xref: /llvm-project/llvm/lib/Transforms/Utils/Local.cpp (revision 93abd7d915fb12c9967fe2433dfc873d3b4e938d)
1 //===-- Local.cpp - Functions to perform local transformations ------------===//
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 family of functions perform various local transformations to the
11 // program.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "llvm/Transforms/Utils/Local.h"
16 #include "llvm/ADT/DenseMap.h"
17 #include "llvm/ADT/DenseSet.h"
18 #include "llvm/ADT/Hashing.h"
19 #include "llvm/ADT/STLExtras.h"
20 #include "llvm/ADT/SetVector.h"
21 #include "llvm/ADT/SmallPtrSet.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/Analysis/EHPersonalities.h"
24 #include "llvm/Analysis/InstructionSimplify.h"
25 #include "llvm/Analysis/LazyValueInfo.h"
26 #include "llvm/Analysis/MemoryBuiltins.h"
27 #include "llvm/Analysis/ValueTracking.h"
28 #include "llvm/IR/CFG.h"
29 #include "llvm/IR/ConstantRange.h"
30 #include "llvm/IR/Constants.h"
31 #include "llvm/IR/DIBuilder.h"
32 #include "llvm/IR/DataLayout.h"
33 #include "llvm/IR/DebugInfo.h"
34 #include "llvm/IR/DerivedTypes.h"
35 #include "llvm/IR/Dominators.h"
36 #include "llvm/IR/GetElementPtrTypeIterator.h"
37 #include "llvm/IR/GlobalAlias.h"
38 #include "llvm/IR/GlobalVariable.h"
39 #include "llvm/IR/IRBuilder.h"
40 #include "llvm/IR/Instructions.h"
41 #include "llvm/IR/IntrinsicInst.h"
42 #include "llvm/IR/Intrinsics.h"
43 #include "llvm/IR/MDBuilder.h"
44 #include "llvm/IR/Metadata.h"
45 #include "llvm/IR/Operator.h"
46 #include "llvm/IR/PatternMatch.h"
47 #include "llvm/IR/ValueHandle.h"
48 #include "llvm/Support/Debug.h"
49 #include "llvm/Support/KnownBits.h"
50 #include "llvm/Support/MathExtras.h"
51 #include "llvm/Support/raw_ostream.h"
52 using namespace llvm;
53 using namespace llvm::PatternMatch;
54 
55 #define DEBUG_TYPE "local"
56 
57 STATISTIC(NumRemoved, "Number of unreachable basic blocks removed");
58 
59 //===----------------------------------------------------------------------===//
60 //  Local constant propagation.
61 //
62 
63 /// ConstantFoldTerminator - If a terminator instruction is predicated on a
64 /// constant value, convert it into an unconditional branch to the constant
65 /// destination.  This is a nontrivial operation because the successors of this
66 /// basic block must have their PHI nodes updated.
67 /// Also calls RecursivelyDeleteTriviallyDeadInstructions() on any branch/switch
68 /// conditions and indirectbr addresses this might make dead if
69 /// DeleteDeadConditions is true.
70 bool llvm::ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions,
71                                   const TargetLibraryInfo *TLI) {
72   TerminatorInst *T = BB->getTerminator();
73   IRBuilder<> Builder(T);
74 
75   // Branch - See if we are conditional jumping on constant
76   if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
77     if (BI->isUnconditional()) return false;  // Can't optimize uncond branch
78     BasicBlock *Dest1 = BI->getSuccessor(0);
79     BasicBlock *Dest2 = BI->getSuccessor(1);
80 
81     if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
82       // Are we branching on constant?
83       // YES.  Change to unconditional branch...
84       BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
85       BasicBlock *OldDest     = Cond->getZExtValue() ? Dest2 : Dest1;
86 
87       //cerr << "Function: " << T->getParent()->getParent()
88       //     << "\nRemoving branch from " << T->getParent()
89       //     << "\n\nTo: " << OldDest << endl;
90 
91       // Let the basic block know that we are letting go of it.  Based on this,
92       // it will adjust it's PHI nodes.
93       OldDest->removePredecessor(BB);
94 
95       // Replace the conditional branch with an unconditional one.
96       Builder.CreateBr(Destination);
97       BI->eraseFromParent();
98       return true;
99     }
100 
101     if (Dest2 == Dest1) {       // Conditional branch to same location?
102       // This branch matches something like this:
103       //     br bool %cond, label %Dest, label %Dest
104       // and changes it into:  br label %Dest
105 
106       // Let the basic block know that we are letting go of one copy of it.
107       assert(BI->getParent() && "Terminator not inserted in block!");
108       Dest1->removePredecessor(BI->getParent());
109 
110       // Replace the conditional branch with an unconditional one.
111       Builder.CreateBr(Dest1);
112       Value *Cond = BI->getCondition();
113       BI->eraseFromParent();
114       if (DeleteDeadConditions)
115         RecursivelyDeleteTriviallyDeadInstructions(Cond, TLI);
116       return true;
117     }
118     return false;
119   }
120 
121   if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
122     // If we are switching on a constant, we can convert the switch to an
123     // unconditional branch.
124     ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
125     BasicBlock *DefaultDest = SI->getDefaultDest();
126     BasicBlock *TheOnlyDest = DefaultDest;
127 
128     // If the default is unreachable, ignore it when searching for TheOnlyDest.
129     if (isa<UnreachableInst>(DefaultDest->getFirstNonPHIOrDbg()) &&
130         SI->getNumCases() > 0) {
131       TheOnlyDest = SI->case_begin()->getCaseSuccessor();
132     }
133 
134     // Figure out which case it goes to.
135     for (auto i = SI->case_begin(), e = SI->case_end(); i != e;) {
136       // Found case matching a constant operand?
137       if (i->getCaseValue() == CI) {
138         TheOnlyDest = i->getCaseSuccessor();
139         break;
140       }
141 
142       // Check to see if this branch is going to the same place as the default
143       // dest.  If so, eliminate it as an explicit compare.
144       if (i->getCaseSuccessor() == DefaultDest) {
145         MDNode *MD = SI->getMetadata(LLVMContext::MD_prof);
146         unsigned NCases = SI->getNumCases();
147         // Fold the case metadata into the default if there will be any branches
148         // left, unless the metadata doesn't match the switch.
149         if (NCases > 1 && MD && MD->getNumOperands() == 2 + NCases) {
150           // Collect branch weights into a vector.
151           SmallVector<uint32_t, 8> Weights;
152           for (unsigned MD_i = 1, MD_e = MD->getNumOperands(); MD_i < MD_e;
153                ++MD_i) {
154             auto *CI = mdconst::extract<ConstantInt>(MD->getOperand(MD_i));
155             Weights.push_back(CI->getValue().getZExtValue());
156           }
157           // Merge weight of this case to the default weight.
158           unsigned idx = i->getCaseIndex();
159           Weights[0] += Weights[idx+1];
160           // Remove weight for this case.
161           std::swap(Weights[idx+1], Weights.back());
162           Weights.pop_back();
163           SI->setMetadata(LLVMContext::MD_prof,
164                           MDBuilder(BB->getContext()).
165                           createBranchWeights(Weights));
166         }
167         // Remove this entry.
168         DefaultDest->removePredecessor(SI->getParent());
169         i = SI->removeCase(i);
170         e = SI->case_end();
171         continue;
172       }
173 
174       // Otherwise, check to see if the switch only branches to one destination.
175       // We do this by reseting "TheOnlyDest" to null when we find two non-equal
176       // destinations.
177       if (i->getCaseSuccessor() != TheOnlyDest)
178         TheOnlyDest = nullptr;
179 
180       // Increment this iterator as we haven't removed the case.
181       ++i;
182     }
183 
184     if (CI && !TheOnlyDest) {
185       // Branching on a constant, but not any of the cases, go to the default
186       // successor.
187       TheOnlyDest = SI->getDefaultDest();
188     }
189 
190     // If we found a single destination that we can fold the switch into, do so
191     // now.
192     if (TheOnlyDest) {
193       // Insert the new branch.
194       Builder.CreateBr(TheOnlyDest);
195       BasicBlock *BB = SI->getParent();
196 
197       // Remove entries from PHI nodes which we no longer branch to...
198       for (BasicBlock *Succ : SI->successors()) {
199         // Found case matching a constant operand?
200         if (Succ == TheOnlyDest)
201           TheOnlyDest = nullptr; // Don't modify the first branch to TheOnlyDest
202         else
203           Succ->removePredecessor(BB);
204       }
205 
206       // Delete the old switch.
207       Value *Cond = SI->getCondition();
208       SI->eraseFromParent();
209       if (DeleteDeadConditions)
210         RecursivelyDeleteTriviallyDeadInstructions(Cond, TLI);
211       return true;
212     }
213 
214     if (SI->getNumCases() == 1) {
215       // Otherwise, we can fold this switch into a conditional branch
216       // instruction if it has only one non-default destination.
217       auto FirstCase = *SI->case_begin();
218       Value *Cond = Builder.CreateICmpEQ(SI->getCondition(),
219           FirstCase.getCaseValue(), "cond");
220 
221       // Insert the new branch.
222       BranchInst *NewBr = Builder.CreateCondBr(Cond,
223                                                FirstCase.getCaseSuccessor(),
224                                                SI->getDefaultDest());
225       MDNode *MD = SI->getMetadata(LLVMContext::MD_prof);
226       if (MD && MD->getNumOperands() == 3) {
227         ConstantInt *SICase =
228             mdconst::dyn_extract<ConstantInt>(MD->getOperand(2));
229         ConstantInt *SIDef =
230             mdconst::dyn_extract<ConstantInt>(MD->getOperand(1));
231         assert(SICase && SIDef);
232         // The TrueWeight should be the weight for the single case of SI.
233         NewBr->setMetadata(LLVMContext::MD_prof,
234                         MDBuilder(BB->getContext()).
235                         createBranchWeights(SICase->getValue().getZExtValue(),
236                                             SIDef->getValue().getZExtValue()));
237       }
238 
239       // Update make.implicit metadata to the newly-created conditional branch.
240       MDNode *MakeImplicitMD = SI->getMetadata(LLVMContext::MD_make_implicit);
241       if (MakeImplicitMD)
242         NewBr->setMetadata(LLVMContext::MD_make_implicit, MakeImplicitMD);
243 
244       // Delete the old switch.
245       SI->eraseFromParent();
246       return true;
247     }
248     return false;
249   }
250 
251   if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(T)) {
252     // indirectbr blockaddress(@F, @BB) -> br label @BB
253     if (BlockAddress *BA =
254           dyn_cast<BlockAddress>(IBI->getAddress()->stripPointerCasts())) {
255       BasicBlock *TheOnlyDest = BA->getBasicBlock();
256       // Insert the new branch.
257       Builder.CreateBr(TheOnlyDest);
258 
259       for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
260         if (IBI->getDestination(i) == TheOnlyDest)
261           TheOnlyDest = nullptr;
262         else
263           IBI->getDestination(i)->removePredecessor(IBI->getParent());
264       }
265       Value *Address = IBI->getAddress();
266       IBI->eraseFromParent();
267       if (DeleteDeadConditions)
268         RecursivelyDeleteTriviallyDeadInstructions(Address, TLI);
269 
270       // If we didn't find our destination in the IBI successor list, then we
271       // have undefined behavior.  Replace the unconditional branch with an
272       // 'unreachable' instruction.
273       if (TheOnlyDest) {
274         BB->getTerminator()->eraseFromParent();
275         new UnreachableInst(BB->getContext(), BB);
276       }
277 
278       return true;
279     }
280   }
281 
282   return false;
283 }
284 
285 
286 //===----------------------------------------------------------------------===//
287 //  Local dead code elimination.
288 //
289 
290 /// isInstructionTriviallyDead - Return true if the result produced by the
291 /// instruction is not used, and the instruction has no side effects.
292 ///
293 bool llvm::isInstructionTriviallyDead(Instruction *I,
294                                       const TargetLibraryInfo *TLI) {
295   if (!I->use_empty())
296     return false;
297   return wouldInstructionBeTriviallyDead(I, TLI);
298 }
299 
300 bool llvm::wouldInstructionBeTriviallyDead(Instruction *I,
301                                            const TargetLibraryInfo *TLI) {
302   if (isa<TerminatorInst>(I))
303     return false;
304 
305   // We don't want the landingpad-like instructions removed by anything this
306   // general.
307   if (I->isEHPad())
308     return false;
309 
310   // We don't want debug info removed by anything this general, unless
311   // debug info is empty.
312   if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(I)) {
313     if (DDI->getAddress())
314       return false;
315     return true;
316   }
317   if (DbgValueInst *DVI = dyn_cast<DbgValueInst>(I)) {
318     if (DVI->getValue())
319       return false;
320     return true;
321   }
322 
323   if (!I->mayHaveSideEffects())
324     return true;
325 
326   // Special case intrinsics that "may have side effects" but can be deleted
327   // when dead.
328   if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
329     // Safe to delete llvm.stacksave if dead.
330     if (II->getIntrinsicID() == Intrinsic::stacksave)
331       return true;
332 
333     // Lifetime intrinsics are dead when their right-hand is undef.
334     if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
335         II->getIntrinsicID() == Intrinsic::lifetime_end)
336       return isa<UndefValue>(II->getArgOperand(1));
337 
338     // Assumptions are dead if their condition is trivially true.  Guards on
339     // true are operationally no-ops.  In the future we can consider more
340     // sophisticated tradeoffs for guards considering potential for check
341     // widening, but for now we keep things simple.
342     if (II->getIntrinsicID() == Intrinsic::assume ||
343         II->getIntrinsicID() == Intrinsic::experimental_guard) {
344       if (ConstantInt *Cond = dyn_cast<ConstantInt>(II->getArgOperand(0)))
345         return !Cond->isZero();
346 
347       return false;
348     }
349   }
350 
351   if (isAllocLikeFn(I, TLI))
352     return true;
353 
354   if (CallInst *CI = isFreeCall(I, TLI))
355     if (Constant *C = dyn_cast<Constant>(CI->getArgOperand(0)))
356       return C->isNullValue() || isa<UndefValue>(C);
357 
358   if (CallSite CS = CallSite(I))
359     if (isMathLibCallNoop(CS, TLI))
360       return true;
361 
362   return false;
363 }
364 
365 /// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a
366 /// trivially dead instruction, delete it.  If that makes any of its operands
367 /// trivially dead, delete them too, recursively.  Return true if any
368 /// instructions were deleted.
369 bool
370 llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V,
371                                                  const TargetLibraryInfo *TLI) {
372   Instruction *I = dyn_cast<Instruction>(V);
373   if (!I || !I->use_empty() || !isInstructionTriviallyDead(I, TLI))
374     return false;
375 
376   SmallVector<Instruction*, 16> DeadInsts;
377   DeadInsts.push_back(I);
378 
379   do {
380     I = DeadInsts.pop_back_val();
381 
382     // Null out all of the instruction's operands to see if any operand becomes
383     // dead as we go.
384     for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
385       Value *OpV = I->getOperand(i);
386       I->setOperand(i, nullptr);
387 
388       if (!OpV->use_empty()) continue;
389 
390       // If the operand is an instruction that became dead as we nulled out the
391       // operand, and if it is 'trivially' dead, delete it in a future loop
392       // iteration.
393       if (Instruction *OpI = dyn_cast<Instruction>(OpV))
394         if (isInstructionTriviallyDead(OpI, TLI))
395           DeadInsts.push_back(OpI);
396     }
397 
398     I->eraseFromParent();
399   } while (!DeadInsts.empty());
400 
401   return true;
402 }
403 
404 /// areAllUsesEqual - Check whether the uses of a value are all the same.
405 /// This is similar to Instruction::hasOneUse() except this will also return
406 /// true when there are no uses or multiple uses that all refer to the same
407 /// value.
408 static bool areAllUsesEqual(Instruction *I) {
409   Value::user_iterator UI = I->user_begin();
410   Value::user_iterator UE = I->user_end();
411   if (UI == UE)
412     return true;
413 
414   User *TheUse = *UI;
415   for (++UI; UI != UE; ++UI) {
416     if (*UI != TheUse)
417       return false;
418   }
419   return true;
420 }
421 
422 /// RecursivelyDeleteDeadPHINode - If the specified value is an effectively
423 /// dead PHI node, due to being a def-use chain of single-use nodes that
424 /// either forms a cycle or is terminated by a trivially dead instruction,
425 /// delete it.  If that makes any of its operands trivially dead, delete them
426 /// too, recursively.  Return true if a change was made.
427 bool llvm::RecursivelyDeleteDeadPHINode(PHINode *PN,
428                                         const TargetLibraryInfo *TLI) {
429   SmallPtrSet<Instruction*, 4> Visited;
430   for (Instruction *I = PN; areAllUsesEqual(I) && !I->mayHaveSideEffects();
431        I = cast<Instruction>(*I->user_begin())) {
432     if (I->use_empty())
433       return RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
434 
435     // If we find an instruction more than once, we're on a cycle that
436     // won't prove fruitful.
437     if (!Visited.insert(I).second) {
438       // Break the cycle and delete the instruction and its operands.
439       I->replaceAllUsesWith(UndefValue::get(I->getType()));
440       (void)RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
441       return true;
442     }
443   }
444   return false;
445 }
446 
447 static bool
448 simplifyAndDCEInstruction(Instruction *I,
449                           SmallSetVector<Instruction *, 16> &WorkList,
450                           const DataLayout &DL,
451                           const TargetLibraryInfo *TLI) {
452   if (isInstructionTriviallyDead(I, TLI)) {
453     // Null out all of the instruction's operands to see if any operand becomes
454     // dead as we go.
455     for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
456       Value *OpV = I->getOperand(i);
457       I->setOperand(i, nullptr);
458 
459       if (!OpV->use_empty() || I == OpV)
460         continue;
461 
462       // If the operand is an instruction that became dead as we nulled out the
463       // operand, and if it is 'trivially' dead, delete it in a future loop
464       // iteration.
465       if (Instruction *OpI = dyn_cast<Instruction>(OpV))
466         if (isInstructionTriviallyDead(OpI, TLI))
467           WorkList.insert(OpI);
468     }
469 
470     I->eraseFromParent();
471 
472     return true;
473   }
474 
475   if (Value *SimpleV = SimplifyInstruction(I, DL)) {
476     // Add the users to the worklist. CAREFUL: an instruction can use itself,
477     // in the case of a phi node.
478     for (User *U : I->users()) {
479       if (U != I) {
480         WorkList.insert(cast<Instruction>(U));
481       }
482     }
483 
484     // Replace the instruction with its simplified value.
485     bool Changed = false;
486     if (!I->use_empty()) {
487       I->replaceAllUsesWith(SimpleV);
488       Changed = true;
489     }
490     if (isInstructionTriviallyDead(I, TLI)) {
491       I->eraseFromParent();
492       Changed = true;
493     }
494     return Changed;
495   }
496   return false;
497 }
498 
499 /// SimplifyInstructionsInBlock - Scan the specified basic block and try to
500 /// simplify any instructions in it and recursively delete dead instructions.
501 ///
502 /// This returns true if it changed the code, note that it can delete
503 /// instructions in other blocks as well in this block.
504 bool llvm::SimplifyInstructionsInBlock(BasicBlock *BB,
505                                        const TargetLibraryInfo *TLI) {
506   bool MadeChange = false;
507   const DataLayout &DL = BB->getModule()->getDataLayout();
508 
509 #ifndef NDEBUG
510   // In debug builds, ensure that the terminator of the block is never replaced
511   // or deleted by these simplifications. The idea of simplification is that it
512   // cannot introduce new instructions, and there is no way to replace the
513   // terminator of a block without introducing a new instruction.
514   AssertingVH<Instruction> TerminatorVH(&BB->back());
515 #endif
516 
517   SmallSetVector<Instruction *, 16> WorkList;
518   // Iterate over the original function, only adding insts to the worklist
519   // if they actually need to be revisited. This avoids having to pre-init
520   // the worklist with the entire function's worth of instructions.
521   for (BasicBlock::iterator BI = BB->begin(), E = std::prev(BB->end());
522        BI != E;) {
523     assert(!BI->isTerminator());
524     Instruction *I = &*BI;
525     ++BI;
526 
527     // We're visiting this instruction now, so make sure it's not in the
528     // worklist from an earlier visit.
529     if (!WorkList.count(I))
530       MadeChange |= simplifyAndDCEInstruction(I, WorkList, DL, TLI);
531   }
532 
533   while (!WorkList.empty()) {
534     Instruction *I = WorkList.pop_back_val();
535     MadeChange |= simplifyAndDCEInstruction(I, WorkList, DL, TLI);
536   }
537   return MadeChange;
538 }
539 
540 //===----------------------------------------------------------------------===//
541 //  Control Flow Graph Restructuring.
542 //
543 
544 
545 /// RemovePredecessorAndSimplify - Like BasicBlock::removePredecessor, this
546 /// method is called when we're about to delete Pred as a predecessor of BB.  If
547 /// BB contains any PHI nodes, this drops the entries in the PHI nodes for Pred.
548 ///
549 /// Unlike the removePredecessor method, this attempts to simplify uses of PHI
550 /// nodes that collapse into identity values.  For example, if we have:
551 ///   x = phi(1, 0, 0, 0)
552 ///   y = and x, z
553 ///
554 /// .. and delete the predecessor corresponding to the '1', this will attempt to
555 /// recursively fold the and to 0.
556 void llvm::RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred) {
557   // This only adjusts blocks with PHI nodes.
558   if (!isa<PHINode>(BB->begin()))
559     return;
560 
561   // Remove the entries for Pred from the PHI nodes in BB, but do not simplify
562   // them down.  This will leave us with single entry phi nodes and other phis
563   // that can be removed.
564   BB->removePredecessor(Pred, true);
565 
566   WeakTrackingVH PhiIt = &BB->front();
567   while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) {
568     PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt));
569     Value *OldPhiIt = PhiIt;
570 
571     if (!recursivelySimplifyInstruction(PN))
572       continue;
573 
574     // If recursive simplification ended up deleting the next PHI node we would
575     // iterate to, then our iterator is invalid, restart scanning from the top
576     // of the block.
577     if (PhiIt != OldPhiIt) PhiIt = &BB->front();
578   }
579 }
580 
581 
582 /// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
583 /// predecessor is known to have one successor (DestBB!).  Eliminate the edge
584 /// between them, moving the instructions in the predecessor into DestBB and
585 /// deleting the predecessor block.
586 ///
587 void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, DominatorTree *DT) {
588   // If BB has single-entry PHI nodes, fold them.
589   while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
590     Value *NewVal = PN->getIncomingValue(0);
591     // Replace self referencing PHI with undef, it must be dead.
592     if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
593     PN->replaceAllUsesWith(NewVal);
594     PN->eraseFromParent();
595   }
596 
597   BasicBlock *PredBB = DestBB->getSinglePredecessor();
598   assert(PredBB && "Block doesn't have a single predecessor!");
599 
600   // Zap anything that took the address of DestBB.  Not doing this will give the
601   // address an invalid value.
602   if (DestBB->hasAddressTaken()) {
603     BlockAddress *BA = BlockAddress::get(DestBB);
604     Constant *Replacement =
605       ConstantInt::get(llvm::Type::getInt32Ty(BA->getContext()), 1);
606     BA->replaceAllUsesWith(ConstantExpr::getIntToPtr(Replacement,
607                                                      BA->getType()));
608     BA->destroyConstant();
609   }
610 
611   // Anything that branched to PredBB now branches to DestBB.
612   PredBB->replaceAllUsesWith(DestBB);
613 
614   // Splice all the instructions from PredBB to DestBB.
615   PredBB->getTerminator()->eraseFromParent();
616   DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
617 
618   // If the PredBB is the entry block of the function, move DestBB up to
619   // become the entry block after we erase PredBB.
620   if (PredBB == &DestBB->getParent()->getEntryBlock())
621     DestBB->moveAfter(PredBB);
622 
623   if (DT) {
624     BasicBlock *PredBBIDom = DT->getNode(PredBB)->getIDom()->getBlock();
625     DT->changeImmediateDominator(DestBB, PredBBIDom);
626     DT->eraseNode(PredBB);
627   }
628   // Nuke BB.
629   PredBB->eraseFromParent();
630 }
631 
632 /// CanMergeValues - Return true if we can choose one of these values to use
633 /// in place of the other. Note that we will always choose the non-undef
634 /// value to keep.
635 static bool CanMergeValues(Value *First, Value *Second) {
636   return First == Second || isa<UndefValue>(First) || isa<UndefValue>(Second);
637 }
638 
639 /// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
640 /// almost-empty BB ending in an unconditional branch to Succ, into Succ.
641 ///
642 /// Assumption: Succ is the single successor for BB.
643 ///
644 static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
645   assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
646 
647   DEBUG(dbgs() << "Looking to fold " << BB->getName() << " into "
648         << Succ->getName() << "\n");
649   // Shortcut, if there is only a single predecessor it must be BB and merging
650   // is always safe
651   if (Succ->getSinglePredecessor()) return true;
652 
653   // Make a list of the predecessors of BB
654   SmallPtrSet<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
655 
656   // Look at all the phi nodes in Succ, to see if they present a conflict when
657   // merging these blocks
658   for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
659     PHINode *PN = cast<PHINode>(I);
660 
661     // If the incoming value from BB is again a PHINode in
662     // BB which has the same incoming value for *PI as PN does, we can
663     // merge the phi nodes and then the blocks can still be merged
664     PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB));
665     if (BBPN && BBPN->getParent() == BB) {
666       for (unsigned PI = 0, PE = PN->getNumIncomingValues(); PI != PE; ++PI) {
667         BasicBlock *IBB = PN->getIncomingBlock(PI);
668         if (BBPreds.count(IBB) &&
669             !CanMergeValues(BBPN->getIncomingValueForBlock(IBB),
670                             PN->getIncomingValue(PI))) {
671           DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in "
672                 << Succ->getName() << " is conflicting with "
673                 << BBPN->getName() << " with regard to common predecessor "
674                 << IBB->getName() << "\n");
675           return false;
676         }
677       }
678     } else {
679       Value* Val = PN->getIncomingValueForBlock(BB);
680       for (unsigned PI = 0, PE = PN->getNumIncomingValues(); PI != PE; ++PI) {
681         // See if the incoming value for the common predecessor is equal to the
682         // one for BB, in which case this phi node will not prevent the merging
683         // of the block.
684         BasicBlock *IBB = PN->getIncomingBlock(PI);
685         if (BBPreds.count(IBB) &&
686             !CanMergeValues(Val, PN->getIncomingValue(PI))) {
687           DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in "
688                 << Succ->getName() << " is conflicting with regard to common "
689                 << "predecessor " << IBB->getName() << "\n");
690           return false;
691         }
692       }
693     }
694   }
695 
696   return true;
697 }
698 
699 typedef SmallVector<BasicBlock *, 16> PredBlockVector;
700 typedef DenseMap<BasicBlock *, Value *> IncomingValueMap;
701 
702 /// \brief Determines the value to use as the phi node input for a block.
703 ///
704 /// Select between \p OldVal any value that we know flows from \p BB
705 /// to a particular phi on the basis of which one (if either) is not
706 /// undef. Update IncomingValues based on the selected value.
707 ///
708 /// \param OldVal The value we are considering selecting.
709 /// \param BB The block that the value flows in from.
710 /// \param IncomingValues A map from block-to-value for other phi inputs
711 /// that we have examined.
712 ///
713 /// \returns the selected value.
714 static Value *selectIncomingValueForBlock(Value *OldVal, BasicBlock *BB,
715                                           IncomingValueMap &IncomingValues) {
716   if (!isa<UndefValue>(OldVal)) {
717     assert((!IncomingValues.count(BB) ||
718             IncomingValues.find(BB)->second == OldVal) &&
719            "Expected OldVal to match incoming value from BB!");
720 
721     IncomingValues.insert(std::make_pair(BB, OldVal));
722     return OldVal;
723   }
724 
725   IncomingValueMap::const_iterator It = IncomingValues.find(BB);
726   if (It != IncomingValues.end()) return It->second;
727 
728   return OldVal;
729 }
730 
731 /// \brief Create a map from block to value for the operands of a
732 /// given phi.
733 ///
734 /// Create a map from block to value for each non-undef value flowing
735 /// into \p PN.
736 ///
737 /// \param PN The phi we are collecting the map for.
738 /// \param IncomingValues [out] The map from block to value for this phi.
739 static void gatherIncomingValuesToPhi(PHINode *PN,
740                                       IncomingValueMap &IncomingValues) {
741   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
742     BasicBlock *BB = PN->getIncomingBlock(i);
743     Value *V = PN->getIncomingValue(i);
744 
745     if (!isa<UndefValue>(V))
746       IncomingValues.insert(std::make_pair(BB, V));
747   }
748 }
749 
750 /// \brief Replace the incoming undef values to a phi with the values
751 /// from a block-to-value map.
752 ///
753 /// \param PN The phi we are replacing the undefs in.
754 /// \param IncomingValues A map from block to value.
755 static void replaceUndefValuesInPhi(PHINode *PN,
756                                     const IncomingValueMap &IncomingValues) {
757   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
758     Value *V = PN->getIncomingValue(i);
759 
760     if (!isa<UndefValue>(V)) continue;
761 
762     BasicBlock *BB = PN->getIncomingBlock(i);
763     IncomingValueMap::const_iterator It = IncomingValues.find(BB);
764     if (It == IncomingValues.end()) continue;
765 
766     PN->setIncomingValue(i, It->second);
767   }
768 }
769 
770 /// \brief Replace a value flowing from a block to a phi with
771 /// potentially multiple instances of that value flowing from the
772 /// block's predecessors to the phi.
773 ///
774 /// \param BB The block with the value flowing into the phi.
775 /// \param BBPreds The predecessors of BB.
776 /// \param PN The phi that we are updating.
777 static void redirectValuesFromPredecessorsToPhi(BasicBlock *BB,
778                                                 const PredBlockVector &BBPreds,
779                                                 PHINode *PN) {
780   Value *OldVal = PN->removeIncomingValue(BB, false);
781   assert(OldVal && "No entry in PHI for Pred BB!");
782 
783   IncomingValueMap IncomingValues;
784 
785   // We are merging two blocks - BB, and the block containing PN - and
786   // as a result we need to redirect edges from the predecessors of BB
787   // to go to the block containing PN, and update PN
788   // accordingly. Since we allow merging blocks in the case where the
789   // predecessor and successor blocks both share some predecessors,
790   // and where some of those common predecessors might have undef
791   // values flowing into PN, we want to rewrite those values to be
792   // consistent with the non-undef values.
793 
794   gatherIncomingValuesToPhi(PN, IncomingValues);
795 
796   // If this incoming value is one of the PHI nodes in BB, the new entries
797   // in the PHI node are the entries from the old PHI.
798   if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
799     PHINode *OldValPN = cast<PHINode>(OldVal);
800     for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i) {
801       // Note that, since we are merging phi nodes and BB and Succ might
802       // have common predecessors, we could end up with a phi node with
803       // identical incoming branches. This will be cleaned up later (and
804       // will trigger asserts if we try to clean it up now, without also
805       // simplifying the corresponding conditional branch).
806       BasicBlock *PredBB = OldValPN->getIncomingBlock(i);
807       Value *PredVal = OldValPN->getIncomingValue(i);
808       Value *Selected = selectIncomingValueForBlock(PredVal, PredBB,
809                                                     IncomingValues);
810 
811       // And add a new incoming value for this predecessor for the
812       // newly retargeted branch.
813       PN->addIncoming(Selected, PredBB);
814     }
815   } else {
816     for (unsigned i = 0, e = BBPreds.size(); i != e; ++i) {
817       // Update existing incoming values in PN for this
818       // predecessor of BB.
819       BasicBlock *PredBB = BBPreds[i];
820       Value *Selected = selectIncomingValueForBlock(OldVal, PredBB,
821                                                     IncomingValues);
822 
823       // And add a new incoming value for this predecessor for the
824       // newly retargeted branch.
825       PN->addIncoming(Selected, PredBB);
826     }
827   }
828 
829   replaceUndefValuesInPhi(PN, IncomingValues);
830 }
831 
832 /// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an
833 /// unconditional branch, and contains no instructions other than PHI nodes,
834 /// potential side-effect free intrinsics and the branch.  If possible,
835 /// eliminate BB by rewriting all the predecessors to branch to the successor
836 /// block and return true.  If we can't transform, return false.
837 bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) {
838   assert(BB != &BB->getParent()->getEntryBlock() &&
839          "TryToSimplifyUncondBranchFromEmptyBlock called on entry block!");
840 
841   // We can't eliminate infinite loops.
842   BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0);
843   if (BB == Succ) return false;
844 
845   // Check to see if merging these blocks would cause conflicts for any of the
846   // phi nodes in BB or Succ. If not, we can safely merge.
847   if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
848 
849   // Check for cases where Succ has multiple predecessors and a PHI node in BB
850   // has uses which will not disappear when the PHI nodes are merged.  It is
851   // possible to handle such cases, but difficult: it requires checking whether
852   // BB dominates Succ, which is non-trivial to calculate in the case where
853   // Succ has multiple predecessors.  Also, it requires checking whether
854   // constructing the necessary self-referential PHI node doesn't introduce any
855   // conflicts; this isn't too difficult, but the previous code for doing this
856   // was incorrect.
857   //
858   // Note that if this check finds a live use, BB dominates Succ, so BB is
859   // something like a loop pre-header (or rarely, a part of an irreducible CFG);
860   // folding the branch isn't profitable in that case anyway.
861   if (!Succ->getSinglePredecessor()) {
862     BasicBlock::iterator BBI = BB->begin();
863     while (isa<PHINode>(*BBI)) {
864       for (Use &U : BBI->uses()) {
865         if (PHINode* PN = dyn_cast<PHINode>(U.getUser())) {
866           if (PN->getIncomingBlock(U) != BB)
867             return false;
868         } else {
869           return false;
870         }
871       }
872       ++BBI;
873     }
874   }
875 
876   DEBUG(dbgs() << "Killing Trivial BB: \n" << *BB);
877 
878   if (isa<PHINode>(Succ->begin())) {
879     // If there is more than one pred of succ, and there are PHI nodes in
880     // the successor, then we need to add incoming edges for the PHI nodes
881     //
882     const PredBlockVector BBPreds(pred_begin(BB), pred_end(BB));
883 
884     // Loop over all of the PHI nodes in the successor of BB.
885     for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
886       PHINode *PN = cast<PHINode>(I);
887 
888       redirectValuesFromPredecessorsToPhi(BB, BBPreds, PN);
889     }
890   }
891 
892   if (Succ->getSinglePredecessor()) {
893     // BB is the only predecessor of Succ, so Succ will end up with exactly
894     // the same predecessors BB had.
895 
896     // Copy over any phi, debug or lifetime instruction.
897     BB->getTerminator()->eraseFromParent();
898     Succ->getInstList().splice(Succ->getFirstNonPHI()->getIterator(),
899                                BB->getInstList());
900   } else {
901     while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
902       // We explicitly check for such uses in CanPropagatePredecessorsForPHIs.
903       assert(PN->use_empty() && "There shouldn't be any uses here!");
904       PN->eraseFromParent();
905     }
906   }
907 
908   // If the unconditional branch we replaced contains llvm.loop metadata, we
909   // add the metadata to the branch instructions in the predecessors.
910   unsigned LoopMDKind = BB->getContext().getMDKindID("llvm.loop");
911   Instruction *TI = BB->getTerminator();
912   if (TI)
913     if (MDNode *LoopMD = TI->getMetadata(LoopMDKind))
914       for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
915         BasicBlock *Pred = *PI;
916         Pred->getTerminator()->setMetadata(LoopMDKind, LoopMD);
917       }
918 
919   // Everything that jumped to BB now goes to Succ.
920   BB->replaceAllUsesWith(Succ);
921   if (!Succ->hasName()) Succ->takeName(BB);
922   BB->eraseFromParent();              // Delete the old basic block.
923   return true;
924 }
925 
926 /// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI
927 /// nodes in this block. This doesn't try to be clever about PHI nodes
928 /// which differ only in the order of the incoming values, but instcombine
929 /// orders them so it usually won't matter.
930 ///
931 bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
932   // This implementation doesn't currently consider undef operands
933   // specially. Theoretically, two phis which are identical except for
934   // one having an undef where the other doesn't could be collapsed.
935 
936   struct PHIDenseMapInfo {
937     static PHINode *getEmptyKey() {
938       return DenseMapInfo<PHINode *>::getEmptyKey();
939     }
940     static PHINode *getTombstoneKey() {
941       return DenseMapInfo<PHINode *>::getTombstoneKey();
942     }
943     static unsigned getHashValue(PHINode *PN) {
944       // Compute a hash value on the operands. Instcombine will likely have
945       // sorted them, which helps expose duplicates, but we have to check all
946       // the operands to be safe in case instcombine hasn't run.
947       return static_cast<unsigned>(hash_combine(
948           hash_combine_range(PN->value_op_begin(), PN->value_op_end()),
949           hash_combine_range(PN->block_begin(), PN->block_end())));
950     }
951     static bool isEqual(PHINode *LHS, PHINode *RHS) {
952       if (LHS == getEmptyKey() || LHS == getTombstoneKey() ||
953           RHS == getEmptyKey() || RHS == getTombstoneKey())
954         return LHS == RHS;
955       return LHS->isIdenticalTo(RHS);
956     }
957   };
958 
959   // Set of unique PHINodes.
960   DenseSet<PHINode *, PHIDenseMapInfo> PHISet;
961 
962   // Examine each PHI.
963   bool Changed = false;
964   for (auto I = BB->begin(); PHINode *PN = dyn_cast<PHINode>(I++);) {
965     auto Inserted = PHISet.insert(PN);
966     if (!Inserted.second) {
967       // A duplicate. Replace this PHI with its duplicate.
968       PN->replaceAllUsesWith(*Inserted.first);
969       PN->eraseFromParent();
970       Changed = true;
971 
972       // The RAUW can change PHIs that we already visited. Start over from the
973       // beginning.
974       PHISet.clear();
975       I = BB->begin();
976     }
977   }
978 
979   return Changed;
980 }
981 
982 /// enforceKnownAlignment - If the specified pointer points to an object that
983 /// we control, modify the object's alignment to PrefAlign. This isn't
984 /// often possible though. If alignment is important, a more reliable approach
985 /// is to simply align all global variables and allocation instructions to
986 /// their preferred alignment from the beginning.
987 ///
988 static unsigned enforceKnownAlignment(Value *V, unsigned Align,
989                                       unsigned PrefAlign,
990                                       const DataLayout &DL) {
991   assert(PrefAlign > Align);
992 
993   V = V->stripPointerCasts();
994 
995   if (AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
996     // TODO: ideally, computeKnownBits ought to have used
997     // AllocaInst::getAlignment() in its computation already, making
998     // the below max redundant. But, as it turns out,
999     // stripPointerCasts recurses through infinite layers of bitcasts,
1000     // while computeKnownBits is not allowed to traverse more than 6
1001     // levels.
1002     Align = std::max(AI->getAlignment(), Align);
1003     if (PrefAlign <= Align)
1004       return Align;
1005 
1006     // If the preferred alignment is greater than the natural stack alignment
1007     // then don't round up. This avoids dynamic stack realignment.
1008     if (DL.exceedsNaturalStackAlignment(PrefAlign))
1009       return Align;
1010     AI->setAlignment(PrefAlign);
1011     return PrefAlign;
1012   }
1013 
1014   if (auto *GO = dyn_cast<GlobalObject>(V)) {
1015     // TODO: as above, this shouldn't be necessary.
1016     Align = std::max(GO->getAlignment(), Align);
1017     if (PrefAlign <= Align)
1018       return Align;
1019 
1020     // If there is a large requested alignment and we can, bump up the alignment
1021     // of the global.  If the memory we set aside for the global may not be the
1022     // memory used by the final program then it is impossible for us to reliably
1023     // enforce the preferred alignment.
1024     if (!GO->canIncreaseAlignment())
1025       return Align;
1026 
1027     GO->setAlignment(PrefAlign);
1028     return PrefAlign;
1029   }
1030 
1031   return Align;
1032 }
1033 
1034 unsigned llvm::getOrEnforceKnownAlignment(Value *V, unsigned PrefAlign,
1035                                           const DataLayout &DL,
1036                                           const Instruction *CxtI,
1037                                           AssumptionCache *AC,
1038                                           const DominatorTree *DT) {
1039   assert(V->getType()->isPointerTy() &&
1040          "getOrEnforceKnownAlignment expects a pointer!");
1041 
1042   KnownBits Known = computeKnownBits(V, DL, 0, AC, CxtI, DT);
1043   unsigned TrailZ = Known.countMinTrailingZeros();
1044 
1045   // Avoid trouble with ridiculously large TrailZ values, such as
1046   // those computed from a null pointer.
1047   TrailZ = std::min(TrailZ, unsigned(sizeof(unsigned) * CHAR_BIT - 1));
1048 
1049   unsigned Align = 1u << std::min(Known.getBitWidth() - 1, TrailZ);
1050 
1051   // LLVM doesn't support alignments larger than this currently.
1052   Align = std::min(Align, +Value::MaximumAlignment);
1053 
1054   if (PrefAlign > Align)
1055     Align = enforceKnownAlignment(V, Align, PrefAlign, DL);
1056 
1057   // We don't need to make any adjustment.
1058   return Align;
1059 }
1060 
1061 ///===---------------------------------------------------------------------===//
1062 ///  Dbg Intrinsic utilities
1063 ///
1064 
1065 /// See if there is a dbg.value intrinsic for DIVar before I.
1066 static bool LdStHasDebugValue(DILocalVariable *DIVar, DIExpression *DIExpr,
1067                               Instruction *I) {
1068   // Since we can't guarantee that the original dbg.declare instrinsic
1069   // is removed by LowerDbgDeclare(), we need to make sure that we are
1070   // not inserting the same dbg.value intrinsic over and over.
1071   llvm::BasicBlock::InstListType::iterator PrevI(I);
1072   if (PrevI != I->getParent()->getInstList().begin()) {
1073     --PrevI;
1074     if (DbgValueInst *DVI = dyn_cast<DbgValueInst>(PrevI))
1075       if (DVI->getValue() == I->getOperand(0) &&
1076           DVI->getOffset() == 0 &&
1077           DVI->getVariable() == DIVar &&
1078           DVI->getExpression() == DIExpr)
1079         return true;
1080   }
1081   return false;
1082 }
1083 
1084 /// See if there is a dbg.value intrinsic for DIVar for the PHI node.
1085 static bool PhiHasDebugValue(DILocalVariable *DIVar,
1086                              DIExpression *DIExpr,
1087                              PHINode *APN) {
1088   // Since we can't guarantee that the original dbg.declare instrinsic
1089   // is removed by LowerDbgDeclare(), we need to make sure that we are
1090   // not inserting the same dbg.value intrinsic over and over.
1091   SmallVector<DbgValueInst *, 1> DbgValues;
1092   findDbgValues(DbgValues, APN);
1093   for (auto *DVI : DbgValues) {
1094     assert(DVI->getValue() == APN);
1095     assert(DVI->getOffset() == 0);
1096     if ((DVI->getVariable() == DIVar) && (DVI->getExpression() == DIExpr))
1097       return true;
1098   }
1099   return false;
1100 }
1101 
1102 /// Inserts a llvm.dbg.value intrinsic before a store to an alloca'd value
1103 /// that has an associated llvm.dbg.decl intrinsic.
1104 void llvm::ConvertDebugDeclareToDebugValue(DbgDeclareInst *DDI,
1105                                            StoreInst *SI, DIBuilder &Builder) {
1106   auto *DIVar = DDI->getVariable();
1107   assert(DIVar && "Missing variable");
1108   auto *DIExpr = DDI->getExpression();
1109   Value *DV = SI->getOperand(0);
1110 
1111   // If an argument is zero extended then use argument directly. The ZExt
1112   // may be zapped by an optimization pass in future.
1113   Argument *ExtendedArg = nullptr;
1114   if (ZExtInst *ZExt = dyn_cast<ZExtInst>(SI->getOperand(0)))
1115     ExtendedArg = dyn_cast<Argument>(ZExt->getOperand(0));
1116   if (SExtInst *SExt = dyn_cast<SExtInst>(SI->getOperand(0)))
1117     ExtendedArg = dyn_cast<Argument>(SExt->getOperand(0));
1118   if (ExtendedArg) {
1119     // If this DDI was already describing only a fragment of a variable, ensure
1120     // that fragment is appropriately narrowed here.
1121     // But if a fragment wasn't used, describe the value as the original
1122     // argument (rather than the zext or sext) so that it remains described even
1123     // if the sext/zext is optimized away. This widens the variable description,
1124     // leaving it up to the consumer to know how the smaller value may be
1125     // represented in a larger register.
1126     if (auto Fragment = DIExpr->getFragmentInfo()) {
1127       unsigned FragmentOffset = Fragment->OffsetInBits;
1128       SmallVector<uint64_t, 3> Ops(DIExpr->elements_begin(),
1129                                    DIExpr->elements_end() - 3);
1130       Ops.push_back(dwarf::DW_OP_LLVM_fragment);
1131       Ops.push_back(FragmentOffset);
1132       const DataLayout &DL = DDI->getModule()->getDataLayout();
1133       Ops.push_back(DL.getTypeSizeInBits(ExtendedArg->getType()));
1134       DIExpr = Builder.createExpression(Ops);
1135     }
1136     DV = ExtendedArg;
1137   }
1138   if (!LdStHasDebugValue(DIVar, DIExpr, SI))
1139     Builder.insertDbgValueIntrinsic(DV, 0, DIVar, DIExpr, DDI->getDebugLoc(),
1140                                     SI);
1141 }
1142 
1143 /// Inserts a llvm.dbg.value intrinsic before a load of an alloca'd value
1144 /// that has an associated llvm.dbg.decl intrinsic.
1145 void llvm::ConvertDebugDeclareToDebugValue(DbgDeclareInst *DDI,
1146                                            LoadInst *LI, DIBuilder &Builder) {
1147   auto *DIVar = DDI->getVariable();
1148   auto *DIExpr = DDI->getExpression();
1149   assert(DIVar && "Missing variable");
1150 
1151   if (LdStHasDebugValue(DIVar, DIExpr, LI))
1152     return;
1153 
1154   // We are now tracking the loaded value instead of the address. In the
1155   // future if multi-location support is added to the IR, it might be
1156   // preferable to keep tracking both the loaded value and the original
1157   // address in case the alloca can not be elided.
1158   Instruction *DbgValue = Builder.insertDbgValueIntrinsic(
1159       LI, 0, DIVar, DIExpr, DDI->getDebugLoc(), (Instruction *)nullptr);
1160   DbgValue->insertAfter(LI);
1161 }
1162 
1163 /// Inserts a llvm.dbg.value intrinsic after a phi
1164 /// that has an associated llvm.dbg.decl intrinsic.
1165 void llvm::ConvertDebugDeclareToDebugValue(DbgDeclareInst *DDI,
1166                                            PHINode *APN, DIBuilder &Builder) {
1167   auto *DIVar = DDI->getVariable();
1168   auto *DIExpr = DDI->getExpression();
1169   assert(DIVar && "Missing variable");
1170 
1171   if (PhiHasDebugValue(DIVar, DIExpr, APN))
1172     return;
1173 
1174   BasicBlock *BB = APN->getParent();
1175   auto InsertionPt = BB->getFirstInsertionPt();
1176 
1177   // The block may be a catchswitch block, which does not have a valid
1178   // insertion point.
1179   // FIXME: Insert dbg.value markers in the successors when appropriate.
1180   if (InsertionPt != BB->end())
1181     Builder.insertDbgValueIntrinsic(APN, 0, DIVar, DIExpr, DDI->getDebugLoc(),
1182                                     &*InsertionPt);
1183 }
1184 
1185 /// Determine whether this alloca is either a VLA or an array.
1186 static bool isArray(AllocaInst *AI) {
1187   return AI->isArrayAllocation() ||
1188     AI->getType()->getElementType()->isArrayTy();
1189 }
1190 
1191 /// LowerDbgDeclare - Lowers llvm.dbg.declare intrinsics into appropriate set
1192 /// of llvm.dbg.value intrinsics.
1193 bool llvm::LowerDbgDeclare(Function &F) {
1194   DIBuilder DIB(*F.getParent(), /*AllowUnresolved*/ false);
1195   SmallVector<DbgDeclareInst *, 4> Dbgs;
1196   for (auto &FI : F)
1197     for (Instruction &BI : FI)
1198       if (auto DDI = dyn_cast<DbgDeclareInst>(&BI))
1199         Dbgs.push_back(DDI);
1200 
1201   if (Dbgs.empty())
1202     return false;
1203 
1204   for (auto &I : Dbgs) {
1205     DbgDeclareInst *DDI = I;
1206     AllocaInst *AI = dyn_cast_or_null<AllocaInst>(DDI->getAddress());
1207     // If this is an alloca for a scalar variable, insert a dbg.value
1208     // at each load and store to the alloca and erase the dbg.declare.
1209     // The dbg.values allow tracking a variable even if it is not
1210     // stored on the stack, while the dbg.declare can only describe
1211     // the stack slot (and at a lexical-scope granularity). Later
1212     // passes will attempt to elide the stack slot.
1213     if (AI && !isArray(AI)) {
1214       for (auto &AIUse : AI->uses()) {
1215         User *U = AIUse.getUser();
1216         if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
1217           if (AIUse.getOperandNo() == 1)
1218             ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
1219         } else if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
1220           ConvertDebugDeclareToDebugValue(DDI, LI, DIB);
1221         } else if (CallInst *CI = dyn_cast<CallInst>(U)) {
1222           // This is a call by-value or some other instruction that
1223           // takes a pointer to the variable. Insert a *value*
1224           // intrinsic that describes the alloca.
1225           DIB.insertDbgValueIntrinsic(AI, 0, DDI->getVariable(),
1226                                       DDI->getExpression(), DDI->getDebugLoc(),
1227                                       CI);
1228         }
1229       }
1230       DDI->eraseFromParent();
1231     }
1232   }
1233   return true;
1234 }
1235 
1236 /// FindAllocaDbgDeclare - Finds the llvm.dbg.declare intrinsic describing the
1237 /// alloca 'V', if any.
1238 DbgDeclareInst *llvm::FindAllocaDbgDeclare(Value *V) {
1239   if (auto *L = LocalAsMetadata::getIfExists(V))
1240     if (auto *MDV = MetadataAsValue::getIfExists(V->getContext(), L))
1241       for (User *U : MDV->users())
1242         if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(U))
1243           return DDI;
1244 
1245   return nullptr;
1246 }
1247 
1248 void llvm::findDbgValues(SmallVectorImpl<DbgValueInst *> &DbgValues, Value *V) {
1249   if (auto *L = LocalAsMetadata::getIfExists(V))
1250     if (auto *MDV = MetadataAsValue::getIfExists(V->getContext(), L))
1251       for (User *U : MDV->users())
1252         if (DbgValueInst *DVI = dyn_cast<DbgValueInst>(U))
1253           DbgValues.push_back(DVI);
1254 }
1255 
1256 
1257 bool llvm::replaceDbgDeclare(Value *Address, Value *NewAddress,
1258                              Instruction *InsertBefore, DIBuilder &Builder,
1259                              bool Deref, int Offset) {
1260   DbgDeclareInst *DDI = FindAllocaDbgDeclare(Address);
1261   if (!DDI)
1262     return false;
1263   DebugLoc Loc = DDI->getDebugLoc();
1264   auto *DIVar = DDI->getVariable();
1265   auto *DIExpr = DDI->getExpression();
1266   assert(DIVar && "Missing variable");
1267   DIExpr = DIExpression::prepend(DIExpr, Deref, Offset);
1268   // Insert llvm.dbg.declare immediately after the original alloca, and remove
1269   // old llvm.dbg.declare.
1270   Builder.insertDeclare(NewAddress, DIVar, DIExpr, Loc, InsertBefore);
1271   DDI->eraseFromParent();
1272   return true;
1273 }
1274 
1275 bool llvm::replaceDbgDeclareForAlloca(AllocaInst *AI, Value *NewAllocaAddress,
1276                                       DIBuilder &Builder, bool Deref, int Offset) {
1277   return replaceDbgDeclare(AI, NewAllocaAddress, AI->getNextNode(), Builder,
1278                            Deref, Offset);
1279 }
1280 
1281 static void replaceOneDbgValueForAlloca(DbgValueInst *DVI, Value *NewAddress,
1282                                         DIBuilder &Builder, int Offset) {
1283   DebugLoc Loc = DVI->getDebugLoc();
1284   auto *DIVar = DVI->getVariable();
1285   auto *DIExpr = DVI->getExpression();
1286   assert(DIVar && "Missing variable");
1287 
1288   // This is an alloca-based llvm.dbg.value. The first thing it should do with
1289   // the alloca pointer is dereference it. Otherwise we don't know how to handle
1290   // it and give up.
1291   if (!DIExpr || DIExpr->getNumElements() < 1 ||
1292       DIExpr->getElement(0) != dwarf::DW_OP_deref)
1293     return;
1294 
1295   // Insert the offset immediately after the first deref.
1296   // We could just change the offset argument of dbg.value, but it's unsigned...
1297   if (Offset) {
1298     SmallVector<uint64_t, 4> Ops;
1299     Ops.push_back(dwarf::DW_OP_deref);
1300     DIExpression::appendOffset(Ops, Offset);
1301     Ops.append(DIExpr->elements_begin() + 1, DIExpr->elements_end());
1302     DIExpr = Builder.createExpression(Ops);
1303   }
1304 
1305   Builder.insertDbgValueIntrinsic(NewAddress, DVI->getOffset(), DIVar, DIExpr,
1306                                   Loc, DVI);
1307   DVI->eraseFromParent();
1308 }
1309 
1310 void llvm::replaceDbgValueForAlloca(AllocaInst *AI, Value *NewAllocaAddress,
1311                                     DIBuilder &Builder, int Offset) {
1312   if (auto *L = LocalAsMetadata::getIfExists(AI))
1313     if (auto *MDV = MetadataAsValue::getIfExists(AI->getContext(), L))
1314       for (auto UI = MDV->use_begin(), UE = MDV->use_end(); UI != UE;) {
1315         Use &U = *UI++;
1316         if (auto *DVI = dyn_cast<DbgValueInst>(U.getUser()))
1317           replaceOneDbgValueForAlloca(DVI, NewAllocaAddress, Builder, Offset);
1318       }
1319 }
1320 
1321 void llvm::salvageDebugInfo(Instruction &I) {
1322   SmallVector<DbgValueInst *, 1> DbgValues;
1323   auto &M = *I.getModule();
1324 
1325   auto MDWrap = [&](Value *V) {
1326     return MetadataAsValue::get(I.getContext(), ValueAsMetadata::get(V));
1327   };
1328 
1329   if (isa<BitCastInst>(&I)) {
1330     findDbgValues(DbgValues, &I);
1331     for (auto *DVI : DbgValues) {
1332       // Bitcasts are entirely irrelevant for debug info. Rewrite the dbg.value
1333       // to use the cast's source.
1334       DVI->setOperand(0, MDWrap(I.getOperand(0)));
1335       DEBUG(dbgs() << "SALVAGE: " << *DVI << '\n');
1336     }
1337   } else if (auto *GEP = dyn_cast<GetElementPtrInst>(&I)) {
1338     findDbgValues(DbgValues, &I);
1339     for (auto *DVI : DbgValues) {
1340       unsigned BitWidth =
1341           M.getDataLayout().getPointerSizeInBits(GEP->getPointerAddressSpace());
1342       APInt Offset(BitWidth, 0);
1343       // Rewrite a constant GEP into a DIExpression.  Since we are performing
1344       // arithmetic to compute the variable's *value* in the DIExpression, we
1345       // need to mark the expression with a DW_OP_stack_value.
1346       if (GEP->accumulateConstantOffset(M.getDataLayout(), Offset)) {
1347         auto *DIExpr = DVI->getExpression();
1348         DIBuilder DIB(M, /*AllowUnresolved*/ false);
1349         // GEP offsets are i32 and thus always fit into an int64_t.
1350         DIExpr = DIExpression::prepend(DIExpr, DIExpression::NoDeref,
1351                                        Offset.getSExtValue(),
1352                                        DIExpression::WithStackValue);
1353         DVI->setOperand(0, MDWrap(I.getOperand(0)));
1354         DVI->setOperand(3, MetadataAsValue::get(I.getContext(), DIExpr));
1355         DEBUG(dbgs() << "SALVAGE: " << *DVI << '\n');
1356       }
1357     }
1358   } else if (isa<LoadInst>(&I)) {
1359     findDbgValues(DbgValues, &I);
1360     for (auto *DVI : DbgValues) {
1361       // Rewrite the load into DW_OP_deref.
1362       auto *DIExpr = DVI->getExpression();
1363       DIBuilder DIB(M, /*AllowUnresolved*/ false);
1364       DIExpr = DIExpression::prepend(DIExpr, DIExpression::WithDeref);
1365       DVI->setOperand(0, MDWrap(I.getOperand(0)));
1366       DVI->setOperand(3, MetadataAsValue::get(I.getContext(), DIExpr));
1367       DEBUG(dbgs() << "SALVAGE:  " << *DVI << '\n');
1368     }
1369   }
1370 }
1371 
1372 unsigned llvm::removeAllNonTerminatorAndEHPadInstructions(BasicBlock *BB) {
1373   unsigned NumDeadInst = 0;
1374   // Delete the instructions backwards, as it has a reduced likelihood of
1375   // having to update as many def-use and use-def chains.
1376   Instruction *EndInst = BB->getTerminator(); // Last not to be deleted.
1377   while (EndInst != &BB->front()) {
1378     // Delete the next to last instruction.
1379     Instruction *Inst = &*--EndInst->getIterator();
1380     if (!Inst->use_empty() && !Inst->getType()->isTokenTy())
1381       Inst->replaceAllUsesWith(UndefValue::get(Inst->getType()));
1382     if (Inst->isEHPad() || Inst->getType()->isTokenTy()) {
1383       EndInst = Inst;
1384       continue;
1385     }
1386     if (!isa<DbgInfoIntrinsic>(Inst))
1387       ++NumDeadInst;
1388     Inst->eraseFromParent();
1389   }
1390   return NumDeadInst;
1391 }
1392 
1393 unsigned llvm::changeToUnreachable(Instruction *I, bool UseLLVMTrap,
1394                                    bool PreserveLCSSA) {
1395   BasicBlock *BB = I->getParent();
1396   // Loop over all of the successors, removing BB's entry from any PHI
1397   // nodes.
1398   for (BasicBlock *Successor : successors(BB))
1399     Successor->removePredecessor(BB, PreserveLCSSA);
1400 
1401   // Insert a call to llvm.trap right before this.  This turns the undefined
1402   // behavior into a hard fail instead of falling through into random code.
1403   if (UseLLVMTrap) {
1404     Function *TrapFn =
1405       Intrinsic::getDeclaration(BB->getParent()->getParent(), Intrinsic::trap);
1406     CallInst *CallTrap = CallInst::Create(TrapFn, "", I);
1407     CallTrap->setDebugLoc(I->getDebugLoc());
1408   }
1409   new UnreachableInst(I->getContext(), I);
1410 
1411   // All instructions after this are dead.
1412   unsigned NumInstrsRemoved = 0;
1413   BasicBlock::iterator BBI = I->getIterator(), BBE = BB->end();
1414   while (BBI != BBE) {
1415     if (!BBI->use_empty())
1416       BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
1417     BB->getInstList().erase(BBI++);
1418     ++NumInstrsRemoved;
1419   }
1420   return NumInstrsRemoved;
1421 }
1422 
1423 /// changeToCall - Convert the specified invoke into a normal call.
1424 static void changeToCall(InvokeInst *II) {
1425   SmallVector<Value*, 8> Args(II->arg_begin(), II->arg_end());
1426   SmallVector<OperandBundleDef, 1> OpBundles;
1427   II->getOperandBundlesAsDefs(OpBundles);
1428   CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args, OpBundles,
1429                                        "", II);
1430   NewCall->takeName(II);
1431   NewCall->setCallingConv(II->getCallingConv());
1432   NewCall->setAttributes(II->getAttributes());
1433   NewCall->setDebugLoc(II->getDebugLoc());
1434   II->replaceAllUsesWith(NewCall);
1435 
1436   // Follow the call by a branch to the normal destination.
1437   BranchInst::Create(II->getNormalDest(), II);
1438 
1439   // Update PHI nodes in the unwind destination
1440   II->getUnwindDest()->removePredecessor(II->getParent());
1441   II->eraseFromParent();
1442 }
1443 
1444 BasicBlock *llvm::changeToInvokeAndSplitBasicBlock(CallInst *CI,
1445                                                    BasicBlock *UnwindEdge) {
1446   BasicBlock *BB = CI->getParent();
1447 
1448   // Convert this function call into an invoke instruction.  First, split the
1449   // basic block.
1450   BasicBlock *Split =
1451       BB->splitBasicBlock(CI->getIterator(), CI->getName() + ".noexc");
1452 
1453   // Delete the unconditional branch inserted by splitBasicBlock
1454   BB->getInstList().pop_back();
1455 
1456   // Create the new invoke instruction.
1457   SmallVector<Value *, 8> InvokeArgs(CI->arg_begin(), CI->arg_end());
1458   SmallVector<OperandBundleDef, 1> OpBundles;
1459 
1460   CI->getOperandBundlesAsDefs(OpBundles);
1461 
1462   // Note: we're round tripping operand bundles through memory here, and that
1463   // can potentially be avoided with a cleverer API design that we do not have
1464   // as of this time.
1465 
1466   InvokeInst *II = InvokeInst::Create(CI->getCalledValue(), Split, UnwindEdge,
1467                                       InvokeArgs, OpBundles, CI->getName(), BB);
1468   II->setDebugLoc(CI->getDebugLoc());
1469   II->setCallingConv(CI->getCallingConv());
1470   II->setAttributes(CI->getAttributes());
1471 
1472   // Make sure that anything using the call now uses the invoke!  This also
1473   // updates the CallGraph if present, because it uses a WeakTrackingVH.
1474   CI->replaceAllUsesWith(II);
1475 
1476   // Delete the original call
1477   Split->getInstList().pop_front();
1478   return Split;
1479 }
1480 
1481 static bool markAliveBlocks(Function &F,
1482                             SmallPtrSetImpl<BasicBlock*> &Reachable) {
1483 
1484   SmallVector<BasicBlock*, 128> Worklist;
1485   BasicBlock *BB = &F.front();
1486   Worklist.push_back(BB);
1487   Reachable.insert(BB);
1488   bool Changed = false;
1489   do {
1490     BB = Worklist.pop_back_val();
1491 
1492     // Do a quick scan of the basic block, turning any obviously unreachable
1493     // instructions into LLVM unreachable insts.  The instruction combining pass
1494     // canonicalizes unreachable insts into stores to null or undef.
1495     for (Instruction &I : *BB) {
1496       // Assumptions that are known to be false are equivalent to unreachable.
1497       // Also, if the condition is undefined, then we make the choice most
1498       // beneficial to the optimizer, and choose that to also be unreachable.
1499       if (auto *II = dyn_cast<IntrinsicInst>(&I)) {
1500         if (II->getIntrinsicID() == Intrinsic::assume) {
1501           if (match(II->getArgOperand(0), m_CombineOr(m_Zero(), m_Undef()))) {
1502             // Don't insert a call to llvm.trap right before the unreachable.
1503             changeToUnreachable(II, false);
1504             Changed = true;
1505             break;
1506           }
1507         }
1508 
1509         if (II->getIntrinsicID() == Intrinsic::experimental_guard) {
1510           // A call to the guard intrinsic bails out of the current compilation
1511           // unit if the predicate passed to it is false.  If the predicate is a
1512           // constant false, then we know the guard will bail out of the current
1513           // compile unconditionally, so all code following it is dead.
1514           //
1515           // Note: unlike in llvm.assume, it is not "obviously profitable" for
1516           // guards to treat `undef` as `false` since a guard on `undef` can
1517           // still be useful for widening.
1518           if (match(II->getArgOperand(0), m_Zero()))
1519             if (!isa<UnreachableInst>(II->getNextNode())) {
1520               changeToUnreachable(II->getNextNode(), /*UseLLVMTrap=*/ false);
1521               Changed = true;
1522               break;
1523             }
1524         }
1525       }
1526 
1527       if (auto *CI = dyn_cast<CallInst>(&I)) {
1528         Value *Callee = CI->getCalledValue();
1529         if (isa<ConstantPointerNull>(Callee) || isa<UndefValue>(Callee)) {
1530           changeToUnreachable(CI, /*UseLLVMTrap=*/false);
1531           Changed = true;
1532           break;
1533         }
1534         if (CI->doesNotReturn()) {
1535           // If we found a call to a no-return function, insert an unreachable
1536           // instruction after it.  Make sure there isn't *already* one there
1537           // though.
1538           if (!isa<UnreachableInst>(CI->getNextNode())) {
1539             // Don't insert a call to llvm.trap right before the unreachable.
1540             changeToUnreachable(CI->getNextNode(), false);
1541             Changed = true;
1542           }
1543           break;
1544         }
1545       }
1546 
1547       // Store to undef and store to null are undefined and used to signal that
1548       // they should be changed to unreachable by passes that can't modify the
1549       // CFG.
1550       if (auto *SI = dyn_cast<StoreInst>(&I)) {
1551         // Don't touch volatile stores.
1552         if (SI->isVolatile()) continue;
1553 
1554         Value *Ptr = SI->getOperand(1);
1555 
1556         if (isa<UndefValue>(Ptr) ||
1557             (isa<ConstantPointerNull>(Ptr) &&
1558              SI->getPointerAddressSpace() == 0)) {
1559           changeToUnreachable(SI, true);
1560           Changed = true;
1561           break;
1562         }
1563       }
1564     }
1565 
1566     TerminatorInst *Terminator = BB->getTerminator();
1567     if (auto *II = dyn_cast<InvokeInst>(Terminator)) {
1568       // Turn invokes that call 'nounwind' functions into ordinary calls.
1569       Value *Callee = II->getCalledValue();
1570       if (isa<ConstantPointerNull>(Callee) || isa<UndefValue>(Callee)) {
1571         changeToUnreachable(II, true);
1572         Changed = true;
1573       } else if (II->doesNotThrow() && canSimplifyInvokeNoUnwind(&F)) {
1574         if (II->use_empty() && II->onlyReadsMemory()) {
1575           // jump to the normal destination branch.
1576           BranchInst::Create(II->getNormalDest(), II);
1577           II->getUnwindDest()->removePredecessor(II->getParent());
1578           II->eraseFromParent();
1579         } else
1580           changeToCall(II);
1581         Changed = true;
1582       }
1583     } else if (auto *CatchSwitch = dyn_cast<CatchSwitchInst>(Terminator)) {
1584       // Remove catchpads which cannot be reached.
1585       struct CatchPadDenseMapInfo {
1586         static CatchPadInst *getEmptyKey() {
1587           return DenseMapInfo<CatchPadInst *>::getEmptyKey();
1588         }
1589         static CatchPadInst *getTombstoneKey() {
1590           return DenseMapInfo<CatchPadInst *>::getTombstoneKey();
1591         }
1592         static unsigned getHashValue(CatchPadInst *CatchPad) {
1593           return static_cast<unsigned>(hash_combine_range(
1594               CatchPad->value_op_begin(), CatchPad->value_op_end()));
1595         }
1596         static bool isEqual(CatchPadInst *LHS, CatchPadInst *RHS) {
1597           if (LHS == getEmptyKey() || LHS == getTombstoneKey() ||
1598               RHS == getEmptyKey() || RHS == getTombstoneKey())
1599             return LHS == RHS;
1600           return LHS->isIdenticalTo(RHS);
1601         }
1602       };
1603 
1604       // Set of unique CatchPads.
1605       SmallDenseMap<CatchPadInst *, detail::DenseSetEmpty, 4,
1606                     CatchPadDenseMapInfo, detail::DenseSetPair<CatchPadInst *>>
1607           HandlerSet;
1608       detail::DenseSetEmpty Empty;
1609       for (CatchSwitchInst::handler_iterator I = CatchSwitch->handler_begin(),
1610                                              E = CatchSwitch->handler_end();
1611            I != E; ++I) {
1612         BasicBlock *HandlerBB = *I;
1613         auto *CatchPad = cast<CatchPadInst>(HandlerBB->getFirstNonPHI());
1614         if (!HandlerSet.insert({CatchPad, Empty}).second) {
1615           CatchSwitch->removeHandler(I);
1616           --I;
1617           --E;
1618           Changed = true;
1619         }
1620       }
1621     }
1622 
1623     Changed |= ConstantFoldTerminator(BB, true);
1624     for (BasicBlock *Successor : successors(BB))
1625       if (Reachable.insert(Successor).second)
1626         Worklist.push_back(Successor);
1627   } while (!Worklist.empty());
1628   return Changed;
1629 }
1630 
1631 void llvm::removeUnwindEdge(BasicBlock *BB) {
1632   TerminatorInst *TI = BB->getTerminator();
1633 
1634   if (auto *II = dyn_cast<InvokeInst>(TI)) {
1635     changeToCall(II);
1636     return;
1637   }
1638 
1639   TerminatorInst *NewTI;
1640   BasicBlock *UnwindDest;
1641 
1642   if (auto *CRI = dyn_cast<CleanupReturnInst>(TI)) {
1643     NewTI = CleanupReturnInst::Create(CRI->getCleanupPad(), nullptr, CRI);
1644     UnwindDest = CRI->getUnwindDest();
1645   } else if (auto *CatchSwitch = dyn_cast<CatchSwitchInst>(TI)) {
1646     auto *NewCatchSwitch = CatchSwitchInst::Create(
1647         CatchSwitch->getParentPad(), nullptr, CatchSwitch->getNumHandlers(),
1648         CatchSwitch->getName(), CatchSwitch);
1649     for (BasicBlock *PadBB : CatchSwitch->handlers())
1650       NewCatchSwitch->addHandler(PadBB);
1651 
1652     NewTI = NewCatchSwitch;
1653     UnwindDest = CatchSwitch->getUnwindDest();
1654   } else {
1655     llvm_unreachable("Could not find unwind successor");
1656   }
1657 
1658   NewTI->takeName(TI);
1659   NewTI->setDebugLoc(TI->getDebugLoc());
1660   UnwindDest->removePredecessor(BB);
1661   TI->replaceAllUsesWith(NewTI);
1662   TI->eraseFromParent();
1663 }
1664 
1665 /// removeUnreachableBlocks - Remove blocks that are not reachable, even
1666 /// if they are in a dead cycle.  Return true if a change was made, false
1667 /// otherwise. If `LVI` is passed, this function preserves LazyValueInfo
1668 /// after modifying the CFG.
1669 bool llvm::removeUnreachableBlocks(Function &F, LazyValueInfo *LVI) {
1670   SmallPtrSet<BasicBlock*, 16> Reachable;
1671   bool Changed = markAliveBlocks(F, Reachable);
1672 
1673   // If there are unreachable blocks in the CFG...
1674   if (Reachable.size() == F.size())
1675     return Changed;
1676 
1677   assert(Reachable.size() < F.size());
1678   NumRemoved += F.size()-Reachable.size();
1679 
1680   // Loop over all of the basic blocks that are not reachable, dropping all of
1681   // their internal references...
1682   for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
1683     if (Reachable.count(&*BB))
1684       continue;
1685 
1686     for (BasicBlock *Successor : successors(&*BB))
1687       if (Reachable.count(Successor))
1688         Successor->removePredecessor(&*BB);
1689     if (LVI)
1690       LVI->eraseBlock(&*BB);
1691     BB->dropAllReferences();
1692   }
1693 
1694   for (Function::iterator I = ++F.begin(); I != F.end();)
1695     if (!Reachable.count(&*I))
1696       I = F.getBasicBlockList().erase(I);
1697     else
1698       ++I;
1699 
1700   return true;
1701 }
1702 
1703 void llvm::combineMetadata(Instruction *K, const Instruction *J,
1704                            ArrayRef<unsigned> KnownIDs) {
1705   SmallVector<std::pair<unsigned, MDNode *>, 4> Metadata;
1706   K->dropUnknownNonDebugMetadata(KnownIDs);
1707   K->getAllMetadataOtherThanDebugLoc(Metadata);
1708   for (const auto &MD : Metadata) {
1709     unsigned Kind = MD.first;
1710     MDNode *JMD = J->getMetadata(Kind);
1711     MDNode *KMD = MD.second;
1712 
1713     switch (Kind) {
1714       default:
1715         K->setMetadata(Kind, nullptr); // Remove unknown metadata
1716         break;
1717       case LLVMContext::MD_dbg:
1718         llvm_unreachable("getAllMetadataOtherThanDebugLoc returned a MD_dbg");
1719       case LLVMContext::MD_tbaa:
1720         K->setMetadata(Kind, MDNode::getMostGenericTBAA(JMD, KMD));
1721         break;
1722       case LLVMContext::MD_alias_scope:
1723         K->setMetadata(Kind, MDNode::getMostGenericAliasScope(JMD, KMD));
1724         break;
1725       case LLVMContext::MD_noalias:
1726       case LLVMContext::MD_mem_parallel_loop_access:
1727         K->setMetadata(Kind, MDNode::intersect(JMD, KMD));
1728         break;
1729       case LLVMContext::MD_range:
1730         K->setMetadata(Kind, MDNode::getMostGenericRange(JMD, KMD));
1731         break;
1732       case LLVMContext::MD_fpmath:
1733         K->setMetadata(Kind, MDNode::getMostGenericFPMath(JMD, KMD));
1734         break;
1735       case LLVMContext::MD_invariant_load:
1736         // Only set the !invariant.load if it is present in both instructions.
1737         K->setMetadata(Kind, JMD);
1738         break;
1739       case LLVMContext::MD_nonnull:
1740         // Only set the !nonnull if it is present in both instructions.
1741         K->setMetadata(Kind, JMD);
1742         break;
1743       case LLVMContext::MD_invariant_group:
1744         // Preserve !invariant.group in K.
1745         break;
1746       case LLVMContext::MD_align:
1747         K->setMetadata(Kind,
1748           MDNode::getMostGenericAlignmentOrDereferenceable(JMD, KMD));
1749         break;
1750       case LLVMContext::MD_dereferenceable:
1751       case LLVMContext::MD_dereferenceable_or_null:
1752         K->setMetadata(Kind,
1753           MDNode::getMostGenericAlignmentOrDereferenceable(JMD, KMD));
1754         break;
1755     }
1756   }
1757   // Set !invariant.group from J if J has it. If both instructions have it
1758   // then we will just pick it from J - even when they are different.
1759   // Also make sure that K is load or store - f.e. combining bitcast with load
1760   // could produce bitcast with invariant.group metadata, which is invalid.
1761   // FIXME: we should try to preserve both invariant.group md if they are
1762   // different, but right now instruction can only have one invariant.group.
1763   if (auto *JMD = J->getMetadata(LLVMContext::MD_invariant_group))
1764     if (isa<LoadInst>(K) || isa<StoreInst>(K))
1765       K->setMetadata(LLVMContext::MD_invariant_group, JMD);
1766 }
1767 
1768 void llvm::combineMetadataForCSE(Instruction *K, const Instruction *J) {
1769   unsigned KnownIDs[] = {
1770       LLVMContext::MD_tbaa,            LLVMContext::MD_alias_scope,
1771       LLVMContext::MD_noalias,         LLVMContext::MD_range,
1772       LLVMContext::MD_invariant_load,  LLVMContext::MD_nonnull,
1773       LLVMContext::MD_invariant_group, LLVMContext::MD_align,
1774       LLVMContext::MD_dereferenceable,
1775       LLVMContext::MD_dereferenceable_or_null};
1776   combineMetadata(K, J, KnownIDs);
1777 }
1778 
1779 template <typename RootType, typename DominatesFn>
1780 static unsigned replaceDominatedUsesWith(Value *From, Value *To,
1781                                          const RootType &Root,
1782                                          const DominatesFn &Dominates) {
1783   assert(From->getType() == To->getType());
1784 
1785   unsigned Count = 0;
1786   for (Value::use_iterator UI = From->use_begin(), UE = From->use_end();
1787        UI != UE;) {
1788     Use &U = *UI++;
1789     if (!Dominates(Root, U))
1790       continue;
1791     U.set(To);
1792     DEBUG(dbgs() << "Replace dominated use of '" << From->getName() << "' as "
1793                  << *To << " in " << *U << "\n");
1794     ++Count;
1795   }
1796   return Count;
1797 }
1798 
1799 unsigned llvm::replaceNonLocalUsesWith(Instruction *From, Value *To) {
1800    assert(From->getType() == To->getType());
1801    auto *BB = From->getParent();
1802    unsigned Count = 0;
1803 
1804   for (Value::use_iterator UI = From->use_begin(), UE = From->use_end();
1805        UI != UE;) {
1806     Use &U = *UI++;
1807     auto *I = cast<Instruction>(U.getUser());
1808     if (I->getParent() == BB)
1809       continue;
1810     U.set(To);
1811     ++Count;
1812   }
1813   return Count;
1814 }
1815 
1816 unsigned llvm::replaceDominatedUsesWith(Value *From, Value *To,
1817                                         DominatorTree &DT,
1818                                         const BasicBlockEdge &Root) {
1819   auto Dominates = [&DT](const BasicBlockEdge &Root, const Use &U) {
1820     return DT.dominates(Root, U);
1821   };
1822   return ::replaceDominatedUsesWith(From, To, Root, Dominates);
1823 }
1824 
1825 unsigned llvm::replaceDominatedUsesWith(Value *From, Value *To,
1826                                         DominatorTree &DT,
1827                                         const BasicBlock *BB) {
1828   auto ProperlyDominates = [&DT](const BasicBlock *BB, const Use &U) {
1829     auto *I = cast<Instruction>(U.getUser())->getParent();
1830     return DT.properlyDominates(BB, I);
1831   };
1832   return ::replaceDominatedUsesWith(From, To, BB, ProperlyDominates);
1833 }
1834 
1835 bool llvm::callsGCLeafFunction(ImmutableCallSite CS) {
1836   // Check if the function is specifically marked as a gc leaf function.
1837   if (CS.hasFnAttr("gc-leaf-function"))
1838     return true;
1839   if (const Function *F = CS.getCalledFunction()) {
1840     if (F->hasFnAttribute("gc-leaf-function"))
1841       return true;
1842 
1843     if (auto IID = F->getIntrinsicID())
1844       // Most LLVM intrinsics do not take safepoints.
1845       return IID != Intrinsic::experimental_gc_statepoint &&
1846              IID != Intrinsic::experimental_deoptimize;
1847   }
1848 
1849   return false;
1850 }
1851 
1852 void llvm::copyNonnullMetadata(const LoadInst &OldLI, MDNode *N,
1853                                LoadInst &NewLI) {
1854   auto *NewTy = NewLI.getType();
1855 
1856   // This only directly applies if the new type is also a pointer.
1857   if (NewTy->isPointerTy()) {
1858     NewLI.setMetadata(LLVMContext::MD_nonnull, N);
1859     return;
1860   }
1861 
1862   // The only other translation we can do is to integral loads with !range
1863   // metadata.
1864   if (!NewTy->isIntegerTy())
1865     return;
1866 
1867   MDBuilder MDB(NewLI.getContext());
1868   const Value *Ptr = OldLI.getPointerOperand();
1869   auto *ITy = cast<IntegerType>(NewTy);
1870   auto *NullInt = ConstantExpr::getPtrToInt(
1871       ConstantPointerNull::get(cast<PointerType>(Ptr->getType())), ITy);
1872   auto *NonNullInt = ConstantExpr::getAdd(NullInt, ConstantInt::get(ITy, 1));
1873   NewLI.setMetadata(LLVMContext::MD_range,
1874                     MDB.createRange(NonNullInt, NullInt));
1875 }
1876 
1877 void llvm::copyRangeMetadata(const DataLayout &DL, const LoadInst &OldLI,
1878                              MDNode *N, LoadInst &NewLI) {
1879   auto *NewTy = NewLI.getType();
1880 
1881   // Give up unless it is converted to a pointer where there is a single very
1882   // valuable mapping we can do reliably.
1883   // FIXME: It would be nice to propagate this in more ways, but the type
1884   // conversions make it hard.
1885   if (!NewTy->isPointerTy())
1886     return;
1887 
1888   unsigned BitWidth = DL.getTypeSizeInBits(NewTy);
1889   if (!getConstantRangeFromMetadata(*N).contains(APInt(BitWidth, 0))) {
1890     MDNode *NN = MDNode::get(OldLI.getContext(), None);
1891     NewLI.setMetadata(LLVMContext::MD_nonnull, NN);
1892   }
1893 }
1894 
1895 namespace {
1896 /// A potential constituent of a bitreverse or bswap expression. See
1897 /// collectBitParts for a fuller explanation.
1898 struct BitPart {
1899   BitPart(Value *P, unsigned BW) : Provider(P) {
1900     Provenance.resize(BW);
1901   }
1902 
1903   /// The Value that this is a bitreverse/bswap of.
1904   Value *Provider;
1905   /// The "provenance" of each bit. Provenance[A] = B means that bit A
1906   /// in Provider becomes bit B in the result of this expression.
1907   SmallVector<int8_t, 32> Provenance; // int8_t means max size is i128.
1908 
1909   enum { Unset = -1 };
1910 };
1911 } // end anonymous namespace
1912 
1913 /// Analyze the specified subexpression and see if it is capable of providing
1914 /// pieces of a bswap or bitreverse. The subexpression provides a potential
1915 /// piece of a bswap or bitreverse if it can be proven that each non-zero bit in
1916 /// the output of the expression came from a corresponding bit in some other
1917 /// value. This function is recursive, and the end result is a mapping of
1918 /// bitnumber to bitnumber. It is the caller's responsibility to validate that
1919 /// the bitnumber to bitnumber mapping is correct for a bswap or bitreverse.
1920 ///
1921 /// For example, if the current subexpression if "(shl i32 %X, 24)" then we know
1922 /// that the expression deposits the low byte of %X into the high byte of the
1923 /// result and that all other bits are zero. This expression is accepted and a
1924 /// BitPart is returned with Provider set to %X and Provenance[24-31] set to
1925 /// [0-7].
1926 ///
1927 /// To avoid revisiting values, the BitPart results are memoized into the
1928 /// provided map. To avoid unnecessary copying of BitParts, BitParts are
1929 /// constructed in-place in the \c BPS map. Because of this \c BPS needs to
1930 /// store BitParts objects, not pointers. As we need the concept of a nullptr
1931 /// BitParts (Value has been analyzed and the analysis failed), we an Optional
1932 /// type instead to provide the same functionality.
1933 ///
1934 /// Because we pass around references into \c BPS, we must use a container that
1935 /// does not invalidate internal references (std::map instead of DenseMap).
1936 ///
1937 static const Optional<BitPart> &
1938 collectBitParts(Value *V, bool MatchBSwaps, bool MatchBitReversals,
1939                 std::map<Value *, Optional<BitPart>> &BPS) {
1940   auto I = BPS.find(V);
1941   if (I != BPS.end())
1942     return I->second;
1943 
1944   auto &Result = BPS[V] = None;
1945   auto BitWidth = cast<IntegerType>(V->getType())->getBitWidth();
1946 
1947   if (Instruction *I = dyn_cast<Instruction>(V)) {
1948     // If this is an or instruction, it may be an inner node of the bswap.
1949     if (I->getOpcode() == Instruction::Or) {
1950       auto &A = collectBitParts(I->getOperand(0), MatchBSwaps,
1951                                 MatchBitReversals, BPS);
1952       auto &B = collectBitParts(I->getOperand(1), MatchBSwaps,
1953                                 MatchBitReversals, BPS);
1954       if (!A || !B)
1955         return Result;
1956 
1957       // Try and merge the two together.
1958       if (!A->Provider || A->Provider != B->Provider)
1959         return Result;
1960 
1961       Result = BitPart(A->Provider, BitWidth);
1962       for (unsigned i = 0; i < A->Provenance.size(); ++i) {
1963         if (A->Provenance[i] != BitPart::Unset &&
1964             B->Provenance[i] != BitPart::Unset &&
1965             A->Provenance[i] != B->Provenance[i])
1966           return Result = None;
1967 
1968         if (A->Provenance[i] == BitPart::Unset)
1969           Result->Provenance[i] = B->Provenance[i];
1970         else
1971           Result->Provenance[i] = A->Provenance[i];
1972       }
1973 
1974       return Result;
1975     }
1976 
1977     // If this is a logical shift by a constant, recurse then shift the result.
1978     if (I->isLogicalShift() && isa<ConstantInt>(I->getOperand(1))) {
1979       unsigned BitShift =
1980           cast<ConstantInt>(I->getOperand(1))->getLimitedValue(~0U);
1981       // Ensure the shift amount is defined.
1982       if (BitShift > BitWidth)
1983         return Result;
1984 
1985       auto &Res = collectBitParts(I->getOperand(0), MatchBSwaps,
1986                                   MatchBitReversals, BPS);
1987       if (!Res)
1988         return Result;
1989       Result = Res;
1990 
1991       // Perform the "shift" on BitProvenance.
1992       auto &P = Result->Provenance;
1993       if (I->getOpcode() == Instruction::Shl) {
1994         P.erase(std::prev(P.end(), BitShift), P.end());
1995         P.insert(P.begin(), BitShift, BitPart::Unset);
1996       } else {
1997         P.erase(P.begin(), std::next(P.begin(), BitShift));
1998         P.insert(P.end(), BitShift, BitPart::Unset);
1999       }
2000 
2001       return Result;
2002     }
2003 
2004     // If this is a logical 'and' with a mask that clears bits, recurse then
2005     // unset the appropriate bits.
2006     if (I->getOpcode() == Instruction::And &&
2007         isa<ConstantInt>(I->getOperand(1))) {
2008       APInt Bit(I->getType()->getPrimitiveSizeInBits(), 1);
2009       const APInt &AndMask = cast<ConstantInt>(I->getOperand(1))->getValue();
2010 
2011       // Check that the mask allows a multiple of 8 bits for a bswap, for an
2012       // early exit.
2013       unsigned NumMaskedBits = AndMask.countPopulation();
2014       if (!MatchBitReversals && NumMaskedBits % 8 != 0)
2015         return Result;
2016 
2017       auto &Res = collectBitParts(I->getOperand(0), MatchBSwaps,
2018                                   MatchBitReversals, BPS);
2019       if (!Res)
2020         return Result;
2021       Result = Res;
2022 
2023       for (unsigned i = 0; i < BitWidth; ++i, Bit <<= 1)
2024         // If the AndMask is zero for this bit, clear the bit.
2025         if ((AndMask & Bit) == 0)
2026           Result->Provenance[i] = BitPart::Unset;
2027       return Result;
2028     }
2029 
2030     // If this is a zext instruction zero extend the result.
2031     if (I->getOpcode() == Instruction::ZExt) {
2032       auto &Res = collectBitParts(I->getOperand(0), MatchBSwaps,
2033                                   MatchBitReversals, BPS);
2034       if (!Res)
2035         return Result;
2036 
2037       Result = BitPart(Res->Provider, BitWidth);
2038       auto NarrowBitWidth =
2039           cast<IntegerType>(cast<ZExtInst>(I)->getSrcTy())->getBitWidth();
2040       for (unsigned i = 0; i < NarrowBitWidth; ++i)
2041         Result->Provenance[i] = Res->Provenance[i];
2042       for (unsigned i = NarrowBitWidth; i < BitWidth; ++i)
2043         Result->Provenance[i] = BitPart::Unset;
2044       return Result;
2045     }
2046   }
2047 
2048   // Okay, we got to something that isn't a shift, 'or' or 'and'.  This must be
2049   // the input value to the bswap/bitreverse.
2050   Result = BitPart(V, BitWidth);
2051   for (unsigned i = 0; i < BitWidth; ++i)
2052     Result->Provenance[i] = i;
2053   return Result;
2054 }
2055 
2056 static bool bitTransformIsCorrectForBSwap(unsigned From, unsigned To,
2057                                           unsigned BitWidth) {
2058   if (From % 8 != To % 8)
2059     return false;
2060   // Convert from bit indices to byte indices and check for a byte reversal.
2061   From >>= 3;
2062   To >>= 3;
2063   BitWidth >>= 3;
2064   return From == BitWidth - To - 1;
2065 }
2066 
2067 static bool bitTransformIsCorrectForBitReverse(unsigned From, unsigned To,
2068                                                unsigned BitWidth) {
2069   return From == BitWidth - To - 1;
2070 }
2071 
2072 /// Given an OR instruction, check to see if this is a bitreverse
2073 /// idiom. If so, insert the new intrinsic and return true.
2074 bool llvm::recognizeBSwapOrBitReverseIdiom(
2075     Instruction *I, bool MatchBSwaps, bool MatchBitReversals,
2076     SmallVectorImpl<Instruction *> &InsertedInsts) {
2077   if (Operator::getOpcode(I) != Instruction::Or)
2078     return false;
2079   if (!MatchBSwaps && !MatchBitReversals)
2080     return false;
2081   IntegerType *ITy = dyn_cast<IntegerType>(I->getType());
2082   if (!ITy || ITy->getBitWidth() > 128)
2083     return false;   // Can't do vectors or integers > 128 bits.
2084   unsigned BW = ITy->getBitWidth();
2085 
2086   unsigned DemandedBW = BW;
2087   IntegerType *DemandedTy = ITy;
2088   if (I->hasOneUse()) {
2089     if (TruncInst *Trunc = dyn_cast<TruncInst>(I->user_back())) {
2090       DemandedTy = cast<IntegerType>(Trunc->getType());
2091       DemandedBW = DemandedTy->getBitWidth();
2092     }
2093   }
2094 
2095   // Try to find all the pieces corresponding to the bswap.
2096   std::map<Value *, Optional<BitPart>> BPS;
2097   auto Res = collectBitParts(I, MatchBSwaps, MatchBitReversals, BPS);
2098   if (!Res)
2099     return false;
2100   auto &BitProvenance = Res->Provenance;
2101 
2102   // Now, is the bit permutation correct for a bswap or a bitreverse? We can
2103   // only byteswap values with an even number of bytes.
2104   bool OKForBSwap = DemandedBW % 16 == 0, OKForBitReverse = true;
2105   for (unsigned i = 0; i < DemandedBW; ++i) {
2106     OKForBSwap &=
2107         bitTransformIsCorrectForBSwap(BitProvenance[i], i, DemandedBW);
2108     OKForBitReverse &=
2109         bitTransformIsCorrectForBitReverse(BitProvenance[i], i, DemandedBW);
2110   }
2111 
2112   Intrinsic::ID Intrin;
2113   if (OKForBSwap && MatchBSwaps)
2114     Intrin = Intrinsic::bswap;
2115   else if (OKForBitReverse && MatchBitReversals)
2116     Intrin = Intrinsic::bitreverse;
2117   else
2118     return false;
2119 
2120   if (ITy != DemandedTy) {
2121     Function *F = Intrinsic::getDeclaration(I->getModule(), Intrin, DemandedTy);
2122     Value *Provider = Res->Provider;
2123     IntegerType *ProviderTy = cast<IntegerType>(Provider->getType());
2124     // We may need to truncate the provider.
2125     if (DemandedTy != ProviderTy) {
2126       auto *Trunc = CastInst::Create(Instruction::Trunc, Provider, DemandedTy,
2127                                      "trunc", I);
2128       InsertedInsts.push_back(Trunc);
2129       Provider = Trunc;
2130     }
2131     auto *CI = CallInst::Create(F, Provider, "rev", I);
2132     InsertedInsts.push_back(CI);
2133     auto *ExtInst = CastInst::Create(Instruction::ZExt, CI, ITy, "zext", I);
2134     InsertedInsts.push_back(ExtInst);
2135     return true;
2136   }
2137 
2138   Function *F = Intrinsic::getDeclaration(I->getModule(), Intrin, ITy);
2139   InsertedInsts.push_back(CallInst::Create(F, Res->Provider, "rev", I));
2140   return true;
2141 }
2142 
2143 // CodeGen has special handling for some string functions that may replace
2144 // them with target-specific intrinsics.  Since that'd skip our interceptors
2145 // in ASan/MSan/TSan/DFSan, and thus make us miss some memory accesses,
2146 // we mark affected calls as NoBuiltin, which will disable optimization
2147 // in CodeGen.
2148 void llvm::maybeMarkSanitizerLibraryCallNoBuiltin(
2149     CallInst *CI, const TargetLibraryInfo *TLI) {
2150   Function *F = CI->getCalledFunction();
2151   LibFunc Func;
2152   if (F && !F->hasLocalLinkage() && F->hasName() &&
2153       TLI->getLibFunc(F->getName(), Func) && TLI->hasOptimizedCodeGen(Func) &&
2154       !F->doesNotAccessMemory())
2155     CI->addAttribute(AttributeList::FunctionIndex, Attribute::NoBuiltin);
2156 }
2157 
2158 bool llvm::canReplaceOperandWithVariable(const Instruction *I, unsigned OpIdx) {
2159   // We can't have a PHI with a metadata type.
2160   if (I->getOperand(OpIdx)->getType()->isMetadataTy())
2161     return false;
2162 
2163   // Early exit.
2164   if (!isa<Constant>(I->getOperand(OpIdx)))
2165     return true;
2166 
2167   switch (I->getOpcode()) {
2168   default:
2169     return true;
2170   case Instruction::Call:
2171   case Instruction::Invoke:
2172     // Can't handle inline asm. Skip it.
2173     if (isa<InlineAsm>(ImmutableCallSite(I).getCalledValue()))
2174       return false;
2175     // Many arithmetic intrinsics have no issue taking a
2176     // variable, however it's hard to distingish these from
2177     // specials such as @llvm.frameaddress that require a constant.
2178     if (isa<IntrinsicInst>(I))
2179       return false;
2180 
2181     // Constant bundle operands may need to retain their constant-ness for
2182     // correctness.
2183     if (ImmutableCallSite(I).isBundleOperand(OpIdx))
2184       return false;
2185     return true;
2186   case Instruction::ShuffleVector:
2187     // Shufflevector masks are constant.
2188     return OpIdx != 2;
2189   case Instruction::Switch:
2190   case Instruction::ExtractValue:
2191     // All operands apart from the first are constant.
2192     return OpIdx == 0;
2193   case Instruction::InsertValue:
2194     // All operands apart from the first and the second are constant.
2195     return OpIdx < 2;
2196   case Instruction::Alloca:
2197     // Static allocas (constant size in the entry block) are handled by
2198     // prologue/epilogue insertion so they're free anyway. We definitely don't
2199     // want to make them non-constant.
2200     return !dyn_cast<AllocaInst>(I)->isStaticAlloca();
2201   case Instruction::GetElementPtr:
2202     if (OpIdx == 0)
2203       return true;
2204     gep_type_iterator It = gep_type_begin(I);
2205     for (auto E = std::next(It, OpIdx); It != E; ++It)
2206       if (It.isStruct())
2207         return false;
2208     return true;
2209   }
2210 }
2211