1 //===- AssumeBundleBuilder.cpp - tools to preserve informations -*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #include "llvm/Transforms/Utils/AssumeBundleBuilder.h"
10 #include "llvm/ADT/DepthFirstIterator.h"
11 #include "llvm/ADT/MapVector.h"
12 #include "llvm/ADT/Statistic.h"
13 #include "llvm/Analysis/AssumeBundleQueries.h"
14 #include "llvm/Analysis/AssumptionCache.h"
15 #include "llvm/Analysis/ValueTracking.h"
16 #include "llvm/IR/Dominators.h"
17 #include "llvm/IR/Function.h"
18 #include "llvm/IR/InstIterator.h"
19 #include "llvm/IR/IntrinsicInst.h"
20 #include "llvm/IR/Module.h"
21 #include "llvm/IR/Operator.h"
22 #include "llvm/Support/CommandLine.h"
23 #include "llvm/Support/DebugCounter.h"
24 #include "llvm/Transforms/Utils/Local.h"
25 
26 using namespace llvm;
27 
28 namespace llvm {
29 cl::opt<bool> ShouldPreserveAllAttributes(
30     "assume-preserve-all", cl::init(false), cl::Hidden,
31     cl::desc("enable preservation of all attrbitues. even those that are "
32              "unlikely to be usefull"));
33 
34 cl::opt<bool> EnableKnowledgeRetention(
35     "enable-knowledge-retention", cl::init(false), cl::Hidden,
36     cl::desc(
37         "enable preservation of attributes throughout code transformation"));
38 } // namespace llvm
39 
40 #define DEBUG_TYPE "assume-builder"
41 
42 STATISTIC(NumAssumeBuilt, "Number of assume built by the assume builder");
43 STATISTIC(NumBundlesInAssumes, "Total number of Bundles in the assume built");
44 STATISTIC(NumAssumesMerged,
45           "Number of assume merged by the assume simplify pass");
46 STATISTIC(NumAssumesRemoved,
47           "Number of assume removed by the assume simplify pass");
48 
49 DEBUG_COUNTER(BuildAssumeCounter, "assume-builder-counter",
50               "Controls which assumes gets created");
51 
52 namespace {
53 
54 bool isUsefullToPreserve(Attribute::AttrKind Kind) {
55   switch (Kind) {
56     case Attribute::NonNull:
57     case Attribute::NoUndef:
58     case Attribute::Alignment:
59     case Attribute::Dereferenceable:
60     case Attribute::DereferenceableOrNull:
61     case Attribute::Cold:
62       return true;
63     default:
64       return false;
65   }
66 }
67 
68 /// This function will try to transform the given knowledge into a more
69 /// canonical one. the canonical knowledge maybe the given one.
70 RetainedKnowledge canonicalizedKnowledge(RetainedKnowledge RK,
71                                          const DataLayout &DL) {
72   switch (RK.AttrKind) {
73   default:
74     return RK;
75   case Attribute::NonNull:
76     RK.WasOn = getUnderlyingObject(RK.WasOn);
77     return RK;
78   case Attribute::Alignment: {
79     Value *V = RK.WasOn->stripInBoundsOffsets([&](const Value *Strip) {
80       if (auto *GEP = dyn_cast<GEPOperator>(Strip))
81         RK.ArgValue =
82             MinAlign(RK.ArgValue, GEP->getMaxPreservedAlignment(DL).value());
83     });
84     RK.WasOn = V;
85     return RK;
86   }
87   case Attribute::Dereferenceable:
88   case Attribute::DereferenceableOrNull: {
89     int64_t Offset = 0;
90     Value *V = GetPointerBaseWithConstantOffset(RK.WasOn, Offset, DL,
91                                                 /*AllowNonInBounds*/ false);
92     if (Offset < 0)
93       return RK;
94     RK.ArgValue = RK.ArgValue + Offset;
95     RK.WasOn = V;
96   }
97   }
98   return RK;
99 }
100 
101 /// This class contain all knowledge that have been gather while building an
102 /// llvm.assume and the function to manipulate it.
103 struct AssumeBuilderState {
104   Module *M;
105 
106   using MapKey = std::pair<Value *, Attribute::AttrKind>;
107   SmallMapVector<MapKey, uint64_t, 8> AssumedKnowledgeMap;
108   Instruction *InstBeingModified = nullptr;
109   AssumptionCache* AC = nullptr;
110   DominatorTree* DT = nullptr;
111 
112   AssumeBuilderState(Module *M, Instruction *I = nullptr,
113                      AssumptionCache *AC = nullptr, DominatorTree *DT = nullptr)
114       : M(M), InstBeingModified(I), AC(AC), DT(DT) {}
115 
116   bool tryToPreserveWithoutAddingAssume(RetainedKnowledge RK) {
117     if (!InstBeingModified || !RK.WasOn)
118       return false;
119     bool HasBeenPreserved = false;
120     Use* ToUpdate = nullptr;
121     getKnowledgeForValue(
122         RK.WasOn, {RK.AttrKind}, AC,
123         [&](RetainedKnowledge RKOther, Instruction *Assume,
124             const CallInst::BundleOpInfo *Bundle) {
125           if (!isValidAssumeForContext(Assume, InstBeingModified, DT))
126             return false;
127           if (RKOther.ArgValue >= RK.ArgValue) {
128             HasBeenPreserved = true;
129             return true;
130           } else if (isValidAssumeForContext(InstBeingModified, Assume, DT)) {
131             HasBeenPreserved = true;
132             IntrinsicInst *Intr = cast<IntrinsicInst>(Assume);
133             ToUpdate = &Intr->op_begin()[Bundle->Begin + ABA_Argument];
134             return true;
135           }
136           return false;
137         });
138     if (ToUpdate)
139       ToUpdate->set(
140           ConstantInt::get(Type::getInt64Ty(M->getContext()), RK.ArgValue));
141     return HasBeenPreserved;
142   }
143 
144   bool isKnowledgeWorthPreserving(RetainedKnowledge RK) {
145     if (!RK)
146       return false;
147     if (!RK.WasOn)
148       return true;
149     if (RK.WasOn->getType()->isPointerTy()) {
150       Value *UnderlyingPtr = getUnderlyingObject(RK.WasOn);
151       if (isa<AllocaInst>(UnderlyingPtr) || isa<GlobalValue>(UnderlyingPtr))
152         return false;
153     }
154     if (auto *Arg = dyn_cast<Argument>(RK.WasOn)) {
155       if (Arg->hasAttribute(RK.AttrKind) &&
156           (!Attribute::isIntAttrKind(RK.AttrKind) ||
157            Arg->getAttribute(RK.AttrKind).getValueAsInt() >= RK.ArgValue))
158         return false;
159       return true;
160     }
161     if (auto *Inst = dyn_cast<Instruction>(RK.WasOn))
162       if (wouldInstructionBeTriviallyDead(Inst)) {
163         if (RK.WasOn->use_empty())
164           return false;
165         Use *SingleUse = RK.WasOn->getSingleUndroppableUse();
166         if (SingleUse && SingleUse->getUser() == InstBeingModified)
167           return false;
168       }
169     return true;
170   }
171 
172   void addKnowledge(RetainedKnowledge RK) {
173     RK = canonicalizedKnowledge(RK, M->getDataLayout());
174 
175     if (!isKnowledgeWorthPreserving(RK))
176       return;
177 
178     if (tryToPreserveWithoutAddingAssume(RK))
179       return;
180     MapKey Key{RK.WasOn, RK.AttrKind};
181     auto Lookup = AssumedKnowledgeMap.find(Key);
182     if (Lookup == AssumedKnowledgeMap.end()) {
183       AssumedKnowledgeMap[Key] = RK.ArgValue;
184       return;
185     }
186     assert(((Lookup->second == 0 && RK.ArgValue == 0) ||
187             (Lookup->second != 0 && RK.ArgValue != 0)) &&
188            "inconsistent argument value");
189 
190     /// This is only desirable because for all attributes taking an argument
191     /// higher is better.
192     Lookup->second = std::max(Lookup->second, RK.ArgValue);
193   }
194 
195   void addAttribute(Attribute Attr, Value *WasOn) {
196     if (Attr.isTypeAttribute() || Attr.isStringAttribute() ||
197         (!ShouldPreserveAllAttributes &&
198          !isUsefullToPreserve(Attr.getKindAsEnum())))
199       return;
200     uint64_t AttrArg = 0;
201     if (Attr.isIntAttribute())
202       AttrArg = Attr.getValueAsInt();
203     addKnowledge({Attr.getKindAsEnum(), AttrArg, WasOn});
204   }
205 
206   void addCall(const CallBase *Call) {
207     auto addAttrList = [&](AttributeList AttrList, unsigned NumArgs) {
208       for (unsigned Idx = 0; Idx < NumArgs; Idx++)
209         for (Attribute Attr : AttrList.getParamAttrs(Idx)) {
210           bool IsPoisonAttr = Attr.hasAttribute(Attribute::NonNull) ||
211                               Attr.hasAttribute(Attribute::Alignment);
212           if (!IsPoisonAttr || Call->isPassingUndefUB(Idx))
213             addAttribute(Attr, Call->getArgOperand(Idx));
214         }
215       for (Attribute Attr : AttrList.getFnAttrs())
216         addAttribute(Attr, nullptr);
217     };
218     addAttrList(Call->getAttributes(), Call->arg_size());
219     if (Function *Fn = Call->getCalledFunction())
220       addAttrList(Fn->getAttributes(), Fn->arg_size());
221   }
222 
223   AssumeInst *build() {
224     if (AssumedKnowledgeMap.empty())
225       return nullptr;
226     if (!DebugCounter::shouldExecute(BuildAssumeCounter))
227       return nullptr;
228     Function *FnAssume = Intrinsic::getDeclaration(M, Intrinsic::assume);
229     LLVMContext &C = M->getContext();
230     SmallVector<OperandBundleDef, 8> OpBundle;
231     for (auto &MapElem : AssumedKnowledgeMap) {
232       SmallVector<Value *, 2> Args;
233       if (MapElem.first.first)
234         Args.push_back(MapElem.first.first);
235 
236       /// This is only valid because for all attribute that currently exist a
237       /// value of 0 is useless. and should not be preserved.
238       if (MapElem.second)
239         Args.push_back(ConstantInt::get(Type::getInt64Ty(M->getContext()),
240                                         MapElem.second));
241       OpBundle.push_back(OperandBundleDefT<Value *>(
242           std::string(Attribute::getNameFromAttrKind(MapElem.first.second)),
243           Args));
244       NumBundlesInAssumes++;
245     }
246     NumAssumeBuilt++;
247     return cast<AssumeInst>(CallInst::Create(
248         FnAssume, ArrayRef<Value *>({ConstantInt::getTrue(C)}), OpBundle));
249   }
250 
251   void addAccessedPtr(Instruction *MemInst, Value *Pointer, Type *AccType,
252                       MaybeAlign MA) {
253     unsigned DerefSize = MemInst->getModule()
254                              ->getDataLayout()
255                              .getTypeStoreSize(AccType)
256                              .getKnownMinValue();
257     if (DerefSize != 0) {
258       addKnowledge({Attribute::Dereferenceable, DerefSize, Pointer});
259       if (!NullPointerIsDefined(MemInst->getFunction(),
260                                 Pointer->getType()->getPointerAddressSpace()))
261         addKnowledge({Attribute::NonNull, 0u, Pointer});
262     }
263     if (MA.valueOrOne() > 1)
264       addKnowledge({Attribute::Alignment, MA.valueOrOne().value(), Pointer});
265   }
266 
267   void addInstruction(Instruction *I) {
268     if (auto *Call = dyn_cast<CallBase>(I))
269       return addCall(Call);
270     if (auto *Load = dyn_cast<LoadInst>(I))
271       return addAccessedPtr(I, Load->getPointerOperand(), Load->getType(),
272                             Load->getAlign());
273     if (auto *Store = dyn_cast<StoreInst>(I))
274       return addAccessedPtr(I, Store->getPointerOperand(),
275                             Store->getValueOperand()->getType(),
276                             Store->getAlign());
277     // TODO: Add support for the other Instructions.
278     // TODO: Maybe we should look around and merge with other llvm.assume.
279   }
280 };
281 
282 } // namespace
283 
284 AssumeInst *llvm::buildAssumeFromInst(Instruction *I) {
285   if (!EnableKnowledgeRetention)
286     return nullptr;
287   AssumeBuilderState Builder(I->getModule());
288   Builder.addInstruction(I);
289   return Builder.build();
290 }
291 
292 bool llvm::salvageKnowledge(Instruction *I, AssumptionCache *AC,
293                             DominatorTree *DT) {
294   if (!EnableKnowledgeRetention || I->isTerminator())
295     return false;
296   bool Changed = false;
297   AssumeBuilderState Builder(I->getModule(), I, AC, DT);
298   Builder.addInstruction(I);
299   if (auto *Intr = Builder.build()) {
300     Intr->insertBefore(I);
301     Changed = true;
302     if (AC)
303       AC->registerAssumption(Intr);
304   }
305   return Changed;
306 }
307 
308 AssumeInst *
309 llvm::buildAssumeFromKnowledge(ArrayRef<RetainedKnowledge> Knowledge,
310                                Instruction *CtxI, AssumptionCache *AC,
311                                DominatorTree *DT) {
312   AssumeBuilderState Builder(CtxI->getModule(), CtxI, AC, DT);
313   for (const RetainedKnowledge &RK : Knowledge)
314     Builder.addKnowledge(RK);
315   return Builder.build();
316 }
317 
318 RetainedKnowledge llvm::simplifyRetainedKnowledge(AssumeInst *Assume,
319                                                   RetainedKnowledge RK,
320                                                   AssumptionCache *AC,
321                                                   DominatorTree *DT) {
322   AssumeBuilderState Builder(Assume->getModule(), Assume, AC, DT);
323   RK = canonicalizedKnowledge(RK, Assume->getModule()->getDataLayout());
324 
325   if (!Builder.isKnowledgeWorthPreserving(RK))
326     return RetainedKnowledge::none();
327 
328   if (Builder.tryToPreserveWithoutAddingAssume(RK))
329     return RetainedKnowledge::none();
330   return RK;
331 }
332 
333 namespace {
334 
335 struct AssumeSimplify {
336   Function &F;
337   AssumptionCache &AC;
338   DominatorTree *DT;
339   LLVMContext &C;
340   SmallDenseSet<IntrinsicInst *> CleanupToDo;
341   StringMapEntry<uint32_t> *IgnoreTag;
342   SmallDenseMap<BasicBlock *, SmallVector<IntrinsicInst *, 4>, 8> BBToAssume;
343   bool MadeChange = false;
344 
345   AssumeSimplify(Function &F, AssumptionCache &AC, DominatorTree *DT,
346                  LLVMContext &C)
347       : F(F), AC(AC), DT(DT), C(C),
348         IgnoreTag(C.getOrInsertBundleTag(IgnoreBundleTag)) {}
349 
350   void buildMapping(bool FilterBooleanArgument) {
351     BBToAssume.clear();
352     for (Value *V : AC.assumptions()) {
353       if (!V)
354         continue;
355       IntrinsicInst *Assume = cast<IntrinsicInst>(V);
356       if (FilterBooleanArgument) {
357         auto *Arg = dyn_cast<ConstantInt>(Assume->getOperand(0));
358         if (!Arg || Arg->isZero())
359           continue;
360       }
361       BBToAssume[Assume->getParent()].push_back(Assume);
362     }
363 
364     for (auto &Elem : BBToAssume) {
365       llvm::sort(Elem.second,
366                  [](const IntrinsicInst *LHS, const IntrinsicInst *RHS) {
367                    return LHS->comesBefore(RHS);
368                  });
369     }
370   }
371 
372   /// Remove all asumes in CleanupToDo if there boolean argument is true and
373   /// ForceCleanup is set or the assume doesn't hold valuable knowledge.
374   void RunCleanup(bool ForceCleanup) {
375     for (IntrinsicInst *Assume : CleanupToDo) {
376       auto *Arg = dyn_cast<ConstantInt>(Assume->getOperand(0));
377       if (!Arg || Arg->isZero() ||
378           (!ForceCleanup &&
379            !isAssumeWithEmptyBundle(cast<AssumeInst>(*Assume))))
380         continue;
381       MadeChange = true;
382       if (ForceCleanup)
383         NumAssumesMerged++;
384       else
385         NumAssumesRemoved++;
386       Assume->eraseFromParent();
387     }
388     CleanupToDo.clear();
389   }
390 
391   /// Remove knowledge stored in assume when it is already know by an attribute
392   /// or an other assume. This can when valid update an existing knowledge in an
393   /// attribute or an other assume.
394   void dropRedundantKnowledge() {
395     struct MapValue {
396       IntrinsicInst *Assume;
397       uint64_t ArgValue;
398       CallInst::BundleOpInfo *BOI;
399     };
400     buildMapping(false);
401     SmallDenseMap<std::pair<Value *, Attribute::AttrKind>,
402                   SmallVector<MapValue, 2>, 16>
403         Knowledge;
404     for (BasicBlock *BB : depth_first(&F))
405       for (Value *V : BBToAssume[BB]) {
406         if (!V)
407           continue;
408         IntrinsicInst *Assume = cast<IntrinsicInst>(V);
409         for (CallInst::BundleOpInfo &BOI : Assume->bundle_op_infos()) {
410           auto RemoveFromAssume = [&]() {
411             CleanupToDo.insert(Assume);
412             if (BOI.Begin != BOI.End) {
413               Use *U = &Assume->op_begin()[BOI.Begin + ABA_WasOn];
414               U->set(UndefValue::get(U->get()->getType()));
415             }
416             BOI.Tag = IgnoreTag;
417           };
418           if (BOI.Tag == IgnoreTag) {
419             CleanupToDo.insert(Assume);
420             continue;
421           }
422           RetainedKnowledge RK =
423             getKnowledgeFromBundle(cast<AssumeInst>(*Assume), BOI);
424           if (auto *Arg = dyn_cast_or_null<Argument>(RK.WasOn)) {
425             bool HasSameKindAttr = Arg->hasAttribute(RK.AttrKind);
426             if (HasSameKindAttr)
427               if (!Attribute::isIntAttrKind(RK.AttrKind) ||
428                   Arg->getAttribute(RK.AttrKind).getValueAsInt() >=
429                       RK.ArgValue) {
430                 RemoveFromAssume();
431                 continue;
432               }
433             if (isValidAssumeForContext(
434                     Assume, &*F.getEntryBlock().getFirstInsertionPt()) ||
435                 Assume == &*F.getEntryBlock().getFirstInsertionPt()) {
436               if (HasSameKindAttr)
437                 Arg->removeAttr(RK.AttrKind);
438               Arg->addAttr(Attribute::get(C, RK.AttrKind, RK.ArgValue));
439               MadeChange = true;
440               RemoveFromAssume();
441               continue;
442             }
443           }
444           auto &Lookup = Knowledge[{RK.WasOn, RK.AttrKind}];
445           for (MapValue &Elem : Lookup) {
446             if (!isValidAssumeForContext(Elem.Assume, Assume, DT))
447               continue;
448             if (Elem.ArgValue >= RK.ArgValue) {
449               RemoveFromAssume();
450               continue;
451             } else if (isValidAssumeForContext(Assume, Elem.Assume, DT)) {
452               Elem.Assume->op_begin()[Elem.BOI->Begin + ABA_Argument].set(
453                   ConstantInt::get(Type::getInt64Ty(C), RK.ArgValue));
454               MadeChange = true;
455               RemoveFromAssume();
456               continue;
457             }
458           }
459           Lookup.push_back({Assume, RK.ArgValue, &BOI});
460         }
461       }
462   }
463 
464   using MergeIterator = SmallVectorImpl<IntrinsicInst *>::iterator;
465 
466   /// Merge all Assumes from Begin to End in and insert the resulting assume as
467   /// high as possible in the basicblock.
468   void mergeRange(BasicBlock *BB, MergeIterator Begin, MergeIterator End) {
469     if (Begin == End || std::next(Begin) == End)
470       return;
471     /// Provide no additional information so that AssumeBuilderState doesn't
472     /// try to do any punning since it already has been done better.
473     AssumeBuilderState Builder(F.getParent());
474 
475     /// For now it is initialized to the best value it could have
476     Instruction *InsertPt = BB->getFirstNonPHI();
477     if (isa<LandingPadInst>(InsertPt))
478       InsertPt = InsertPt->getNextNode();
479     for (IntrinsicInst *I : make_range(Begin, End)) {
480       CleanupToDo.insert(I);
481       for (CallInst::BundleOpInfo &BOI : I->bundle_op_infos()) {
482         RetainedKnowledge RK =
483           getKnowledgeFromBundle(cast<AssumeInst>(*I), BOI);
484         if (!RK)
485           continue;
486         Builder.addKnowledge(RK);
487         if (auto *I = dyn_cast_or_null<Instruction>(RK.WasOn))
488           if (I->getParent() == InsertPt->getParent() &&
489               (InsertPt->comesBefore(I) || InsertPt == I))
490             InsertPt = I->getNextNode();
491       }
492     }
493 
494     /// Adjust InsertPt if it is before Begin, since mergeAssumes only
495     /// guarantees we can place the resulting assume between Begin and End.
496     if (InsertPt->comesBefore(*Begin))
497       for (auto It = (*Begin)->getIterator(), E = InsertPt->getIterator();
498            It != E; --It)
499         if (!isGuaranteedToTransferExecutionToSuccessor(&*It)) {
500           InsertPt = It->getNextNode();
501           break;
502         }
503     auto *MergedAssume = Builder.build();
504     if (!MergedAssume)
505       return;
506     MadeChange = true;
507     MergedAssume->insertBefore(InsertPt);
508     AC.registerAssumption(MergedAssume);
509   }
510 
511   /// Merge assume when they are in the same BasicBlock and for all instruction
512   /// between them isGuaranteedToTransferExecutionToSuccessor returns true.
513   void mergeAssumes() {
514     buildMapping(true);
515 
516     SmallVector<MergeIterator, 4> SplitPoints;
517     for (auto &Elem : BBToAssume) {
518       SmallVectorImpl<IntrinsicInst *> &AssumesInBB = Elem.second;
519       if (AssumesInBB.size() < 2)
520         continue;
521       /// AssumesInBB is already sorted by order in the block.
522 
523       BasicBlock::iterator It = AssumesInBB.front()->getIterator();
524       BasicBlock::iterator E = AssumesInBB.back()->getIterator();
525       SplitPoints.push_back(AssumesInBB.begin());
526       MergeIterator LastSplit = AssumesInBB.begin();
527       for (; It != E; ++It)
528         if (!isGuaranteedToTransferExecutionToSuccessor(&*It)) {
529           for (; (*LastSplit)->comesBefore(&*It); ++LastSplit)
530             ;
531           if (SplitPoints.back() != LastSplit)
532             SplitPoints.push_back(LastSplit);
533         }
534       SplitPoints.push_back(AssumesInBB.end());
535       for (auto SplitIt = SplitPoints.begin();
536            SplitIt != std::prev(SplitPoints.end()); SplitIt++) {
537         mergeRange(Elem.first, *SplitIt, *(SplitIt + 1));
538       }
539       SplitPoints.clear();
540     }
541   }
542 };
543 
544 bool simplifyAssumes(Function &F, AssumptionCache *AC, DominatorTree *DT) {
545   AssumeSimplify AS(F, *AC, DT, F.getContext());
546 
547   /// Remove knowledge that is already known by a dominating other assume or an
548   /// attribute.
549   AS.dropRedundantKnowledge();
550 
551   /// Remove assume that are empty.
552   AS.RunCleanup(false);
553 
554   /// Merge assume in the same basicblock when possible.
555   AS.mergeAssumes();
556 
557   /// Remove assume that were merged.
558   AS.RunCleanup(true);
559   return AS.MadeChange;
560 }
561 
562 } // namespace
563 
564 PreservedAnalyses AssumeSimplifyPass::run(Function &F,
565                                           FunctionAnalysisManager &AM) {
566   if (!EnableKnowledgeRetention)
567     return PreservedAnalyses::all();
568   if (!simplifyAssumes(F, &AM.getResult<AssumptionAnalysis>(F),
569                        AM.getCachedResult<DominatorTreeAnalysis>(F)))
570     return PreservedAnalyses::all();
571   PreservedAnalyses PA;
572   PA.preserveSet<CFGAnalyses>();
573   return PA;
574 }
575 
576 PreservedAnalyses AssumeBuilderPass::run(Function &F,
577                                          FunctionAnalysisManager &AM) {
578   AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F);
579   DominatorTree* DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
580   bool Changed = false;
581   for (Instruction &I : instructions(F))
582     Changed |= salvageKnowledge(&I, AC, DT);
583   if (!Changed)
584     PreservedAnalyses::all();
585   PreservedAnalyses PA;
586   PA.preserveSet<CFGAnalyses>();
587   return PA;
588 }
589