorOpValues[i].first)->isZero())
1289  ++NumFalse;
1290  else
1291  ++NumTrue;
1292  }
1293 
1294  // Determine which value to split on, true, false, or undef if neither.
1295  ConstantInt *SplitVal = nullptr;
1296  if (NumTrue > NumFalse)
1297  SplitVal = ConstantInt::getTrue(BB->getContext());
1298  else if (NumTrue != 0 || NumFalse != 0)
1299  SplitVal = ConstantInt::getFalse(BB->getContext());
1300 
1301  // Collect all of the blocks that this can be folded into so that we can
1302  // factor this once and clone it once.
1303  SmallVector<BasicBlock*, 8> BlocksToFoldInto;
1304  for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1305  if (XorOpValues[i].first != SplitVal &&
1306  !isa<UndefValue>(XorOpValues[i].first))
1307  continue;
1308 
1309  BlocksToFoldInto.push_back(XorOpValues[i].second);
1310  }
1311 
1312  // If we inferred a value for all of the predecessors, then duplication won't
1313  // help us. However, we can just replace the LHS or RHS with the constant.
1314  if (BlocksToFoldInto.size() ==
1315  cast<PHINode>(BB->front()).getNumIncomingValues()) {
1316  if (!SplitVal) {
1317  // If all preds provide undef, just nuke the xor, because it is undef too.
1319  BO->eraseFromParent();
1320  } else if (SplitVal->isZero()) {
1321  // If all preds provide 0, replace the xor with the other input.
1322  BO->replaceAllUsesWith(BO->getOperand(isLHS));
1323  BO->eraseFromParent();
1324  } else {
1325  // If all preds provide 1, set the computed value to 1.
1326  BO->setOperand(!isLHS, SplitVal);
1327  }
1328 
1329  return true;
1330  }
1331 
1332  // Try to duplicate BB into PredBB.
1333  return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
1334 }
1335 
1336 
1337 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1338 /// predecessor to the PHIBB block. If it has PHI nodes, add entries for
1339 /// NewPred using the entries from OldPred (suitably mapped).
1341  BasicBlock *OldPred,
1342  BasicBlock *NewPred,
1344  for (BasicBlock::iterator PNI = PHIBB->begin();
1345  PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1346  // Ok, we have a PHI node. Figure out what the incoming value was for the
1347  // DestBlock.
1348  Value *IV = PN->getIncomingValueForBlock(OldPred);
1349 
1350  // Remap the value if necessary.
1351  if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1353  if (I != ValueMap.end())
1354  IV = I->second;
1355  }
1356 
1357  PN->addIncoming(IV, NewPred);
1358  }
1359 }
1360 
1361 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1362 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1363 /// across BB. Transform the IR to reflect this change.
1364 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1365  const SmallVectorImpl<BasicBlock*> &PredBBs,
1366  BasicBlock *SuccBB) {
1367  // If threading to the same block as we come from, we would infinite loop.
1368  if (SuccBB == BB) {
1369  DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
1370  << "' - would thread to self!\n");
1371  return false;
1372  }
1373 
1374  // If threading this would thread across a loop header, don't thread the edge.
1375  // See the comments above FindLoopHeaders for justifications and caveats.
1376  if (LoopHeaders.count(BB)) {
1377  DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName()
1378  << "' to dest BB '" << SuccBB->getName()
1379  << "' - it might create an irreducible loop!\n");
1380  return false;
1381  }
1382 
1383  unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB, BBDupThreshold);
1384  if (JumpThreadCost > BBDupThreshold) {
1385  DEBUG(dbgs() << " Not threading BB '" << BB->getName()
1386  << "' - Cost is too high: " << JumpThreadCost << "\n");
1387  return false;
1388  }
1389 
1390  // And finally, do it! Start by factoring the predecessors is needed.
1391  BasicBlock *PredBB;
1392  if (PredBBs.size() == 1)
1393  PredBB = PredBBs[0];
1394  else {
1395  DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1396  << " common predecessors.\n");
1397  PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm");
1398  }
1399 
1400  // And finally, do it!
1401  DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '"
1402  << SuccBB->getName() << "' with cost: " << JumpThreadCost
1403  << ", across block:\n "
1404  << *BB << "\n");
1405 
1406  LVI->threadEdge(PredBB, BB, SuccBB);
1407 
1408  // We are going to have to map operands from the original BB block to the new
1409  // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
1410  // account for entry from PredBB.
1411  DenseMap<Instruction*, Value*> ValueMapping;
1412 
1413  BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1414  BB->getName()+".thread",
1415  BB->getParent(), BB);
1416  NewBB->moveAfter(PredBB);
1417 
1418  BasicBlock::iterator BI = BB->begin();
1419  for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1420  ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1421 
1422  // Clone the non-phi instructions of BB into NewBB, keeping track of the
1423  // mapping and using it to remap operands in the cloned instructions.
1424  for (; !isa<TerminatorInst>(BI); ++BI) {
1425  Instruction *New = BI->clone();
1426  New->setName(BI->getName());
1427  NewBB->getInstList().push_back(New);
1428  ValueMapping[BI] = New;
1429 
1430  // Remap operands to patch up intra-block references.
1431  for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1432  if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1433  DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1434  if (I != ValueMapping.end())
1435  New->setOperand(i, I->second);
1436  }
1437  }
1438 
1439  // We didn't copy the terminator from BB over to NewBB, because there is now
1440  // an unconditional jump to SuccBB. Insert the unconditional jump.
1441  BranchInst *NewBI =BranchInst::Create(SuccBB, NewBB);
1442  NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
1443 
1444  // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1445  // PHI nodes for NewBB now.
1446  AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1447 
1448  // If there were values defined in BB that are used outside the block, then we
1449  // now have to update all uses of the value to use either the original value,
1450  // the cloned value, or some PHI derived value. This can require arbitrary
1451  // PHI insertion, of which we are prepared to do, clean these up now.
1452  SSAUpdater SSAUpdate;
1453  SmallVector<Use*, 16> UsesToRename;
1454  for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1455  // Scan all uses of this instruction to see if it is used outside of its
1456  // block, and if so, record them in UsesToRename.
1457  for (Use &U : I->uses()) {
1458  Instruction *User = cast<Instruction>(U.getUser());
1459  if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1460  if (UserPN->getIncomingBlock(U) == BB)
1461  continue;
1462  } else if (User->getParent() == BB)
1463  continue;
1464 
1465  UsesToRename.push_back(&U);
1466  }
1467 
1468  // If there are no uses outside the block, we're done with this instruction.
1469  if (UsesToRename.empty())
1470  continue;
1471 
1472  DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1473 
1474  // We found a use of I outside of BB. Rename all uses of I that are outside
1475  // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1476  // with the two values we know.
1477  SSAUpdate.Initialize(I->getType(), I->getName());
1478  SSAUpdate.AddAvailableValue(BB, I);
1479  SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1480 
1481  while (!UsesToRename.empty())
1482  SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1483  DEBUG(dbgs() << "\n");
1484  }
1485 
1486 
1487  // Ok, NewBB is good to go. Update the terminator of PredBB to jump to
1488  // NewBB instead of BB. This eliminates predecessors from BB, which requires
1489  // us to simplify any PHI nodes in BB.
1490  TerminatorInst *PredTerm = PredBB->getTerminator();
1491  for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1492  if (PredTerm->getSuccessor(i) == BB) {
1493  BB->removePredecessor(PredBB, true);
1494  PredTerm->setSuccessor(i, NewBB);
1495  }
1496 
1497  // At this point, the IR is fully up to date and consistent. Do a quick scan
1498  // over the new instructions and zap any that are constants or dead. This
1499  // frequently happens because of phi translation.
1500  SimplifyInstructionsInBlock(NewBB, TLI);
1501 
1502  // Threaded an edge!
1503  ++NumThreads;
1504  return true;
1505 }
1506 
1507 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1508 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1509 /// If we can duplicate the contents of BB up into PredBB do so now, this
1510 /// improves the odds that the branch will be on an analyzable instruction like
1511 /// a compare.
1512 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1513  const SmallVectorImpl<BasicBlock *> &PredBBs) {
1514  assert(!PredBBs.empty() && "Can't handle an empty set");
1515 
1516  // If BB is a loop header, then duplicating this block outside the loop would
1517  // cause us to transform this into an irreducible loop, don't do this.
1518  // See the comments above FindLoopHeaders for justifications and caveats.
1519  if (LoopHeaders.count(BB)) {
1520  DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
1521  << "' into predecessor block '" << PredBBs[0]->getName()
1522  << "' - it might create an irreducible loop!\n");
1523  return false;
1524  }
1525 
1526  unsigned DuplicationCost = getJumpThreadDuplicationCost(BB, BBDupThreshold);
1527  if (DuplicationCost > BBDupThreshold) {
1528  DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
1529  << "' - Cost is too high: " << DuplicationCost << "\n");
1530  return false;
1531  }
1532 
1533  // And finally, do it! Start by factoring the predecessors is needed.
1534  BasicBlock *PredBB;
1535  if (PredBBs.size() == 1)
1536  PredBB = PredBBs[0];
1537  else {
1538  DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1539  << " common predecessors.\n");
1540  PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm");
1541  }
1542 
1543  // Okay, we decided to do this! Clone all the instructions in BB onto the end
1544  // of PredBB.
1545  DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '"
1546  << PredBB->getName() << "' to eliminate branch on phi. Cost: "
1547  << DuplicationCost << " block is:" << *BB << "\n");
1548 
1549  // Unless PredBB ends with an unconditional branch, split the edge so that we
1550  // can just clone the bits from BB into the end of the new PredBB.
1551  BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
1552 
1553  if (!OldPredBranch || !OldPredBranch->isUnconditional()) {
1554  PredBB = SplitEdge(PredBB, BB);
1555  OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1556  }
1557 
1558  // We are going to have to map operands from the original BB block into the
1559  // PredBB block. Evaluate PHI nodes in BB.
1560  DenseMap<Instruction*, Value*> ValueMapping;
1561 
1562  BasicBlock::iterator BI = BB->begin();
1563  for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1564  ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1565  // Clone the non-phi instructions of BB into PredBB, keeping track of the
1566  // mapping and using it to remap operands in the cloned instructions.
1567  for (; BI != BB->end(); ++BI) {
1568  Instruction *New = BI->clone();
1569 
1570  // Remap operands to patch up intra-block references.
1571  for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1572  if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1573  DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1574  if (I != ValueMapping.end())
1575  New->setOperand(i, I->second);
1576  }
1577 
1578  // If this instruction can be simplified after the operands are updated,
1579  // just use the simplified value instead. This frequently happens due to
1580  // phi translation.
1581  if (Value *IV =
1582  SimplifyInstruction(New, BB->getModule()->getDataLayout())) {
1583  delete New;
1584  ValueMapping[BI] = IV;
1585  } else {
1586  // Otherwise, insert the new instruction into the block.
1587  New->setName(BI->getName());
1588  PredBB->getInstList().insert(OldPredBranch, New);
1589  ValueMapping[BI] = New;
1590  }
1591  }
1592 
1593  // Check to see if the targets of the branch had PHI nodes. If so, we need to
1594  // add entries to the PHI nodes for branch from PredBB now.
1595  BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1596  AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1597  ValueMapping);
1598  AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1599  ValueMapping);
1600 
1601  // If there were values defined in BB that are used outside the block, then we
1602  // now have to update all uses of the value to use either the original value,
1603  // the cloned value, or some PHI derived value. This can require arbitrary
1604  // PHI insertion, of which we are prepared to do, clean these up now.
1605  SSAUpdater SSAUpdate;
1606  SmallVector<Use*, 16> UsesToRename;
1607  for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1608  // Scan all uses of this instruction to see if it is used outside of its
1609  // block, and if so, record them in UsesToRename.
1610  for (Use &U : I->uses()) {
1611  Instruction *User = cast<Instruction>(U.getUser());
1612  if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1613  if (UserPN->getIncomingBlock(U) == BB)
1614  continue;
1615  } else if (User->getParent() == BB)
1616  continue;
1617 
1618  UsesToRename.push_back(&U);
1619  }
1620 
1621  // If there are no uses outside the block, we're done with this instruction.
1622  if (UsesToRename.empty())
1623  continue;
1624 
1625  DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1626 
1627  // We found a use of I outside of BB. Rename all uses of I that are outside
1628  // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1629  // with the two values we know.
1630  SSAUpdate.Initialize(I->getType(), I->getName());
1631  SSAUpdate.AddAvailableValue(BB, I);
1632  SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1633 
1634  while (!UsesToRename.empty())
1635  SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1636  DEBUG(dbgs() << "\n");
1637  }
1638 
1639  // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1640  // that we nuked.
1641  BB->removePredecessor(PredBB, true);
1642 
1643  // Remove the unconditional branch at the end of the PredBB block.
1644  OldPredBranch->eraseFromParent();
1645 
1646  ++NumDupes;
1647  return true;
1648 }
1649 
1650 /// TryToUnfoldSelect - Look for blocks of the form
1651 /// bb1:
1652 /// %a = select
1653 /// br bb
1654 ///
1655 /// bb2:
1656 /// %p = phi [%a, %bb] ...
1657 /// %c = icmp %p
1658 /// br i1 %c
1659 ///
1660 /// And expand the select into a branch structure if one of its arms allows %c
1661 /// to be folded. This later enables threading from bb1 over bb2.
1662 bool JumpThreading::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
1663  BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
1664  PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0));
1665  Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1));
1666 
1667  if (!CondBr || !CondBr->isConditional() || !CondLHS ||
1668  CondLHS->getParent() != BB)
1669  return false;
1670 
1671  for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) {
1672  BasicBlock *Pred = CondLHS->getIncomingBlock(I);
1673  SelectInst *SI = dyn_cast<SelectInst>(CondLHS->getIncomingValue(I));
1674 
1675  // Look if one of the incoming values is a select in the corresponding
1676  // predecessor.
1677  if (!SI || SI->getParent() != Pred || !SI->hasOneUse())
1678  continue;
1679 
1680  BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
1681  if (!PredTerm || !PredTerm->isUnconditional())
1682  continue;
1683 
1684  // Now check if one of the select values would allow us to constant fold the
1685  // terminator in BB. We don't do the transform if both sides fold, those
1686  // cases will be threaded in any case.
1687  LazyValueInfo::Tristate LHSFolds =
1688  LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(1),
1689  CondRHS, Pred, BB, CondCmp);
1690  LazyValueInfo::Tristate RHSFolds =
1691  LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(2),
1692  CondRHS, Pred, BB, CondCmp);
1693  if ((LHSFolds != LazyValueInfo::Unknown ||
1694  RHSFolds != LazyValueInfo::Unknown) &&
1695  LHSFolds != RHSFolds) {
1696  // Expand the select.
1697  //
1698  // Pred --
1699  // | v
1700  // | NewBB
1701  // | |
1702  // |-----
1703  // v
1704  // BB
1705  BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold",
1706  BB->getParent(), BB);
1707  // Move the unconditional branch to NewBB.
1708  PredTerm->removeFromParent();
1709  NewBB->getInstList().insert(NewBB->end(), PredTerm);
1710  // Create a conditional branch and update PHI nodes.
1711  BranchInst::Create(NewBB, BB, SI->getCondition(), Pred);
1712  CondLHS->setIncomingValue(I, SI->getFalseValue());
1713  CondLHS->addIncoming(SI->getTrueValue(), NewBB);
1714  // The select is now dead.
1715  SI->eraseFromParent();
1716 
1717  // Update any other PHI nodes in BB.
1718  for (BasicBlock::iterator BI = BB->begin();
1719  PHINode *Phi = dyn_cast<PHINode>(BI); ++BI)
1720  if (Phi != CondLHS)
1721  Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB);
1722  return true;
1723  }
1724  }
1725  return false;
1726 }
iplist< Instruction >::iterator eraseFromParent()
eraseFromParent - This method unlinks 'this' from the containing basic block and deletes it...
Definition: Instruction.cpp:70
A parsed version of the target data layout string in and methods for querying it. ...
Definition: DataLayout.h:104
static ConstantInt * getFalse(LLVMContext &Context)
Definition: Constants.cpp:537
This class is the base class for the comparison instructions.
Definition: InstrTypes.h:679
AnalysisUsage & addPreserved()
Add the specified Pass class to the set of analyses preserved by this pass.
void removePredecessor(BasicBlock *Pred, bool DontDeleteUselessPHIs=false)
Notify the BasicBlock that the predecessor Pred is no longer able to reach it.
Definition: BasicBlock.cpp:266
static IntegerType * getInt1Ty(LLVMContext &C)
Definition: Type.cpp:236
Helper class for SSA formation on a set of values defined in multiple blocks.
Definition: SSAUpdater.h:38
void addIncoming(Value *V, BasicBlock *BB)
addIncoming - Add an incoming value to the end of the PHI list
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
STATISTIC(NumFunctions,"Total number of functions")
void Initialize(Type *Ty, StringRef Name)
Reset this object to get ready for a new set of SSA updates with type 'Ty'.
Definition: SSAUpdater.cpp:45
static CastInst * CreateBitOrPointerCast(Value *S, Type *Ty, const Twine &Name="", Instruction *InsertBefore=0)
Create a BitCast, a PtrToInt, or an IntToPTr cast instruction.
iterator end()
Definition: Function.h:459
void initializeJumpThreadingPass(PassRegistry &)
DenseSet - This implements a dense probed hash-table based set.
Definition: DenseSet.h:39
unsigned getNumOperands() const
Definition: User.h:138
void DeleteDeadBlock(BasicBlock *BB)
DeleteDeadBlock - Delete the specified block, which must have no predecessors.
void AddAvailableValue(BasicBlock *BB, Value *V)
Indicate that a rewritten value is available in the specified block with the specified value...
Definition: SSAUpdater.cpp:58
CallInst - This class represents a function call, abstracting a target machine's calling convention...
This file contains the declarations for metadata subclasses.
size_type count(PtrType Ptr) const
count - Return 1 if the specified pointer is in the set, 0 otherwise.
Definition: SmallPtrSet.h:276
void MergeBasicBlockIntoOnlyPred(BasicBlock *BB, DominatorTree *DT=nullptr)
MergeBasicBlockIntoOnlyPred - BB is a block with one predecessor and its predecessor is known to have...
Definition: Local.cpp:500
const Function * getParent() const
Return the enclosing method, or null if none.
Definition: BasicBlock.h:111
bool SimplifyInstructionsInBlock(BasicBlock *BB, const TargetLibraryInfo *TLI=nullptr)
SimplifyInstructionsInBlock - Scan the specified basic block and try to simplify any instructions in ...
Definition: Local.cpp:422
const Instruction & front() const
Definition: BasicBlock.h:243
F(f)
LoadInst - an instruction for reading from memory.
Definition: Instructions.h:177
static Constant * getCompare(unsigned short pred, Constant *C1, Constant *C2, bool OnlyIfReduced=false)
Return an ICmp or FCmp comparison operator constant expression.
Definition: Constants.cpp:1990
bool isSimple() const
Definition: Instructions.h:279
INITIALIZE_PASS_BEGIN(JumpThreading,"jump-threading","Jump Threading", false, false) INITIALIZE_PASS_END(JumpThreading
FunctionPass * createJumpThreadingPass(int Threshold=-1)
StringRef getName() const
Return a constant reference to the value's name.
Definition: Value.cpp:188
iterator begin()
Instruction iterator methods.
Definition: BasicBlock.h:231
Instruction * getFirstNonPHIOrDbg()
Returns a pointer to the first instruction in this block that is not a PHINode or a debug intrinsic...
Definition: BasicBlock.cpp:172
BlockAddress - The address of a basic block.
Definition: Constants.h:802
AnalysisUsage & addRequired()
#define INITIALIZE_PASS_DEPENDENCY(depName)
Definition: PassSupport.h:70
bool isUnconditional() const
void push_back(NodeTy *val)
Definition: ilist.h:554
static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB, BasicBlock *OldPred, BasicBlock *NewPred, DenseMap< Instruction *, Value * > &ValueMap)
AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new predecessor to the PHIBB block...
SelectInst - This class represents the LLVM 'select' instruction.
static cl::opt< unsigned > BBDuplicateThreshold("jump-threading-threshold", cl::desc("Max block size to duplicate for jump threading"), cl::init(6), cl::Hidden)
UndefValue - 'undef' values are things that do not have specified contents.
Definition: Constants.h:1220
const Module * getModule() const
Return the module owning the function this basic block belongs to, or nullptr it the function does no...
Definition: BasicBlock.cpp:116
T LLVM_ATTRIBUTE_UNUSED_RESULT pop_back_val()
Definition: SmallVector.h:406
A Use represents the edge between a Value definition and its users.
Definition: Use.h:69
bool ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions=false, const TargetLibraryInfo *TLI=nullptr)
ConstantFoldTerminator - If a terminator instruction is predicated on a constant value, convert it into an unconditional branch to the constant destination.
Definition: Local.cpp:64
#define INITIALIZE_PASS_END(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:75
This class consists of common code factored out of the SmallVector class to reduce code duplication b...
Definition: APInt.h:33
Instruction * getFirstNonPHI()
Returns a pointer to the first instruction in this block that is not a PHINode instruction.
Definition: BasicBlock.cpp:165
jump threading
bool hasAddressTaken() const
Returns true if there are any uses of this basic block other than direct branches, switches, etc.
Definition: BasicBlock.h:306
void setName(const Twine &Name)
Change the name of the value.
Definition: Value.cpp:250
static Constant * get(unsigned Opcode, Constant *C1, Constant *C2, unsigned Flags=0, Type *OnlyIfReducedTy=nullptr)
get - Return a binary or shift operator constant expression, folding if possible. ...
Definition: Constants.cpp:1868
bool LLVM_ATTRIBUTE_UNUSED_RESULT empty() const
Definition: SmallVector.h:57
BasicBlock * getSuccessor(unsigned i) const
void setSuccessor(unsigned idx, BasicBlock *B)
Update the specified successor to point at the provided block.
Definition: InstrTypes.h:67
void replaceAllUsesWith(Value *V)
Change all uses of this to point to a new Value.
Definition: Value.cpp:351
void takeName(Value *V)
Transfer the name from V to this value.
Definition: Value.cpp:256
iterator begin()
Definition: Function.h:457
static Constant * getKnownConstant(Value *Val, ConstantPreference Preference)
getKnownConstant - Helper method to determine if we can thread over a terminator with the given value...
unsigned getNumIncomingValues() const
getNumIncomingValues - Return the number of incoming edges
unsigned getNumSuccessors() const
Return the number of successors that this terminator has.
Definition: InstrTypes.h:57
#define P(N)
initializer< Ty > init(const Ty &Val)
Definition: CommandLine.h:325
void array_pod_sort(IteratorTy Start, IteratorTy End)
array_pod_sort - This sorts an array with the specified start and end extent.
Definition: STLExtras.h:287
Subclasses of this class are all able to terminate a basic block.
Definition: InstrTypes.h:35
void setDebugLoc(DebugLoc Loc)
setDebugLoc - Set the debug location information for this instruction.
Definition: Instruction.h:227
LLVM Basic Block Representation.
Definition: BasicBlock.h:65
size_type size() const
Definition: SmallPtrSet.h:79
BasicBlock * getSuccessor(unsigned idx) const
Return the specified successor.
Definition: InstrTypes.h:62
bool TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB)
TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an unconditional branch...
Definition: Local.cpp:750
BranchInst - Conditional or Unconditional Branch instruction.
static BlockAddress * get(Function *F, BasicBlock *BB)
get - Return a BlockAddress for the specified function and basic block.
Definition: Constants.cpp:1496
This is an important base class in LLVM.
Definition: Constant.h:41
const Value * getCondition() const
SmallSet - This maintains a set of unique values, optimizing for the case when the set is small (less...
Definition: SmallSet.h:32
IndirectBrInst - Indirect Branch Instruction.
APInt Or(const APInt &LHS, const APInt &RHS)
Bitwise OR function for APInt.
Definition: APInt.h:1895
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:264
APInt Xor(const APInt &LHS, const APInt &RHS)
Bitwise XOR function for APInt.
Definition: APInt.h:1900
Constant * ConstantFoldInstruction(Instruction *I, const DataLayout &DL, const TargetLibraryInfo *TLI=nullptr)
ConstantFoldInstruction - Try to constant fold the specified instruction.
Interval::pred_iterator pred_begin(Interval *I)
pred_begin/pred_end - define methods so that Intervals may be used just like BasicBlocks can with the...
Definition: Interval.h:114
jump Jump false
const DebugLoc & getDebugLoc() const
getDebugLoc - Return the debug location for this node as a DebugLoc.
Definition: Instruction.h:230
Represent the analysis usage information of a pass.
BasicBlock * getIncomingBlock(unsigned i) const
getIncomingBlock - Return incoming basic block number i.
const InstListType & getInstList() const
Return the underlying instruction list container.
Definition: BasicBlock.h:252
iterator insert(iterator where, NodeTy *New)
Definition: ilist.h:412
jump Jump Threading
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:294
Value * getOperand(unsigned i) const
Definition: User.h:118
Interval::pred_iterator pred_end(Interval *I)
Definition: Interval.h:117
static BasicBlock * Create(LLVMContext &Context, const Twine &Name="", Function *Parent=nullptr, BasicBlock *InsertBefore=nullptr)
Creates a new BasicBlock.
Definition: BasicBlock.h:103
Predicate getPredicate() const
Return the predicate for this instruction.
Definition: InstrTypes.h:760
bool pred_empty(const BasicBlock *BB)
Definition: IR/CFG.h:99
static Constant * getNot(Constant *C)
Definition: Constants.cpp:2252
static UndefValue * get(Type *T)
get() - Static factory methods - Return an 'undef' object of the specified type.
Definition: Constants.cpp:1473
LLVMContext & getContext() const
All values hold a context through their type.
Definition: Value.cpp:519
const Value * getTrueValue() const
static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB)
GetBestDestForBranchOnUndef - If we determine that the specified block ends in an undefined jump...
Tristate
This is used to return true/false/dunno results.
Definition: LazyValueInfo.h:45
bool isTerminator() const
Definition: Instruction.h:115
bool isConditional() const
void moveAfter(BasicBlock *MovePos)
Unlink this basic block from its current function and insert it right after MovePos in the function M...
Definition: BasicBlock.cpp:110
See the file comment.
Definition: ValueMap.h:80
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:299
void FindFunctionBackedges(const Function &F, SmallVectorImpl< std::pair< const BasicBlock *, const BasicBlock * > > &Result)
Analyze the specified function to find all of the loop backedges in the function and return them...
Definition: CFG.cpp:27
This is the shared class of boolean and integer constants.
Definition: Constants.h:47
Value * getIncomingValue(unsigned i) const
getIncomingValue - Return incoming value number x
Value * DoPHITranslation(const BasicBlock *CurBB, const BasicBlock *PredBB)
Translate PHI node to its predecessor from the given basic block.
Definition: Value.cpp:511
iterator end()
Definition: BasicBlock.h:233
const Module * getModule() const
Return the module owning the function this instruction belongs to or nullptr it the function does not...
Definition: Instruction.cpp:57
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:861
Type * getType() const
All values are typed, get the type of this value.
Definition: Value.h:222
Provides information about what library functions are available for the current target.
Value * SimplifyCmpInst(unsigned Predicate, Value *LHS, Value *RHS, const DataLayout &DL, const TargetLibraryInfo *TLI=nullptr, const DominatorTree *DT=nullptr, AssumptionCache *AC=nullptr, const Instruction *CxtI=nullptr)
SimplifyCmpInst - Given operands for a CmpInst, see if we can fold the result.
A collection of metadata nodes that might be associated with a memory access used by the alias-analys...
Definition: Metadata.h:548
Value * stripPointerCasts()
Strip off pointer casts, all-zero GEPs, and aliases.
Definition: Value.cpp:458
static Constant * get(Type *Ty, uint64_t V, bool isSigned=false)
If Ty is a vector type, return a Constant with a splat of the given value.
Definition: Constants.cpp:582
static BranchInst * Create(BasicBlock *IfTrue, Instruction *InsertBefore=nullptr)
bool isZero() const
This is just a convenience method to make client code smaller for a common code.
Definition: Constants.h:161
static PHINode * Create(Type *Ty, unsigned NumReservedValues, const Twine &NameStr="", Instruction *InsertBefore=nullptr)
Constructors - NumReservedValues is a hint for the number of incoming edges that this phi node will h...
const BasicBlock & getEntryBlock() const
Definition: Function.h:442
static ConstantInt * getTrue(LLVMContext &Context)
Definition: Constants.cpp:530
void setOperand(unsigned i, Value *Val)
Definition: User.h:122
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:123
static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB, unsigned Threshold)
getJumpThreadDuplicationCost - Return the cost of duplicating this block to thread across it...
Value * getIncomingValueForBlock(const BasicBlock *BB) const
BasicBlock * getSinglePredecessor()
Return the predecessor of this block if it has a single predecessor block.
Definition: BasicBlock.cpp:211
LLVM_ATTRIBUTE_UNUSED_RESULT std::enable_if< !is_simple_type< Y >::value, typename cast_retty< X, const Y >::ret_type >::type dyn_cast(const Y &Val)
Definition: Casting.h:285
ConstantPreference
APInt And(const APInt &LHS, const APInt &RHS)
Bitwise AND function for APInt.
Definition: APInt.h:1890
void removeFromParent()
removeFromParent - This method unlinks 'this' from the containing basic block, but does not delete it...
Definition: Instruction.cpp:66
static bool isZero(Value *V, const DataLayout &DL, DominatorTree *DT, AssumptionCache *AC)
Definition: Lint.cpp:697
const DataLayout & getDataLayout() const
Get the data layout for the module's target platform.
Definition: Module.cpp:372
Value * getCondition() const
iterator begin()
Definition: DenseMap.h:64
unsigned getAlignment() const
getAlignment - Return the alignment of the access that is being performed
Definition: Instructions.h:243
void getAAMetadata(AAMDNodes &N, bool Merge=false) const
getAAMetadata - Fills the AAMDNodes structure with AA metadata from this instruction.
#define I(x, y, z)
Definition: MD5.cpp:54
TerminatorInst * getTerminator()
Returns the terminator instruction if the block is well formed or null if the block is not well forme...
Definition: BasicBlock.cpp:124
bool isLandingPad() const
Return true if this basic block is a landing pad.
Definition: BasicBlock.cpp:413
bool hasOneUse() const
Return true if there is exactly one user of this value.
Definition: Value.h:311
iterator find(const KeyT &Val)
Definition: DenseMap.h:124
static int const Threshold
TODO: Write a new FunctionPass AliasAnalysis so that it can keep a cache.
SwitchInst - Multiway switch.
This pass computes, caches, and vends lazy value constraint information.
Definition: LazyValueInfo.h:30
bool use_empty() const
Definition: Value.h:275
static BasicBlock * FindMostPopularDest(BasicBlock *BB, const SmallVectorImpl< std::pair< BasicBlock *, BasicBlock * > > &PredToDestList)
FindMostPopularDest - The specified list contains multiple possible threadable destinations.
Value * FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, BasicBlock::iterator &ScanFrom, unsigned MaxInstsToScan=6, AliasAnalysis *AA=nullptr, AAMDNodes *AATags=nullptr)
FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the instruction before ScanFr...
Definition: Loads.cpp:183
LLVMContext & getContext() const
Get the context in which this basic block lives.
Definition: BasicBlock.cpp:32
void removeDeadConstantUsers() const
removeDeadConstantUsers - If there are any dead constant users dangling off of this constant...
Definition: Constants.cpp:487
unsigned getPrimitiveSizeInBits() const LLVM_READONLY
getPrimitiveSizeInBits - Return the basic size of this type if it is a primitive type.
Definition: Type.cpp:121
LLVM Value Representation.
Definition: Value.h:69
unsigned getOpcode() const
getOpcode() returns a member of one of the enums like Instruction::Add.
Definition: Instruction.h:112
static const Function * getParent(const Value *V)
#define DEBUG(X)
Definition: Debug.h:92
BasicBlock * SplitEdge(BasicBlock *From, BasicBlock *To, DominatorTree *DT=nullptr, LoopInfo *LI=nullptr)
SplitEdge - Split the edge connecting specified block.
const Value * getFalseValue() const
BasicBlock * SplitBlockPredecessors(BasicBlock *BB, ArrayRef< BasicBlock * > Preds, const char *Suffix, AliasAnalysis *AA=nullptr, DominatorTree *DT=nullptr, LoopInfo *LI=nullptr, bool PreserveLCSSA=false)
SplitBlockPredecessors - This method introduces at least one new basic block into the function and mo...
bool removeUnreachableBlocks(Function &F)
Remove all blocks that can not be reached from the function's entry.
Definition: Local.cpp:1254
Value * SimplifyInstruction(Instruction *I, const DataLayout &DL, const TargetLibraryInfo *TLI=nullptr, const DominatorTree *DT=nullptr, AssumptionCache *AC=nullptr)
SimplifyInstruction - See if we can compute a simplified version of this instruction.
void RewriteUse(Use &U)
Rewrite a use of the symbolic value.
Definition: SSAUpdater.cpp:178
static bool hasAddressTakenAndUsed(BasicBlock *BB)
const BasicBlock * getParent() const
Definition: Instruction.h:72
void resize(size_type N)
Definition: SmallVector.h:376