xref: /freebsd/contrib/llvm-project/llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp (revision 0d8fe2373503aeac48492f28073049a8bfa4feb5)
1 //===- ObjCARCOpts.cpp - ObjC ARC Optimization ----------------------------===//
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 /// \file
10 /// This file defines ObjC ARC optimizations. ARC stands for Automatic
11 /// Reference Counting and is a system for managing reference counts for objects
12 /// in Objective C.
13 ///
14 /// The optimizations performed include elimination of redundant, partially
15 /// redundant, and inconsequential reference count operations, elimination of
16 /// redundant weak pointer operations, and numerous minor simplifications.
17 ///
18 /// WARNING: This file knows about certain library functions. It recognizes them
19 /// by name, and hardwires knowledge of their semantics.
20 ///
21 /// WARNING: This file knows about how certain Objective-C library functions are
22 /// used. Naive LLVM IR transformations which would otherwise be
23 /// behavior-preserving may break these assumptions.
24 //
25 //===----------------------------------------------------------------------===//
26 
27 #include "ARCRuntimeEntryPoints.h"
28 #include "BlotMapVector.h"
29 #include "DependencyAnalysis.h"
30 #include "ObjCARC.h"
31 #include "ProvenanceAnalysis.h"
32 #include "PtrState.h"
33 #include "llvm/ADT/DenseMap.h"
34 #include "llvm/ADT/None.h"
35 #include "llvm/ADT/STLExtras.h"
36 #include "llvm/ADT/SmallPtrSet.h"
37 #include "llvm/ADT/SmallVector.h"
38 #include "llvm/ADT/Statistic.h"
39 #include "llvm/Analysis/AliasAnalysis.h"
40 #include "llvm/Analysis/EHPersonalities.h"
41 #include "llvm/Analysis/ObjCARCAliasAnalysis.h"
42 #include "llvm/Analysis/ObjCARCAnalysisUtils.h"
43 #include "llvm/Analysis/ObjCARCInstKind.h"
44 #include "llvm/IR/BasicBlock.h"
45 #include "llvm/IR/CFG.h"
46 #include "llvm/IR/Constant.h"
47 #include "llvm/IR/Constants.h"
48 #include "llvm/IR/DerivedTypes.h"
49 #include "llvm/IR/Function.h"
50 #include "llvm/IR/GlobalVariable.h"
51 #include "llvm/IR/InstIterator.h"
52 #include "llvm/IR/InstrTypes.h"
53 #include "llvm/IR/Instruction.h"
54 #include "llvm/IR/Instructions.h"
55 #include "llvm/IR/LLVMContext.h"
56 #include "llvm/IR/Metadata.h"
57 #include "llvm/IR/Type.h"
58 #include "llvm/IR/User.h"
59 #include "llvm/IR/Value.h"
60 #include "llvm/InitializePasses.h"
61 #include "llvm/Pass.h"
62 #include "llvm/Support/Casting.h"
63 #include "llvm/Support/CommandLine.h"
64 #include "llvm/Support/Compiler.h"
65 #include "llvm/Support/Debug.h"
66 #include "llvm/Support/ErrorHandling.h"
67 #include "llvm/Support/raw_ostream.h"
68 #include "llvm/Transforms/ObjCARC.h"
69 #include <cassert>
70 #include <iterator>
71 #include <utility>
72 
73 using namespace llvm;
74 using namespace llvm::objcarc;
75 
76 #define DEBUG_TYPE "objc-arc-opts"
77 
78 static cl::opt<unsigned> MaxPtrStates("arc-opt-max-ptr-states",
79     cl::Hidden,
80     cl::desc("Maximum number of ptr states the optimizer keeps track of"),
81     cl::init(4095));
82 
83 /// \defgroup ARCUtilities Utility declarations/definitions specific to ARC.
84 /// @{
85 
86 /// This is similar to GetRCIdentityRoot but it stops as soon
87 /// as it finds a value with multiple uses.
88 static const Value *FindSingleUseIdentifiedObject(const Value *Arg) {
89   // ConstantData (like ConstantPointerNull and UndefValue) is used across
90   // modules.  It's never a single-use value.
91   if (isa<ConstantData>(Arg))
92     return nullptr;
93 
94   if (Arg->hasOneUse()) {
95     if (const BitCastInst *BC = dyn_cast<BitCastInst>(Arg))
96       return FindSingleUseIdentifiedObject(BC->getOperand(0));
97     if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Arg))
98       if (GEP->hasAllZeroIndices())
99         return FindSingleUseIdentifiedObject(GEP->getPointerOperand());
100     if (IsForwarding(GetBasicARCInstKind(Arg)))
101       return FindSingleUseIdentifiedObject(
102                cast<CallInst>(Arg)->getArgOperand(0));
103     if (!IsObjCIdentifiedObject(Arg))
104       return nullptr;
105     return Arg;
106   }
107 
108   // If we found an identifiable object but it has multiple uses, but they are
109   // trivial uses, we can still consider this to be a single-use value.
110   if (IsObjCIdentifiedObject(Arg)) {
111     for (const User *U : Arg->users())
112       if (!U->use_empty() || GetRCIdentityRoot(U) != Arg)
113          return nullptr;
114 
115     return Arg;
116   }
117 
118   return nullptr;
119 }
120 
121 /// @}
122 ///
123 /// \defgroup ARCOpt ARC Optimization.
124 /// @{
125 
126 // TODO: On code like this:
127 //
128 // objc_retain(%x)
129 // stuff_that_cannot_release()
130 // objc_autorelease(%x)
131 // stuff_that_cannot_release()
132 // objc_retain(%x)
133 // stuff_that_cannot_release()
134 // objc_autorelease(%x)
135 //
136 // The second retain and autorelease can be deleted.
137 
138 // TODO: It should be possible to delete
139 // objc_autoreleasePoolPush and objc_autoreleasePoolPop
140 // pairs if nothing is actually autoreleased between them. Also, autorelease
141 // calls followed by objc_autoreleasePoolPop calls (perhaps in ObjC++ code
142 // after inlining) can be turned into plain release calls.
143 
144 // TODO: Critical-edge splitting. If the optimial insertion point is
145 // a critical edge, the current algorithm has to fail, because it doesn't
146 // know how to split edges. It should be possible to make the optimizer
147 // think in terms of edges, rather than blocks, and then split critical
148 // edges on demand.
149 
150 // TODO: OptimizeSequences could generalized to be Interprocedural.
151 
152 // TODO: Recognize that a bunch of other objc runtime calls have
153 // non-escaping arguments and non-releasing arguments, and may be
154 // non-autoreleasing.
155 
156 // TODO: Sink autorelease calls as far as possible. Unfortunately we
157 // usually can't sink them past other calls, which would be the main
158 // case where it would be useful.
159 
160 // TODO: The pointer returned from objc_loadWeakRetained is retained.
161 
162 // TODO: Delete release+retain pairs (rare).
163 
164 STATISTIC(NumNoops,       "Number of no-op objc calls eliminated");
165 STATISTIC(NumPartialNoops, "Number of partially no-op objc calls eliminated");
166 STATISTIC(NumAutoreleases,"Number of autoreleases converted to releases");
167 STATISTIC(NumRets,        "Number of return value forwarding "
168                           "retain+autoreleases eliminated");
169 STATISTIC(NumRRs,         "Number of retain+release paths eliminated");
170 STATISTIC(NumPeeps,       "Number of calls peephole-optimized");
171 #ifndef NDEBUG
172 STATISTIC(NumRetainsBeforeOpt,
173           "Number of retains before optimization");
174 STATISTIC(NumReleasesBeforeOpt,
175           "Number of releases before optimization");
176 STATISTIC(NumRetainsAfterOpt,
177           "Number of retains after optimization");
178 STATISTIC(NumReleasesAfterOpt,
179           "Number of releases after optimization");
180 #endif
181 
182 namespace {
183 
184   /// Per-BasicBlock state.
185   class BBState {
186     /// The number of unique control paths from the entry which can reach this
187     /// block.
188     unsigned TopDownPathCount = 0;
189 
190     /// The number of unique control paths to exits from this block.
191     unsigned BottomUpPathCount = 0;
192 
193     /// The top-down traversal uses this to record information known about a
194     /// pointer at the bottom of each block.
195     BlotMapVector<const Value *, TopDownPtrState> PerPtrTopDown;
196 
197     /// The bottom-up traversal uses this to record information known about a
198     /// pointer at the top of each block.
199     BlotMapVector<const Value *, BottomUpPtrState> PerPtrBottomUp;
200 
201     /// Effective predecessors of the current block ignoring ignorable edges and
202     /// ignored backedges.
203     SmallVector<BasicBlock *, 2> Preds;
204 
205     /// Effective successors of the current block ignoring ignorable edges and
206     /// ignored backedges.
207     SmallVector<BasicBlock *, 2> Succs;
208 
209   public:
210     static const unsigned OverflowOccurredValue;
211 
212     BBState() = default;
213 
214     using top_down_ptr_iterator = decltype(PerPtrTopDown)::iterator;
215     using const_top_down_ptr_iterator = decltype(PerPtrTopDown)::const_iterator;
216 
217     top_down_ptr_iterator top_down_ptr_begin() { return PerPtrTopDown.begin(); }
218     top_down_ptr_iterator top_down_ptr_end() { return PerPtrTopDown.end(); }
219     const_top_down_ptr_iterator top_down_ptr_begin() const {
220       return PerPtrTopDown.begin();
221     }
222     const_top_down_ptr_iterator top_down_ptr_end() const {
223       return PerPtrTopDown.end();
224     }
225     bool hasTopDownPtrs() const {
226       return !PerPtrTopDown.empty();
227     }
228 
229     unsigned top_down_ptr_list_size() const {
230       return std::distance(top_down_ptr_begin(), top_down_ptr_end());
231     }
232 
233     using bottom_up_ptr_iterator = decltype(PerPtrBottomUp)::iterator;
234     using const_bottom_up_ptr_iterator =
235         decltype(PerPtrBottomUp)::const_iterator;
236 
237     bottom_up_ptr_iterator bottom_up_ptr_begin() {
238       return PerPtrBottomUp.begin();
239     }
240     bottom_up_ptr_iterator bottom_up_ptr_end() { return PerPtrBottomUp.end(); }
241     const_bottom_up_ptr_iterator bottom_up_ptr_begin() const {
242       return PerPtrBottomUp.begin();
243     }
244     const_bottom_up_ptr_iterator bottom_up_ptr_end() const {
245       return PerPtrBottomUp.end();
246     }
247     bool hasBottomUpPtrs() const {
248       return !PerPtrBottomUp.empty();
249     }
250 
251     unsigned bottom_up_ptr_list_size() const {
252       return std::distance(bottom_up_ptr_begin(), bottom_up_ptr_end());
253     }
254 
255     /// Mark this block as being an entry block, which has one path from the
256     /// entry by definition.
257     void SetAsEntry() { TopDownPathCount = 1; }
258 
259     /// Mark this block as being an exit block, which has one path to an exit by
260     /// definition.
261     void SetAsExit()  { BottomUpPathCount = 1; }
262 
263     /// Attempt to find the PtrState object describing the top down state for
264     /// pointer Arg. Return a new initialized PtrState describing the top down
265     /// state for Arg if we do not find one.
266     TopDownPtrState &getPtrTopDownState(const Value *Arg) {
267       return PerPtrTopDown[Arg];
268     }
269 
270     /// Attempt to find the PtrState object describing the bottom up state for
271     /// pointer Arg. Return a new initialized PtrState describing the bottom up
272     /// state for Arg if we do not find one.
273     BottomUpPtrState &getPtrBottomUpState(const Value *Arg) {
274       return PerPtrBottomUp[Arg];
275     }
276 
277     /// Attempt to find the PtrState object describing the bottom up state for
278     /// pointer Arg.
279     bottom_up_ptr_iterator findPtrBottomUpState(const Value *Arg) {
280       return PerPtrBottomUp.find(Arg);
281     }
282 
283     void clearBottomUpPointers() {
284       PerPtrBottomUp.clear();
285     }
286 
287     void clearTopDownPointers() {
288       PerPtrTopDown.clear();
289     }
290 
291     void InitFromPred(const BBState &Other);
292     void InitFromSucc(const BBState &Other);
293     void MergePred(const BBState &Other);
294     void MergeSucc(const BBState &Other);
295 
296     /// Compute the number of possible unique paths from an entry to an exit
297     /// which pass through this block. This is only valid after both the
298     /// top-down and bottom-up traversals are complete.
299     ///
300     /// Returns true if overflow occurred. Returns false if overflow did not
301     /// occur.
302     bool GetAllPathCountWithOverflow(unsigned &PathCount) const {
303       if (TopDownPathCount == OverflowOccurredValue ||
304           BottomUpPathCount == OverflowOccurredValue)
305         return true;
306       unsigned long long Product =
307         (unsigned long long)TopDownPathCount*BottomUpPathCount;
308       // Overflow occurred if any of the upper bits of Product are set or if all
309       // the lower bits of Product are all set.
310       return (Product >> 32) ||
311              ((PathCount = Product) == OverflowOccurredValue);
312     }
313 
314     // Specialized CFG utilities.
315     using edge_iterator = SmallVectorImpl<BasicBlock *>::const_iterator;
316 
317     edge_iterator pred_begin() const { return Preds.begin(); }
318     edge_iterator pred_end() const { return Preds.end(); }
319     edge_iterator succ_begin() const { return Succs.begin(); }
320     edge_iterator succ_end() const { return Succs.end(); }
321 
322     void addSucc(BasicBlock *Succ) { Succs.push_back(Succ); }
323     void addPred(BasicBlock *Pred) { Preds.push_back(Pred); }
324 
325     bool isExit() const { return Succs.empty(); }
326   };
327 
328 } // end anonymous namespace
329 
330 const unsigned BBState::OverflowOccurredValue = 0xffffffff;
331 
332 namespace llvm {
333 
334 raw_ostream &operator<<(raw_ostream &OS,
335                         BBState &BBState) LLVM_ATTRIBUTE_UNUSED;
336 
337 } // end namespace llvm
338 
339 void BBState::InitFromPred(const BBState &Other) {
340   PerPtrTopDown = Other.PerPtrTopDown;
341   TopDownPathCount = Other.TopDownPathCount;
342 }
343 
344 void BBState::InitFromSucc(const BBState &Other) {
345   PerPtrBottomUp = Other.PerPtrBottomUp;
346   BottomUpPathCount = Other.BottomUpPathCount;
347 }
348 
349 /// The top-down traversal uses this to merge information about predecessors to
350 /// form the initial state for a new block.
351 void BBState::MergePred(const BBState &Other) {
352   if (TopDownPathCount == OverflowOccurredValue)
353     return;
354 
355   // Other.TopDownPathCount can be 0, in which case it is either dead or a
356   // loop backedge. Loop backedges are special.
357   TopDownPathCount += Other.TopDownPathCount;
358 
359   // In order to be consistent, we clear the top down pointers when by adding
360   // TopDownPathCount becomes OverflowOccurredValue even though "true" overflow
361   // has not occurred.
362   if (TopDownPathCount == OverflowOccurredValue) {
363     clearTopDownPointers();
364     return;
365   }
366 
367   // Check for overflow. If we have overflow, fall back to conservative
368   // behavior.
369   if (TopDownPathCount < Other.TopDownPathCount) {
370     TopDownPathCount = OverflowOccurredValue;
371     clearTopDownPointers();
372     return;
373   }
374 
375   // For each entry in the other set, if our set has an entry with the same key,
376   // merge the entries. Otherwise, copy the entry and merge it with an empty
377   // entry.
378   for (auto MI = Other.top_down_ptr_begin(), ME = Other.top_down_ptr_end();
379        MI != ME; ++MI) {
380     auto Pair = PerPtrTopDown.insert(*MI);
381     Pair.first->second.Merge(Pair.second ? TopDownPtrState() : MI->second,
382                              /*TopDown=*/true);
383   }
384 
385   // For each entry in our set, if the other set doesn't have an entry with the
386   // same key, force it to merge with an empty entry.
387   for (auto MI = top_down_ptr_begin(), ME = top_down_ptr_end(); MI != ME; ++MI)
388     if (Other.PerPtrTopDown.find(MI->first) == Other.PerPtrTopDown.end())
389       MI->second.Merge(TopDownPtrState(), /*TopDown=*/true);
390 }
391 
392 /// The bottom-up traversal uses this to merge information about successors to
393 /// form the initial state for a new block.
394 void BBState::MergeSucc(const BBState &Other) {
395   if (BottomUpPathCount == OverflowOccurredValue)
396     return;
397 
398   // Other.BottomUpPathCount can be 0, in which case it is either dead or a
399   // loop backedge. Loop backedges are special.
400   BottomUpPathCount += Other.BottomUpPathCount;
401 
402   // In order to be consistent, we clear the top down pointers when by adding
403   // BottomUpPathCount becomes OverflowOccurredValue even though "true" overflow
404   // has not occurred.
405   if (BottomUpPathCount == OverflowOccurredValue) {
406     clearBottomUpPointers();
407     return;
408   }
409 
410   // Check for overflow. If we have overflow, fall back to conservative
411   // behavior.
412   if (BottomUpPathCount < Other.BottomUpPathCount) {
413     BottomUpPathCount = OverflowOccurredValue;
414     clearBottomUpPointers();
415     return;
416   }
417 
418   // For each entry in the other set, if our set has an entry with the
419   // same key, merge the entries. Otherwise, copy the entry and merge
420   // it with an empty entry.
421   for (auto MI = Other.bottom_up_ptr_begin(), ME = Other.bottom_up_ptr_end();
422        MI != ME; ++MI) {
423     auto Pair = PerPtrBottomUp.insert(*MI);
424     Pair.first->second.Merge(Pair.second ? BottomUpPtrState() : MI->second,
425                              /*TopDown=*/false);
426   }
427 
428   // For each entry in our set, if the other set doesn't have an entry
429   // with the same key, force it to merge with an empty entry.
430   for (auto MI = bottom_up_ptr_begin(), ME = bottom_up_ptr_end(); MI != ME;
431        ++MI)
432     if (Other.PerPtrBottomUp.find(MI->first) == Other.PerPtrBottomUp.end())
433       MI->second.Merge(BottomUpPtrState(), /*TopDown=*/false);
434 }
435 
436 raw_ostream &llvm::operator<<(raw_ostream &OS, BBState &BBInfo) {
437   // Dump the pointers we are tracking.
438   OS << "    TopDown State:\n";
439   if (!BBInfo.hasTopDownPtrs()) {
440     LLVM_DEBUG(dbgs() << "        NONE!\n");
441   } else {
442     for (auto I = BBInfo.top_down_ptr_begin(), E = BBInfo.top_down_ptr_end();
443          I != E; ++I) {
444       const PtrState &P = I->second;
445       OS << "        Ptr: " << *I->first
446          << "\n            KnownSafe:        " << (P.IsKnownSafe()?"true":"false")
447          << "\n            ImpreciseRelease: "
448            << (P.IsTrackingImpreciseReleases()?"true":"false") << "\n"
449          << "            HasCFGHazards:    "
450            << (P.IsCFGHazardAfflicted()?"true":"false") << "\n"
451          << "            KnownPositive:    "
452            << (P.HasKnownPositiveRefCount()?"true":"false") << "\n"
453          << "            Seq:              "
454          << P.GetSeq() << "\n";
455     }
456   }
457 
458   OS << "    BottomUp State:\n";
459   if (!BBInfo.hasBottomUpPtrs()) {
460     LLVM_DEBUG(dbgs() << "        NONE!\n");
461   } else {
462     for (auto I = BBInfo.bottom_up_ptr_begin(), E = BBInfo.bottom_up_ptr_end();
463          I != E; ++I) {
464       const PtrState &P = I->second;
465       OS << "        Ptr: " << *I->first
466          << "\n            KnownSafe:        " << (P.IsKnownSafe()?"true":"false")
467          << "\n            ImpreciseRelease: "
468            << (P.IsTrackingImpreciseReleases()?"true":"false") << "\n"
469          << "            HasCFGHazards:    "
470            << (P.IsCFGHazardAfflicted()?"true":"false") << "\n"
471          << "            KnownPositive:    "
472            << (P.HasKnownPositiveRefCount()?"true":"false") << "\n"
473          << "            Seq:              "
474          << P.GetSeq() << "\n";
475     }
476   }
477 
478   return OS;
479 }
480 
481 namespace {
482 
483   /// The main ARC optimization pass.
484 class ObjCARCOpt {
485   bool Changed;
486   ProvenanceAnalysis PA;
487 
488   /// A cache of references to runtime entry point constants.
489   ARCRuntimeEntryPoints EP;
490 
491   /// A cache of MDKinds that can be passed into other functions to propagate
492   /// MDKind identifiers.
493   ARCMDKindCache MDKindCache;
494 
495   /// A flag indicating whether this optimization pass should run.
496   bool Run;
497 
498   /// A flag indicating whether the optimization that removes or moves
499   /// retain/release pairs should be performed.
500   bool DisableRetainReleasePairing = false;
501 
502   /// Flags which determine whether each of the interesting runtime functions
503   /// is in fact used in the current function.
504   unsigned UsedInThisFunction;
505 
506   bool OptimizeRetainRVCall(Function &F, Instruction *RetainRV);
507   void OptimizeAutoreleaseRVCall(Function &F, Instruction *AutoreleaseRV,
508                                  ARCInstKind &Class);
509   void OptimizeIndividualCalls(Function &F);
510 
511   /// Optimize an individual call, optionally passing the
512   /// GetArgRCIdentityRoot if it has already been computed.
513   void OptimizeIndividualCallImpl(
514       Function &F, DenseMap<BasicBlock *, ColorVector> &BlockColors,
515       Instruction *Inst, ARCInstKind Class, const Value *Arg);
516 
517   /// Try to optimize an AutoreleaseRV with a RetainRV or ClaimRV.  If the
518   /// optimization occurs, returns true to indicate that the caller should
519   /// assume the instructions are dead.
520   bool OptimizeInlinedAutoreleaseRVCall(
521       Function &F, DenseMap<BasicBlock *, ColorVector> &BlockColors,
522       Instruction *Inst, const Value *&Arg, ARCInstKind Class,
523       Instruction *AutoreleaseRV, const Value *&AutoreleaseRVArg);
524 
525   void CheckForCFGHazards(const BasicBlock *BB,
526                           DenseMap<const BasicBlock *, BBState> &BBStates,
527                           BBState &MyStates) const;
528   bool VisitInstructionBottomUp(Instruction *Inst, BasicBlock *BB,
529                                 BlotMapVector<Value *, RRInfo> &Retains,
530                                 BBState &MyStates);
531   bool VisitBottomUp(BasicBlock *BB,
532                      DenseMap<const BasicBlock *, BBState> &BBStates,
533                      BlotMapVector<Value *, RRInfo> &Retains);
534   bool VisitInstructionTopDown(Instruction *Inst,
535                                DenseMap<Value *, RRInfo> &Releases,
536                                BBState &MyStates);
537   bool VisitTopDown(BasicBlock *BB,
538                     DenseMap<const BasicBlock *, BBState> &BBStates,
539                     DenseMap<Value *, RRInfo> &Releases);
540   bool Visit(Function &F, DenseMap<const BasicBlock *, BBState> &BBStates,
541              BlotMapVector<Value *, RRInfo> &Retains,
542              DenseMap<Value *, RRInfo> &Releases);
543 
544   void MoveCalls(Value *Arg, RRInfo &RetainsToMove, RRInfo &ReleasesToMove,
545                  BlotMapVector<Value *, RRInfo> &Retains,
546                  DenseMap<Value *, RRInfo> &Releases,
547                  SmallVectorImpl<Instruction *> &DeadInsts, Module *M);
548 
549   bool PairUpRetainsAndReleases(DenseMap<const BasicBlock *, BBState> &BBStates,
550                                 BlotMapVector<Value *, RRInfo> &Retains,
551                                 DenseMap<Value *, RRInfo> &Releases, Module *M,
552                                 Instruction *Retain,
553                                 SmallVectorImpl<Instruction *> &DeadInsts,
554                                 RRInfo &RetainsToMove, RRInfo &ReleasesToMove,
555                                 Value *Arg, bool KnownSafe,
556                                 bool &AnyPairsCompletelyEliminated);
557 
558   bool PerformCodePlacement(DenseMap<const BasicBlock *, BBState> &BBStates,
559                             BlotMapVector<Value *, RRInfo> &Retains,
560                             DenseMap<Value *, RRInfo> &Releases, Module *M);
561 
562   void OptimizeWeakCalls(Function &F);
563 
564   bool OptimizeSequences(Function &F);
565 
566   void OptimizeReturns(Function &F);
567 
568 #ifndef NDEBUG
569   void GatherStatistics(Function &F, bool AfterOptimization = false);
570 #endif
571 
572   public:
573     void init(Module &M);
574     bool run(Function &F, AAResults &AA);
575     void releaseMemory();
576 };
577 
578 /// The main ARC optimization pass.
579 class ObjCARCOptLegacyPass : public FunctionPass {
580 public:
581   ObjCARCOptLegacyPass() : FunctionPass(ID) {
582     initializeObjCARCOptLegacyPassPass(*PassRegistry::getPassRegistry());
583   }
584   void getAnalysisUsage(AnalysisUsage &AU) const override;
585   bool doInitialization(Module &M) override {
586     OCAO.init(M);
587     return false;
588   }
589   bool runOnFunction(Function &F) override {
590     return OCAO.run(F, getAnalysis<AAResultsWrapperPass>().getAAResults());
591   }
592   void releaseMemory() override { OCAO.releaseMemory(); }
593   static char ID;
594 
595 private:
596   ObjCARCOpt OCAO;
597 };
598 } // end anonymous namespace
599 
600 char ObjCARCOptLegacyPass::ID = 0;
601 
602 INITIALIZE_PASS_BEGIN(ObjCARCOptLegacyPass, "objc-arc", "ObjC ARC optimization",
603                       false, false)
604 INITIALIZE_PASS_DEPENDENCY(ObjCARCAAWrapperPass)
605 INITIALIZE_PASS_END(ObjCARCOptLegacyPass, "objc-arc", "ObjC ARC optimization",
606                     false, false)
607 
608 Pass *llvm::createObjCARCOptPass() { return new ObjCARCOptLegacyPass(); }
609 
610 void ObjCARCOptLegacyPass::getAnalysisUsage(AnalysisUsage &AU) const {
611   AU.addRequired<ObjCARCAAWrapperPass>();
612   AU.addRequired<AAResultsWrapperPass>();
613   // ARC optimization doesn't currently split critical edges.
614   AU.setPreservesCFG();
615 }
616 
617 /// Turn objc_retainAutoreleasedReturnValue into objc_retain if the operand is
618 /// not a return value.
619 bool
620 ObjCARCOpt::OptimizeRetainRVCall(Function &F, Instruction *RetainRV) {
621   // Check for the argument being from an immediately preceding call or invoke.
622   const Value *Arg = GetArgRCIdentityRoot(RetainRV);
623   if (const Instruction *Call = dyn_cast<CallBase>(Arg)) {
624     if (Call->getParent() == RetainRV->getParent()) {
625       BasicBlock::const_iterator I(Call);
626       ++I;
627       while (IsNoopInstruction(&*I))
628         ++I;
629       if (&*I == RetainRV)
630         return false;
631     } else if (const InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
632       BasicBlock *RetainRVParent = RetainRV->getParent();
633       if (II->getNormalDest() == RetainRVParent) {
634         BasicBlock::const_iterator I = RetainRVParent->begin();
635         while (IsNoopInstruction(&*I))
636           ++I;
637         if (&*I == RetainRV)
638           return false;
639       }
640     }
641   }
642 
643   // Turn it to a plain objc_retain.
644   Changed = true;
645   ++NumPeeps;
646 
647   LLVM_DEBUG(dbgs() << "Transforming objc_retainAutoreleasedReturnValue => "
648                        "objc_retain since the operand is not a return value.\n"
649                        "Old = "
650                     << *RetainRV << "\n");
651 
652   Function *NewDecl = EP.get(ARCRuntimeEntryPointKind::Retain);
653   cast<CallInst>(RetainRV)->setCalledFunction(NewDecl);
654 
655   LLVM_DEBUG(dbgs() << "New = " << *RetainRV << "\n");
656 
657   return false;
658 }
659 
660 bool ObjCARCOpt::OptimizeInlinedAutoreleaseRVCall(
661     Function &F, DenseMap<BasicBlock *, ColorVector> &BlockColors,
662     Instruction *Inst, const Value *&Arg, ARCInstKind Class,
663     Instruction *AutoreleaseRV, const Value *&AutoreleaseRVArg) {
664   // Must be in the same basic block.
665   assert(Inst->getParent() == AutoreleaseRV->getParent());
666 
667   // Must operate on the same root.
668   Arg = GetArgRCIdentityRoot(Inst);
669   AutoreleaseRVArg = GetArgRCIdentityRoot(AutoreleaseRV);
670   if (Arg != AutoreleaseRVArg) {
671     // If there isn't an exact match, check if we have equivalent PHIs.
672     const PHINode *PN = dyn_cast<PHINode>(Arg);
673     if (!PN)
674       return false;
675 
676     SmallVector<const Value *, 4> ArgUsers;
677     getEquivalentPHIs(*PN, ArgUsers);
678     if (!llvm::is_contained(ArgUsers, AutoreleaseRVArg))
679       return false;
680   }
681 
682   // Okay, this is a match.  Merge them.
683   ++NumPeeps;
684   LLVM_DEBUG(dbgs() << "Found inlined objc_autoreleaseReturnValue '"
685                     << *AutoreleaseRV << "' paired with '" << *Inst << "'\n");
686 
687   // Delete the RV pair, starting with the AutoreleaseRV.
688   AutoreleaseRV->replaceAllUsesWith(
689       cast<CallInst>(AutoreleaseRV)->getArgOperand(0));
690   Changed = true;
691   EraseInstruction(AutoreleaseRV);
692   if (Class == ARCInstKind::RetainRV) {
693     // AutoreleaseRV and RetainRV cancel out.  Delete the RetainRV.
694     Inst->replaceAllUsesWith(cast<CallInst>(Inst)->getArgOperand(0));
695     EraseInstruction(Inst);
696     return true;
697   }
698 
699   // ClaimRV is a frontend peephole for RetainRV + Release.  Since the
700   // AutoreleaseRV and RetainRV cancel out, replace the ClaimRV with a Release.
701   assert(Class == ARCInstKind::ClaimRV);
702   Value *CallArg = cast<CallInst>(Inst)->getArgOperand(0);
703   CallInst *Release = CallInst::Create(
704       EP.get(ARCRuntimeEntryPointKind::Release), CallArg, "", Inst);
705   assert(IsAlwaysTail(ARCInstKind::ClaimRV) &&
706          "Expected ClaimRV to be safe to tail call");
707   Release->setTailCall();
708   Inst->replaceAllUsesWith(CallArg);
709   EraseInstruction(Inst);
710 
711   // Run the normal optimizations on Release.
712   OptimizeIndividualCallImpl(F, BlockColors, Release, ARCInstKind::Release,
713                              Arg);
714   return true;
715 }
716 
717 /// Turn objc_autoreleaseReturnValue into objc_autorelease if the result is not
718 /// used as a return value.
719 void ObjCARCOpt::OptimizeAutoreleaseRVCall(Function &F,
720                                            Instruction *AutoreleaseRV,
721                                            ARCInstKind &Class) {
722   // Check for a return of the pointer value.
723   const Value *Ptr = GetArgRCIdentityRoot(AutoreleaseRV);
724 
725   // If the argument is ConstantPointerNull or UndefValue, its other users
726   // aren't actually interesting to look at.
727   if (isa<ConstantData>(Ptr))
728     return;
729 
730   SmallVector<const Value *, 2> Users;
731   Users.push_back(Ptr);
732 
733   // Add PHIs that are equivalent to Ptr to Users.
734   if (const PHINode *PN = dyn_cast<PHINode>(Ptr))
735     getEquivalentPHIs(*PN, Users);
736 
737   do {
738     Ptr = Users.pop_back_val();
739     for (const User *U : Ptr->users()) {
740       if (isa<ReturnInst>(U) || GetBasicARCInstKind(U) == ARCInstKind::RetainRV)
741         return;
742       if (isa<BitCastInst>(U))
743         Users.push_back(U);
744     }
745   } while (!Users.empty());
746 
747   Changed = true;
748   ++NumPeeps;
749 
750   LLVM_DEBUG(
751       dbgs() << "Transforming objc_autoreleaseReturnValue => "
752                 "objc_autorelease since its operand is not used as a return "
753                 "value.\n"
754                 "Old = "
755              << *AutoreleaseRV << "\n");
756 
757   CallInst *AutoreleaseRVCI = cast<CallInst>(AutoreleaseRV);
758   Function *NewDecl = EP.get(ARCRuntimeEntryPointKind::Autorelease);
759   AutoreleaseRVCI->setCalledFunction(NewDecl);
760   AutoreleaseRVCI->setTailCall(false); // Never tail call objc_autorelease.
761   Class = ARCInstKind::Autorelease;
762 
763   LLVM_DEBUG(dbgs() << "New: " << *AutoreleaseRV << "\n");
764 }
765 
766 namespace {
767 Instruction *
768 CloneCallInstForBB(CallInst &CI, BasicBlock &BB,
769                    const DenseMap<BasicBlock *, ColorVector> &BlockColors) {
770   SmallVector<OperandBundleDef, 1> OpBundles;
771   for (unsigned I = 0, E = CI.getNumOperandBundles(); I != E; ++I) {
772     auto Bundle = CI.getOperandBundleAt(I);
773     // Funclets will be reassociated in the future.
774     if (Bundle.getTagID() == LLVMContext::OB_funclet)
775       continue;
776     OpBundles.emplace_back(Bundle);
777   }
778 
779   if (!BlockColors.empty()) {
780     const ColorVector &CV = BlockColors.find(&BB)->second;
781     assert(CV.size() == 1 && "non-unique color for block!");
782     Instruction *EHPad = CV.front()->getFirstNonPHI();
783     if (EHPad->isEHPad())
784       OpBundles.emplace_back("funclet", EHPad);
785   }
786 
787   return CallInst::Create(&CI, OpBundles);
788 }
789 }
790 
791 /// Visit each call, one at a time, and make simplifications without doing any
792 /// additional analysis.
793 void ObjCARCOpt::OptimizeIndividualCalls(Function &F) {
794   LLVM_DEBUG(dbgs() << "\n== ObjCARCOpt::OptimizeIndividualCalls ==\n");
795   // Reset all the flags in preparation for recomputing them.
796   UsedInThisFunction = 0;
797 
798   DenseMap<BasicBlock *, ColorVector> BlockColors;
799   if (F.hasPersonalityFn() &&
800       isScopedEHPersonality(classifyEHPersonality(F.getPersonalityFn())))
801     BlockColors = colorEHFunclets(F);
802 
803   // Store any delayed AutoreleaseRV intrinsics, so they can be easily paired
804   // with RetainRV and ClaimRV.
805   Instruction *DelayedAutoreleaseRV = nullptr;
806   const Value *DelayedAutoreleaseRVArg = nullptr;
807   auto setDelayedAutoreleaseRV = [&](Instruction *AutoreleaseRV) {
808     assert(!DelayedAutoreleaseRV || !AutoreleaseRV);
809     DelayedAutoreleaseRV = AutoreleaseRV;
810     DelayedAutoreleaseRVArg = nullptr;
811   };
812   auto optimizeDelayedAutoreleaseRV = [&]() {
813     if (!DelayedAutoreleaseRV)
814       return;
815     OptimizeIndividualCallImpl(F, BlockColors, DelayedAutoreleaseRV,
816                                ARCInstKind::AutoreleaseRV,
817                                DelayedAutoreleaseRVArg);
818     setDelayedAutoreleaseRV(nullptr);
819   };
820   auto shouldDelayAutoreleaseRV = [&](Instruction *NonARCInst) {
821     // Nothing to delay, but we may as well skip the logic below.
822     if (!DelayedAutoreleaseRV)
823       return true;
824 
825     // If we hit the end of the basic block we're not going to find an RV-pair.
826     // Stop delaying.
827     if (NonARCInst->isTerminator())
828       return false;
829 
830     // Given the frontend rules for emitting AutoreleaseRV, RetainRV, and
831     // ClaimRV, it's probably safe to skip over even opaque function calls
832     // here since OptimizeInlinedAutoreleaseRVCall will confirm that they
833     // have the same RCIdentityRoot.  However, what really matters is
834     // skipping instructions or intrinsics that the inliner could leave behind;
835     // be conservative for now and don't skip over opaque calls, which could
836     // potentially include other ARC calls.
837     auto *CB = dyn_cast<CallBase>(NonARCInst);
838     if (!CB)
839       return true;
840     return CB->getIntrinsicID() != Intrinsic::not_intrinsic;
841   };
842 
843   // Visit all objc_* calls in F.
844   for (inst_iterator I = inst_begin(&F), E = inst_end(&F); I != E; ) {
845     Instruction *Inst = &*I++;
846 
847     ARCInstKind Class = GetBasicARCInstKind(Inst);
848 
849     // Skip this loop if this instruction isn't itself an ARC intrinsic.
850     const Value *Arg = nullptr;
851     switch (Class) {
852     default:
853       optimizeDelayedAutoreleaseRV();
854       break;
855     case ARCInstKind::CallOrUser:
856     case ARCInstKind::User:
857     case ARCInstKind::None:
858       // This is a non-ARC instruction.  If we're delaying an AutoreleaseRV,
859       // check if it's safe to skip over it; if not, optimize the AutoreleaseRV
860       // now.
861       if (!shouldDelayAutoreleaseRV(Inst))
862         optimizeDelayedAutoreleaseRV();
863       continue;
864     case ARCInstKind::AutoreleaseRV:
865       optimizeDelayedAutoreleaseRV();
866       setDelayedAutoreleaseRV(Inst);
867       continue;
868     case ARCInstKind::RetainRV:
869     case ARCInstKind::ClaimRV:
870       if (DelayedAutoreleaseRV) {
871         // We have a potential RV pair.  Check if they cancel out.
872         if (OptimizeInlinedAutoreleaseRVCall(F, BlockColors, Inst, Arg, Class,
873                                              DelayedAutoreleaseRV,
874                                              DelayedAutoreleaseRVArg)) {
875           setDelayedAutoreleaseRV(nullptr);
876           continue;
877         }
878         optimizeDelayedAutoreleaseRV();
879       }
880       break;
881     }
882 
883     OptimizeIndividualCallImpl(F, BlockColors, Inst, Class, Arg);
884   }
885 
886   // Catch the final delayed AutoreleaseRV.
887   optimizeDelayedAutoreleaseRV();
888 }
889 
890 /// This function returns true if the value is inert. An ObjC ARC runtime call
891 /// taking an inert operand can be safely deleted.
892 static bool isInertARCValue(Value *V, SmallPtrSet<Value *, 1> &VisitedPhis) {
893   V = V->stripPointerCasts();
894 
895   if (IsNullOrUndef(V))
896     return true;
897 
898   // See if this is a global attribute annotated with an 'objc_arc_inert'.
899   if (auto *GV = dyn_cast<GlobalVariable>(V))
900     if (GV->hasAttribute("objc_arc_inert"))
901       return true;
902 
903   if (auto PN = dyn_cast<PHINode>(V)) {
904     // Ignore this phi if it has already been discovered.
905     if (!VisitedPhis.insert(PN).second)
906       return true;
907     // Look through phis's operands.
908     for (Value *Opnd : PN->incoming_values())
909       if (!isInertARCValue(Opnd, VisitedPhis))
910         return false;
911     return true;
912   }
913 
914   return false;
915 }
916 
917 void ObjCARCOpt::OptimizeIndividualCallImpl(
918     Function &F, DenseMap<BasicBlock *, ColorVector> &BlockColors,
919     Instruction *Inst, ARCInstKind Class, const Value *Arg) {
920   LLVM_DEBUG(dbgs() << "Visiting: Class: " << Class << "; " << *Inst << "\n");
921 
922   // We can delete this call if it takes an inert value.
923   SmallPtrSet<Value *, 1> VisitedPhis;
924 
925   if (IsNoopOnGlobal(Class))
926     if (isInertARCValue(Inst->getOperand(0), VisitedPhis)) {
927       if (!Inst->getType()->isVoidTy())
928         Inst->replaceAllUsesWith(Inst->getOperand(0));
929       Inst->eraseFromParent();
930       Changed = true;
931       return;
932     }
933 
934   switch (Class) {
935   default:
936     break;
937 
938   // Delete no-op casts. These function calls have special semantics, but
939   // the semantics are entirely implemented via lowering in the front-end,
940   // so by the time they reach the optimizer, they are just no-op calls
941   // which return their argument.
942   //
943   // There are gray areas here, as the ability to cast reference-counted
944   // pointers to raw void* and back allows code to break ARC assumptions,
945   // however these are currently considered to be unimportant.
946   case ARCInstKind::NoopCast:
947     Changed = true;
948     ++NumNoops;
949     LLVM_DEBUG(dbgs() << "Erasing no-op cast: " << *Inst << "\n");
950     EraseInstruction(Inst);
951     return;
952 
953   // If the pointer-to-weak-pointer is null, it's undefined behavior.
954   case ARCInstKind::StoreWeak:
955   case ARCInstKind::LoadWeak:
956   case ARCInstKind::LoadWeakRetained:
957   case ARCInstKind::InitWeak:
958   case ARCInstKind::DestroyWeak: {
959     CallInst *CI = cast<CallInst>(Inst);
960     if (IsNullOrUndef(CI->getArgOperand(0))) {
961       Changed = true;
962       Type *Ty = CI->getArgOperand(0)->getType();
963       new StoreInst(UndefValue::get(cast<PointerType>(Ty)->getElementType()),
964                     Constant::getNullValue(Ty), CI);
965       Value *NewValue = UndefValue::get(CI->getType());
966       LLVM_DEBUG(
967           dbgs() << "A null pointer-to-weak-pointer is undefined behavior."
968                     "\nOld = "
969                  << *CI << "\nNew = " << *NewValue << "\n");
970       CI->replaceAllUsesWith(NewValue);
971       CI->eraseFromParent();
972       return;
973     }
974     break;
975   }
976   case ARCInstKind::CopyWeak:
977   case ARCInstKind::MoveWeak: {
978     CallInst *CI = cast<CallInst>(Inst);
979     if (IsNullOrUndef(CI->getArgOperand(0)) ||
980         IsNullOrUndef(CI->getArgOperand(1))) {
981       Changed = true;
982       Type *Ty = CI->getArgOperand(0)->getType();
983       new StoreInst(UndefValue::get(cast<PointerType>(Ty)->getElementType()),
984                     Constant::getNullValue(Ty), CI);
985 
986       Value *NewValue = UndefValue::get(CI->getType());
987       LLVM_DEBUG(
988           dbgs() << "A null pointer-to-weak-pointer is undefined behavior."
989                     "\nOld = "
990                  << *CI << "\nNew = " << *NewValue << "\n");
991 
992       CI->replaceAllUsesWith(NewValue);
993       CI->eraseFromParent();
994       return;
995     }
996     break;
997   }
998   case ARCInstKind::RetainRV:
999     if (OptimizeRetainRVCall(F, Inst))
1000       return;
1001     break;
1002   case ARCInstKind::AutoreleaseRV:
1003     OptimizeAutoreleaseRVCall(F, Inst, Class);
1004     break;
1005   }
1006 
1007   // objc_autorelease(x) -> objc_release(x) if x is otherwise unused.
1008   if (IsAutorelease(Class) && Inst->use_empty()) {
1009     CallInst *Call = cast<CallInst>(Inst);
1010     const Value *Arg = Call->getArgOperand(0);
1011     Arg = FindSingleUseIdentifiedObject(Arg);
1012     if (Arg) {
1013       Changed = true;
1014       ++NumAutoreleases;
1015 
1016       // Create the declaration lazily.
1017       LLVMContext &C = Inst->getContext();
1018 
1019       Function *Decl = EP.get(ARCRuntimeEntryPointKind::Release);
1020       CallInst *NewCall =
1021           CallInst::Create(Decl, Call->getArgOperand(0), "", Call);
1022       NewCall->setMetadata(MDKindCache.get(ARCMDKindID::ImpreciseRelease),
1023                            MDNode::get(C, None));
1024 
1025       LLVM_DEBUG(dbgs() << "Replacing autorelease{,RV}(x) with objc_release(x) "
1026                            "since x is otherwise unused.\nOld: "
1027                         << *Call << "\nNew: " << *NewCall << "\n");
1028 
1029       EraseInstruction(Call);
1030       Inst = NewCall;
1031       Class = ARCInstKind::Release;
1032     }
1033   }
1034 
1035   // For functions which can never be passed stack arguments, add
1036   // a tail keyword.
1037   if (IsAlwaysTail(Class) && !cast<CallInst>(Inst)->isNoTailCall()) {
1038     Changed = true;
1039     LLVM_DEBUG(
1040         dbgs() << "Adding tail keyword to function since it can never be "
1041                   "passed stack args: "
1042                << *Inst << "\n");
1043     cast<CallInst>(Inst)->setTailCall();
1044   }
1045 
1046   // Ensure that functions that can never have a "tail" keyword due to the
1047   // semantics of ARC truly do not do so.
1048   if (IsNeverTail(Class)) {
1049     Changed = true;
1050     LLVM_DEBUG(dbgs() << "Removing tail keyword from function: " << *Inst
1051                       << "\n");
1052     cast<CallInst>(Inst)->setTailCall(false);
1053   }
1054 
1055   // Set nounwind as needed.
1056   if (IsNoThrow(Class)) {
1057     Changed = true;
1058     LLVM_DEBUG(dbgs() << "Found no throw class. Setting nounwind on: " << *Inst
1059                       << "\n");
1060     cast<CallInst>(Inst)->setDoesNotThrow();
1061   }
1062 
1063   // Note: This catches instructions unrelated to ARC.
1064   if (!IsNoopOnNull(Class)) {
1065     UsedInThisFunction |= 1 << unsigned(Class);
1066     return;
1067   }
1068 
1069   // If we haven't already looked up the root, look it up now.
1070   if (!Arg)
1071     Arg = GetArgRCIdentityRoot(Inst);
1072 
1073   // ARC calls with null are no-ops. Delete them.
1074   if (IsNullOrUndef(Arg)) {
1075     Changed = true;
1076     ++NumNoops;
1077     LLVM_DEBUG(dbgs() << "ARC calls with  null are no-ops. Erasing: " << *Inst
1078                       << "\n");
1079     EraseInstruction(Inst);
1080     return;
1081   }
1082 
1083   // Keep track of which of retain, release, autorelease, and retain_block
1084   // are actually present in this function.
1085   UsedInThisFunction |= 1 << unsigned(Class);
1086 
1087   // If Arg is a PHI, and one or more incoming values to the
1088   // PHI are null, and the call is control-equivalent to the PHI, and there
1089   // are no relevant side effects between the PHI and the call, and the call
1090   // is not a release that doesn't have the clang.imprecise_release tag, the
1091   // call could be pushed up to just those paths with non-null incoming
1092   // values. For now, don't bother splitting critical edges for this.
1093   if (Class == ARCInstKind::Release &&
1094       !Inst->getMetadata(MDKindCache.get(ARCMDKindID::ImpreciseRelease)))
1095     return;
1096 
1097   SmallVector<std::pair<Instruction *, const Value *>, 4> Worklist;
1098   Worklist.push_back(std::make_pair(Inst, Arg));
1099   do {
1100     std::pair<Instruction *, const Value *> Pair = Worklist.pop_back_val();
1101     Inst = Pair.first;
1102     Arg = Pair.second;
1103 
1104     const PHINode *PN = dyn_cast<PHINode>(Arg);
1105     if (!PN)
1106       continue;
1107 
1108     // Determine if the PHI has any null operands, or any incoming
1109     // critical edges.
1110     bool HasNull = false;
1111     bool HasCriticalEdges = false;
1112     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1113       Value *Incoming = GetRCIdentityRoot(PN->getIncomingValue(i));
1114       if (IsNullOrUndef(Incoming))
1115         HasNull = true;
1116       else if (PN->getIncomingBlock(i)->getTerminator()->getNumSuccessors() !=
1117                1) {
1118         HasCriticalEdges = true;
1119         break;
1120       }
1121     }
1122     // If we have null operands and no critical edges, optimize.
1123     if (HasCriticalEdges)
1124       continue;
1125     if (!HasNull)
1126       continue;
1127 
1128     Instruction *DepInst = nullptr;
1129 
1130     // Check that there is nothing that cares about the reference
1131     // count between the call and the phi.
1132     switch (Class) {
1133     case ARCInstKind::Retain:
1134     case ARCInstKind::RetainBlock:
1135       // These can always be moved up.
1136       break;
1137     case ARCInstKind::Release:
1138       // These can't be moved across things that care about the retain
1139       // count.
1140       DepInst = findSingleDependency(NeedsPositiveRetainCount, Arg,
1141                                      Inst->getParent(), Inst, PA);
1142       break;
1143     case ARCInstKind::Autorelease:
1144       // These can't be moved across autorelease pool scope boundaries.
1145       DepInst = findSingleDependency(AutoreleasePoolBoundary, Arg,
1146                                      Inst->getParent(), Inst, PA);
1147       break;
1148     case ARCInstKind::ClaimRV:
1149     case ARCInstKind::RetainRV:
1150     case ARCInstKind::AutoreleaseRV:
1151       // Don't move these; the RV optimization depends on the autoreleaseRV
1152       // being tail called, and the retainRV being immediately after a call
1153       // (which might still happen if we get lucky with codegen layout, but
1154       // it's not worth taking the chance).
1155       continue;
1156     default:
1157       llvm_unreachable("Invalid dependence flavor");
1158     }
1159 
1160     if (DepInst != PN)
1161       continue;
1162 
1163     Changed = true;
1164     ++NumPartialNoops;
1165     // Clone the call into each predecessor that has a non-null value.
1166     CallInst *CInst = cast<CallInst>(Inst);
1167     Type *ParamTy = CInst->getArgOperand(0)->getType();
1168     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1169       Value *Incoming = GetRCIdentityRoot(PN->getIncomingValue(i));
1170       if (IsNullOrUndef(Incoming))
1171         continue;
1172       Value *Op = PN->getIncomingValue(i);
1173       Instruction *InsertPos = &PN->getIncomingBlock(i)->back();
1174       CallInst *Clone = cast<CallInst>(
1175           CloneCallInstForBB(*CInst, *InsertPos->getParent(), BlockColors));
1176       if (Op->getType() != ParamTy)
1177         Op = new BitCastInst(Op, ParamTy, "", InsertPos);
1178       Clone->setArgOperand(0, Op);
1179       Clone->insertBefore(InsertPos);
1180 
1181       LLVM_DEBUG(dbgs() << "Cloning " << *CInst << "\n"
1182                                                    "And inserting clone at "
1183                         << *InsertPos << "\n");
1184       Worklist.push_back(std::make_pair(Clone, Incoming));
1185     }
1186     // Erase the original call.
1187     LLVM_DEBUG(dbgs() << "Erasing: " << *CInst << "\n");
1188     EraseInstruction(CInst);
1189   } while (!Worklist.empty());
1190 }
1191 
1192 /// If we have a top down pointer in the S_Use state, make sure that there are
1193 /// no CFG hazards by checking the states of various bottom up pointers.
1194 static void CheckForUseCFGHazard(const Sequence SuccSSeq,
1195                                  const bool SuccSRRIKnownSafe,
1196                                  TopDownPtrState &S,
1197                                  bool &SomeSuccHasSame,
1198                                  bool &AllSuccsHaveSame,
1199                                  bool &NotAllSeqEqualButKnownSafe,
1200                                  bool &ShouldContinue) {
1201   switch (SuccSSeq) {
1202   case S_CanRelease: {
1203     if (!S.IsKnownSafe() && !SuccSRRIKnownSafe) {
1204       S.ClearSequenceProgress();
1205       break;
1206     }
1207     S.SetCFGHazardAfflicted(true);
1208     ShouldContinue = true;
1209     break;
1210   }
1211   case S_Use:
1212     SomeSuccHasSame = true;
1213     break;
1214   case S_Stop:
1215   case S_Release:
1216   case S_MovableRelease:
1217     if (!S.IsKnownSafe() && !SuccSRRIKnownSafe)
1218       AllSuccsHaveSame = false;
1219     else
1220       NotAllSeqEqualButKnownSafe = true;
1221     break;
1222   case S_Retain:
1223     llvm_unreachable("bottom-up pointer in retain state!");
1224   case S_None:
1225     llvm_unreachable("This should have been handled earlier.");
1226   }
1227 }
1228 
1229 /// If we have a Top Down pointer in the S_CanRelease state, make sure that
1230 /// there are no CFG hazards by checking the states of various bottom up
1231 /// pointers.
1232 static void CheckForCanReleaseCFGHazard(const Sequence SuccSSeq,
1233                                         const bool SuccSRRIKnownSafe,
1234                                         TopDownPtrState &S,
1235                                         bool &SomeSuccHasSame,
1236                                         bool &AllSuccsHaveSame,
1237                                         bool &NotAllSeqEqualButKnownSafe) {
1238   switch (SuccSSeq) {
1239   case S_CanRelease:
1240     SomeSuccHasSame = true;
1241     break;
1242   case S_Stop:
1243   case S_Release:
1244   case S_MovableRelease:
1245   case S_Use:
1246     if (!S.IsKnownSafe() && !SuccSRRIKnownSafe)
1247       AllSuccsHaveSame = false;
1248     else
1249       NotAllSeqEqualButKnownSafe = true;
1250     break;
1251   case S_Retain:
1252     llvm_unreachable("bottom-up pointer in retain state!");
1253   case S_None:
1254     llvm_unreachable("This should have been handled earlier.");
1255   }
1256 }
1257 
1258 /// Check for critical edges, loop boundaries, irreducible control flow, or
1259 /// other CFG structures where moving code across the edge would result in it
1260 /// being executed more.
1261 void
1262 ObjCARCOpt::CheckForCFGHazards(const BasicBlock *BB,
1263                                DenseMap<const BasicBlock *, BBState> &BBStates,
1264                                BBState &MyStates) const {
1265   // If any top-down local-use or possible-dec has a succ which is earlier in
1266   // the sequence, forget it.
1267   for (auto I = MyStates.top_down_ptr_begin(), E = MyStates.top_down_ptr_end();
1268        I != E; ++I) {
1269     TopDownPtrState &S = I->second;
1270     const Sequence Seq = I->second.GetSeq();
1271 
1272     // We only care about S_Retain, S_CanRelease, and S_Use.
1273     if (Seq == S_None)
1274       continue;
1275 
1276     // Make sure that if extra top down states are added in the future that this
1277     // code is updated to handle it.
1278     assert((Seq == S_Retain || Seq == S_CanRelease || Seq == S_Use) &&
1279            "Unknown top down sequence state.");
1280 
1281     const Value *Arg = I->first;
1282     bool SomeSuccHasSame = false;
1283     bool AllSuccsHaveSame = true;
1284     bool NotAllSeqEqualButKnownSafe = false;
1285 
1286     for (const BasicBlock *Succ : successors(BB)) {
1287       // If VisitBottomUp has pointer information for this successor, take
1288       // what we know about it.
1289       const DenseMap<const BasicBlock *, BBState>::iterator BBI =
1290           BBStates.find(Succ);
1291       assert(BBI != BBStates.end());
1292       const BottomUpPtrState &SuccS = BBI->second.getPtrBottomUpState(Arg);
1293       const Sequence SuccSSeq = SuccS.GetSeq();
1294 
1295       // If bottom up, the pointer is in an S_None state, clear the sequence
1296       // progress since the sequence in the bottom up state finished
1297       // suggesting a mismatch in between retains/releases. This is true for
1298       // all three cases that we are handling here: S_Retain, S_Use, and
1299       // S_CanRelease.
1300       if (SuccSSeq == S_None) {
1301         S.ClearSequenceProgress();
1302         continue;
1303       }
1304 
1305       // If we have S_Use or S_CanRelease, perform our check for cfg hazard
1306       // checks.
1307       const bool SuccSRRIKnownSafe = SuccS.IsKnownSafe();
1308 
1309       // *NOTE* We do not use Seq from above here since we are allowing for
1310       // S.GetSeq() to change while we are visiting basic blocks.
1311       switch(S.GetSeq()) {
1312       case S_Use: {
1313         bool ShouldContinue = false;
1314         CheckForUseCFGHazard(SuccSSeq, SuccSRRIKnownSafe, S, SomeSuccHasSame,
1315                              AllSuccsHaveSame, NotAllSeqEqualButKnownSafe,
1316                              ShouldContinue);
1317         if (ShouldContinue)
1318           continue;
1319         break;
1320       }
1321       case S_CanRelease:
1322         CheckForCanReleaseCFGHazard(SuccSSeq, SuccSRRIKnownSafe, S,
1323                                     SomeSuccHasSame, AllSuccsHaveSame,
1324                                     NotAllSeqEqualButKnownSafe);
1325         break;
1326       case S_Retain:
1327       case S_None:
1328       case S_Stop:
1329       case S_Release:
1330       case S_MovableRelease:
1331         break;
1332       }
1333     }
1334 
1335     // If the state at the other end of any of the successor edges
1336     // matches the current state, require all edges to match. This
1337     // guards against loops in the middle of a sequence.
1338     if (SomeSuccHasSame && !AllSuccsHaveSame) {
1339       S.ClearSequenceProgress();
1340     } else if (NotAllSeqEqualButKnownSafe) {
1341       // If we would have cleared the state foregoing the fact that we are known
1342       // safe, stop code motion. This is because whether or not it is safe to
1343       // remove RR pairs via KnownSafe is an orthogonal concept to whether we
1344       // are allowed to perform code motion.
1345       S.SetCFGHazardAfflicted(true);
1346     }
1347   }
1348 }
1349 
1350 bool ObjCARCOpt::VisitInstructionBottomUp(
1351     Instruction *Inst, BasicBlock *BB, BlotMapVector<Value *, RRInfo> &Retains,
1352     BBState &MyStates) {
1353   bool NestingDetected = false;
1354   ARCInstKind Class = GetARCInstKind(Inst);
1355   const Value *Arg = nullptr;
1356 
1357   LLVM_DEBUG(dbgs() << "        Class: " << Class << "\n");
1358 
1359   switch (Class) {
1360   case ARCInstKind::Release: {
1361     Arg = GetArgRCIdentityRoot(Inst);
1362 
1363     BottomUpPtrState &S = MyStates.getPtrBottomUpState(Arg);
1364     NestingDetected |= S.InitBottomUp(MDKindCache, Inst);
1365     break;
1366   }
1367   case ARCInstKind::RetainBlock:
1368     // In OptimizeIndividualCalls, we have strength reduced all optimizable
1369     // objc_retainBlocks to objc_retains. Thus at this point any
1370     // objc_retainBlocks that we see are not optimizable.
1371     break;
1372   case ARCInstKind::Retain:
1373   case ARCInstKind::RetainRV: {
1374     Arg = GetArgRCIdentityRoot(Inst);
1375     BottomUpPtrState &S = MyStates.getPtrBottomUpState(Arg);
1376     if (S.MatchWithRetain()) {
1377       // Don't do retain+release tracking for ARCInstKind::RetainRV, because
1378       // it's better to let it remain as the first instruction after a call.
1379       if (Class != ARCInstKind::RetainRV) {
1380         LLVM_DEBUG(dbgs() << "        Matching with: " << *Inst << "\n");
1381         Retains[Inst] = S.GetRRInfo();
1382       }
1383       S.ClearSequenceProgress();
1384     }
1385     // A retain moving bottom up can be a use.
1386     break;
1387   }
1388   case ARCInstKind::AutoreleasepoolPop:
1389     // Conservatively, clear MyStates for all known pointers.
1390     MyStates.clearBottomUpPointers();
1391     return NestingDetected;
1392   case ARCInstKind::AutoreleasepoolPush:
1393   case ARCInstKind::None:
1394     // These are irrelevant.
1395     return NestingDetected;
1396   default:
1397     break;
1398   }
1399 
1400   // Consider any other possible effects of this instruction on each
1401   // pointer being tracked.
1402   for (auto MI = MyStates.bottom_up_ptr_begin(),
1403             ME = MyStates.bottom_up_ptr_end();
1404        MI != ME; ++MI) {
1405     const Value *Ptr = MI->first;
1406     if (Ptr == Arg)
1407       continue; // Handled above.
1408     BottomUpPtrState &S = MI->second;
1409 
1410     if (S.HandlePotentialAlterRefCount(Inst, Ptr, PA, Class))
1411       continue;
1412 
1413     S.HandlePotentialUse(BB, Inst, Ptr, PA, Class);
1414   }
1415 
1416   return NestingDetected;
1417 }
1418 
1419 bool ObjCARCOpt::VisitBottomUp(BasicBlock *BB,
1420                                DenseMap<const BasicBlock *, BBState> &BBStates,
1421                                BlotMapVector<Value *, RRInfo> &Retains) {
1422   LLVM_DEBUG(dbgs() << "\n== ObjCARCOpt::VisitBottomUp ==\n");
1423 
1424   bool NestingDetected = false;
1425   BBState &MyStates = BBStates[BB];
1426 
1427   // Merge the states from each successor to compute the initial state
1428   // for the current block.
1429   BBState::edge_iterator SI(MyStates.succ_begin()),
1430                          SE(MyStates.succ_end());
1431   if (SI != SE) {
1432     const BasicBlock *Succ = *SI;
1433     DenseMap<const BasicBlock *, BBState>::iterator I = BBStates.find(Succ);
1434     assert(I != BBStates.end());
1435     MyStates.InitFromSucc(I->second);
1436     ++SI;
1437     for (; SI != SE; ++SI) {
1438       Succ = *SI;
1439       I = BBStates.find(Succ);
1440       assert(I != BBStates.end());
1441       MyStates.MergeSucc(I->second);
1442     }
1443   }
1444 
1445   LLVM_DEBUG(dbgs() << "Before:\n"
1446                     << BBStates[BB] << "\n"
1447                     << "Performing Dataflow:\n");
1448 
1449   // Visit all the instructions, bottom-up.
1450   for (BasicBlock::iterator I = BB->end(), E = BB->begin(); I != E; --I) {
1451     Instruction *Inst = &*std::prev(I);
1452 
1453     // Invoke instructions are visited as part of their successors (below).
1454     if (isa<InvokeInst>(Inst))
1455       continue;
1456 
1457     LLVM_DEBUG(dbgs() << "    Visiting " << *Inst << "\n");
1458 
1459     NestingDetected |= VisitInstructionBottomUp(Inst, BB, Retains, MyStates);
1460 
1461     // Bail out if the number of pointers being tracked becomes too large so
1462     // that this pass can complete in a reasonable amount of time.
1463     if (MyStates.bottom_up_ptr_list_size() > MaxPtrStates) {
1464       DisableRetainReleasePairing = true;
1465       return false;
1466     }
1467   }
1468 
1469   // If there's a predecessor with an invoke, visit the invoke as if it were
1470   // part of this block, since we can't insert code after an invoke in its own
1471   // block, and we don't want to split critical edges.
1472   for (BBState::edge_iterator PI(MyStates.pred_begin()),
1473        PE(MyStates.pred_end()); PI != PE; ++PI) {
1474     BasicBlock *Pred = *PI;
1475     if (InvokeInst *II = dyn_cast<InvokeInst>(&Pred->back()))
1476       NestingDetected |= VisitInstructionBottomUp(II, BB, Retains, MyStates);
1477   }
1478 
1479   LLVM_DEBUG(dbgs() << "\nFinal State:\n" << BBStates[BB] << "\n");
1480 
1481   return NestingDetected;
1482 }
1483 
1484 bool
1485 ObjCARCOpt::VisitInstructionTopDown(Instruction *Inst,
1486                                     DenseMap<Value *, RRInfo> &Releases,
1487                                     BBState &MyStates) {
1488   bool NestingDetected = false;
1489   ARCInstKind Class = GetARCInstKind(Inst);
1490   const Value *Arg = nullptr;
1491 
1492   LLVM_DEBUG(dbgs() << "        Class: " << Class << "\n");
1493 
1494   switch (Class) {
1495   case ARCInstKind::RetainBlock:
1496     // In OptimizeIndividualCalls, we have strength reduced all optimizable
1497     // objc_retainBlocks to objc_retains. Thus at this point any
1498     // objc_retainBlocks that we see are not optimizable. We need to break since
1499     // a retain can be a potential use.
1500     break;
1501   case ARCInstKind::Retain:
1502   case ARCInstKind::RetainRV: {
1503     Arg = GetArgRCIdentityRoot(Inst);
1504     TopDownPtrState &S = MyStates.getPtrTopDownState(Arg);
1505     NestingDetected |= S.InitTopDown(Class, Inst);
1506     // A retain can be a potential use; proceed to the generic checking
1507     // code below.
1508     break;
1509   }
1510   case ARCInstKind::Release: {
1511     Arg = GetArgRCIdentityRoot(Inst);
1512     TopDownPtrState &S = MyStates.getPtrTopDownState(Arg);
1513     // Try to form a tentative pair in between this release instruction and the
1514     // top down pointers that we are tracking.
1515     if (S.MatchWithRelease(MDKindCache, Inst)) {
1516       // If we succeed, copy S's RRInfo into the Release -> {Retain Set
1517       // Map}. Then we clear S.
1518       LLVM_DEBUG(dbgs() << "        Matching with: " << *Inst << "\n");
1519       Releases[Inst] = S.GetRRInfo();
1520       S.ClearSequenceProgress();
1521     }
1522     break;
1523   }
1524   case ARCInstKind::AutoreleasepoolPop:
1525     // Conservatively, clear MyStates for all known pointers.
1526     MyStates.clearTopDownPointers();
1527     return false;
1528   case ARCInstKind::AutoreleasepoolPush:
1529   case ARCInstKind::None:
1530     // These can not be uses of
1531     return false;
1532   default:
1533     break;
1534   }
1535 
1536   // Consider any other possible effects of this instruction on each
1537   // pointer being tracked.
1538   for (auto MI = MyStates.top_down_ptr_begin(),
1539             ME = MyStates.top_down_ptr_end();
1540        MI != ME; ++MI) {
1541     const Value *Ptr = MI->first;
1542     if (Ptr == Arg)
1543       continue; // Handled above.
1544     TopDownPtrState &S = MI->second;
1545     if (S.HandlePotentialAlterRefCount(Inst, Ptr, PA, Class))
1546       continue;
1547 
1548     S.HandlePotentialUse(Inst, Ptr, PA, Class);
1549   }
1550 
1551   return NestingDetected;
1552 }
1553 
1554 bool
1555 ObjCARCOpt::VisitTopDown(BasicBlock *BB,
1556                          DenseMap<const BasicBlock *, BBState> &BBStates,
1557                          DenseMap<Value *, RRInfo> &Releases) {
1558   LLVM_DEBUG(dbgs() << "\n== ObjCARCOpt::VisitTopDown ==\n");
1559   bool NestingDetected = false;
1560   BBState &MyStates = BBStates[BB];
1561 
1562   // Merge the states from each predecessor to compute the initial state
1563   // for the current block.
1564   BBState::edge_iterator PI(MyStates.pred_begin()),
1565                          PE(MyStates.pred_end());
1566   if (PI != PE) {
1567     const BasicBlock *Pred = *PI;
1568     DenseMap<const BasicBlock *, BBState>::iterator I = BBStates.find(Pred);
1569     assert(I != BBStates.end());
1570     MyStates.InitFromPred(I->second);
1571     ++PI;
1572     for (; PI != PE; ++PI) {
1573       Pred = *PI;
1574       I = BBStates.find(Pred);
1575       assert(I != BBStates.end());
1576       MyStates.MergePred(I->second);
1577     }
1578   }
1579 
1580   // Check that BB and MyStates have the same number of predecessors. This
1581   // prevents retain calls that live outside a loop from being moved into the
1582   // loop.
1583   if (!BB->hasNPredecessors(MyStates.pred_end() - MyStates.pred_begin()))
1584     for (auto I = MyStates.top_down_ptr_begin(),
1585               E = MyStates.top_down_ptr_end();
1586          I != E; ++I)
1587       I->second.SetCFGHazardAfflicted(true);
1588 
1589   LLVM_DEBUG(dbgs() << "Before:\n"
1590                     << BBStates[BB] << "\n"
1591                     << "Performing Dataflow:\n");
1592 
1593   // Visit all the instructions, top-down.
1594   for (Instruction &Inst : *BB) {
1595     LLVM_DEBUG(dbgs() << "    Visiting " << Inst << "\n");
1596 
1597     NestingDetected |= VisitInstructionTopDown(&Inst, Releases, MyStates);
1598 
1599     // Bail out if the number of pointers being tracked becomes too large so
1600     // that this pass can complete in a reasonable amount of time.
1601     if (MyStates.top_down_ptr_list_size() > MaxPtrStates) {
1602       DisableRetainReleasePairing = true;
1603       return false;
1604     }
1605   }
1606 
1607   LLVM_DEBUG(dbgs() << "\nState Before Checking for CFG Hazards:\n"
1608                     << BBStates[BB] << "\n\n");
1609   CheckForCFGHazards(BB, BBStates, MyStates);
1610   LLVM_DEBUG(dbgs() << "Final State:\n" << BBStates[BB] << "\n");
1611   return NestingDetected;
1612 }
1613 
1614 static void
1615 ComputePostOrders(Function &F,
1616                   SmallVectorImpl<BasicBlock *> &PostOrder,
1617                   SmallVectorImpl<BasicBlock *> &ReverseCFGPostOrder,
1618                   unsigned NoObjCARCExceptionsMDKind,
1619                   DenseMap<const BasicBlock *, BBState> &BBStates) {
1620   /// The visited set, for doing DFS walks.
1621   SmallPtrSet<BasicBlock *, 16> Visited;
1622 
1623   // Do DFS, computing the PostOrder.
1624   SmallPtrSet<BasicBlock *, 16> OnStack;
1625   SmallVector<std::pair<BasicBlock *, succ_iterator>, 16> SuccStack;
1626 
1627   // Functions always have exactly one entry block, and we don't have
1628   // any other block that we treat like an entry block.
1629   BasicBlock *EntryBB = &F.getEntryBlock();
1630   BBState &MyStates = BBStates[EntryBB];
1631   MyStates.SetAsEntry();
1632   Instruction *EntryTI = EntryBB->getTerminator();
1633   SuccStack.push_back(std::make_pair(EntryBB, succ_iterator(EntryTI)));
1634   Visited.insert(EntryBB);
1635   OnStack.insert(EntryBB);
1636   do {
1637   dfs_next_succ:
1638     BasicBlock *CurrBB = SuccStack.back().first;
1639     succ_iterator SE(CurrBB->getTerminator(), false);
1640 
1641     while (SuccStack.back().second != SE) {
1642       BasicBlock *SuccBB = *SuccStack.back().second++;
1643       if (Visited.insert(SuccBB).second) {
1644         SuccStack.push_back(
1645             std::make_pair(SuccBB, succ_iterator(SuccBB->getTerminator())));
1646         BBStates[CurrBB].addSucc(SuccBB);
1647         BBState &SuccStates = BBStates[SuccBB];
1648         SuccStates.addPred(CurrBB);
1649         OnStack.insert(SuccBB);
1650         goto dfs_next_succ;
1651       }
1652 
1653       if (!OnStack.count(SuccBB)) {
1654         BBStates[CurrBB].addSucc(SuccBB);
1655         BBStates[SuccBB].addPred(CurrBB);
1656       }
1657     }
1658     OnStack.erase(CurrBB);
1659     PostOrder.push_back(CurrBB);
1660     SuccStack.pop_back();
1661   } while (!SuccStack.empty());
1662 
1663   Visited.clear();
1664 
1665   // Do reverse-CFG DFS, computing the reverse-CFG PostOrder.
1666   // Functions may have many exits, and there also blocks which we treat
1667   // as exits due to ignored edges.
1668   SmallVector<std::pair<BasicBlock *, BBState::edge_iterator>, 16> PredStack;
1669   for (BasicBlock &ExitBB : F) {
1670     BBState &MyStates = BBStates[&ExitBB];
1671     if (!MyStates.isExit())
1672       continue;
1673 
1674     MyStates.SetAsExit();
1675 
1676     PredStack.push_back(std::make_pair(&ExitBB, MyStates.pred_begin()));
1677     Visited.insert(&ExitBB);
1678     while (!PredStack.empty()) {
1679     reverse_dfs_next_succ:
1680       BBState::edge_iterator PE = BBStates[PredStack.back().first].pred_end();
1681       while (PredStack.back().second != PE) {
1682         BasicBlock *BB = *PredStack.back().second++;
1683         if (Visited.insert(BB).second) {
1684           PredStack.push_back(std::make_pair(BB, BBStates[BB].pred_begin()));
1685           goto reverse_dfs_next_succ;
1686         }
1687       }
1688       ReverseCFGPostOrder.push_back(PredStack.pop_back_val().first);
1689     }
1690   }
1691 }
1692 
1693 // Visit the function both top-down and bottom-up.
1694 bool ObjCARCOpt::Visit(Function &F,
1695                        DenseMap<const BasicBlock *, BBState> &BBStates,
1696                        BlotMapVector<Value *, RRInfo> &Retains,
1697                        DenseMap<Value *, RRInfo> &Releases) {
1698   // Use reverse-postorder traversals, because we magically know that loops
1699   // will be well behaved, i.e. they won't repeatedly call retain on a single
1700   // pointer without doing a release. We can't use the ReversePostOrderTraversal
1701   // class here because we want the reverse-CFG postorder to consider each
1702   // function exit point, and we want to ignore selected cycle edges.
1703   SmallVector<BasicBlock *, 16> PostOrder;
1704   SmallVector<BasicBlock *, 16> ReverseCFGPostOrder;
1705   ComputePostOrders(F, PostOrder, ReverseCFGPostOrder,
1706                     MDKindCache.get(ARCMDKindID::NoObjCARCExceptions),
1707                     BBStates);
1708 
1709   // Use reverse-postorder on the reverse CFG for bottom-up.
1710   bool BottomUpNestingDetected = false;
1711   for (BasicBlock *BB : llvm::reverse(ReverseCFGPostOrder)) {
1712     BottomUpNestingDetected |= VisitBottomUp(BB, BBStates, Retains);
1713     if (DisableRetainReleasePairing)
1714       return false;
1715   }
1716 
1717   // Use reverse-postorder for top-down.
1718   bool TopDownNestingDetected = false;
1719   for (BasicBlock *BB : llvm::reverse(PostOrder)) {
1720     TopDownNestingDetected |= VisitTopDown(BB, BBStates, Releases);
1721     if (DisableRetainReleasePairing)
1722       return false;
1723   }
1724 
1725   return TopDownNestingDetected && BottomUpNestingDetected;
1726 }
1727 
1728 /// Move the calls in RetainsToMove and ReleasesToMove.
1729 void ObjCARCOpt::MoveCalls(Value *Arg, RRInfo &RetainsToMove,
1730                            RRInfo &ReleasesToMove,
1731                            BlotMapVector<Value *, RRInfo> &Retains,
1732                            DenseMap<Value *, RRInfo> &Releases,
1733                            SmallVectorImpl<Instruction *> &DeadInsts,
1734                            Module *M) {
1735   Type *ArgTy = Arg->getType();
1736   Type *ParamTy = PointerType::getUnqual(Type::getInt8Ty(ArgTy->getContext()));
1737 
1738   LLVM_DEBUG(dbgs() << "== ObjCARCOpt::MoveCalls ==\n");
1739 
1740   // Insert the new retain and release calls.
1741   for (Instruction *InsertPt : ReleasesToMove.ReverseInsertPts) {
1742     Value *MyArg = ArgTy == ParamTy ? Arg :
1743                    new BitCastInst(Arg, ParamTy, "", InsertPt);
1744     Function *Decl = EP.get(ARCRuntimeEntryPointKind::Retain);
1745     CallInst *Call = CallInst::Create(Decl, MyArg, "", InsertPt);
1746     Call->setDoesNotThrow();
1747     Call->setTailCall();
1748 
1749     LLVM_DEBUG(dbgs() << "Inserting new Retain: " << *Call
1750                       << "\n"
1751                          "At insertion point: "
1752                       << *InsertPt << "\n");
1753   }
1754   for (Instruction *InsertPt : RetainsToMove.ReverseInsertPts) {
1755     Value *MyArg = ArgTy == ParamTy ? Arg :
1756                    new BitCastInst(Arg, ParamTy, "", InsertPt);
1757     Function *Decl = EP.get(ARCRuntimeEntryPointKind::Release);
1758     CallInst *Call = CallInst::Create(Decl, MyArg, "", InsertPt);
1759     // Attach a clang.imprecise_release metadata tag, if appropriate.
1760     if (MDNode *M = ReleasesToMove.ReleaseMetadata)
1761       Call->setMetadata(MDKindCache.get(ARCMDKindID::ImpreciseRelease), M);
1762     Call->setDoesNotThrow();
1763     if (ReleasesToMove.IsTailCallRelease)
1764       Call->setTailCall();
1765 
1766     LLVM_DEBUG(dbgs() << "Inserting new Release: " << *Call
1767                       << "\n"
1768                          "At insertion point: "
1769                       << *InsertPt << "\n");
1770   }
1771 
1772   // Delete the original retain and release calls.
1773   for (Instruction *OrigRetain : RetainsToMove.Calls) {
1774     Retains.blot(OrigRetain);
1775     DeadInsts.push_back(OrigRetain);
1776     LLVM_DEBUG(dbgs() << "Deleting retain: " << *OrigRetain << "\n");
1777   }
1778   for (Instruction *OrigRelease : ReleasesToMove.Calls) {
1779     Releases.erase(OrigRelease);
1780     DeadInsts.push_back(OrigRelease);
1781     LLVM_DEBUG(dbgs() << "Deleting release: " << *OrigRelease << "\n");
1782   }
1783 }
1784 
1785 bool ObjCARCOpt::PairUpRetainsAndReleases(
1786     DenseMap<const BasicBlock *, BBState> &BBStates,
1787     BlotMapVector<Value *, RRInfo> &Retains,
1788     DenseMap<Value *, RRInfo> &Releases, Module *M,
1789     Instruction *Retain,
1790     SmallVectorImpl<Instruction *> &DeadInsts, RRInfo &RetainsToMove,
1791     RRInfo &ReleasesToMove, Value *Arg, bool KnownSafe,
1792     bool &AnyPairsCompletelyEliminated) {
1793   // If a pair happens in a region where it is known that the reference count
1794   // is already incremented, we can similarly ignore possible decrements unless
1795   // we are dealing with a retainable object with multiple provenance sources.
1796   bool KnownSafeTD = true, KnownSafeBU = true;
1797   bool CFGHazardAfflicted = false;
1798 
1799   // Connect the dots between the top-down-collected RetainsToMove and
1800   // bottom-up-collected ReleasesToMove to form sets of related calls.
1801   // This is an iterative process so that we connect multiple releases
1802   // to multiple retains if needed.
1803   unsigned OldDelta = 0;
1804   unsigned NewDelta = 0;
1805   unsigned OldCount = 0;
1806   unsigned NewCount = 0;
1807   bool FirstRelease = true;
1808   for (SmallVector<Instruction *, 4> NewRetains{Retain};;) {
1809     SmallVector<Instruction *, 4> NewReleases;
1810     for (Instruction *NewRetain : NewRetains) {
1811       auto It = Retains.find(NewRetain);
1812       assert(It != Retains.end());
1813       const RRInfo &NewRetainRRI = It->second;
1814       KnownSafeTD &= NewRetainRRI.KnownSafe;
1815       CFGHazardAfflicted |= NewRetainRRI.CFGHazardAfflicted;
1816       for (Instruction *NewRetainRelease : NewRetainRRI.Calls) {
1817         auto Jt = Releases.find(NewRetainRelease);
1818         if (Jt == Releases.end())
1819           return false;
1820         const RRInfo &NewRetainReleaseRRI = Jt->second;
1821 
1822         // If the release does not have a reference to the retain as well,
1823         // something happened which is unaccounted for. Do not do anything.
1824         //
1825         // This can happen if we catch an additive overflow during path count
1826         // merging.
1827         if (!NewRetainReleaseRRI.Calls.count(NewRetain))
1828           return false;
1829 
1830         if (ReleasesToMove.Calls.insert(NewRetainRelease).second) {
1831           // If we overflow when we compute the path count, don't remove/move
1832           // anything.
1833           const BBState &NRRBBState = BBStates[NewRetainRelease->getParent()];
1834           unsigned PathCount = BBState::OverflowOccurredValue;
1835           if (NRRBBState.GetAllPathCountWithOverflow(PathCount))
1836             return false;
1837           assert(PathCount != BBState::OverflowOccurredValue &&
1838                  "PathCount at this point can not be "
1839                  "OverflowOccurredValue.");
1840           OldDelta -= PathCount;
1841 
1842           // Merge the ReleaseMetadata and IsTailCallRelease values.
1843           if (FirstRelease) {
1844             ReleasesToMove.ReleaseMetadata =
1845               NewRetainReleaseRRI.ReleaseMetadata;
1846             ReleasesToMove.IsTailCallRelease =
1847               NewRetainReleaseRRI.IsTailCallRelease;
1848             FirstRelease = false;
1849           } else {
1850             if (ReleasesToMove.ReleaseMetadata !=
1851                 NewRetainReleaseRRI.ReleaseMetadata)
1852               ReleasesToMove.ReleaseMetadata = nullptr;
1853             if (ReleasesToMove.IsTailCallRelease !=
1854                 NewRetainReleaseRRI.IsTailCallRelease)
1855               ReleasesToMove.IsTailCallRelease = false;
1856           }
1857 
1858           // Collect the optimal insertion points.
1859           if (!KnownSafe)
1860             for (Instruction *RIP : NewRetainReleaseRRI.ReverseInsertPts) {
1861               if (ReleasesToMove.ReverseInsertPts.insert(RIP).second) {
1862                 // If we overflow when we compute the path count, don't
1863                 // remove/move anything.
1864                 const BBState &RIPBBState = BBStates[RIP->getParent()];
1865                 PathCount = BBState::OverflowOccurredValue;
1866                 if (RIPBBState.GetAllPathCountWithOverflow(PathCount))
1867                   return false;
1868                 assert(PathCount != BBState::OverflowOccurredValue &&
1869                        "PathCount at this point can not be "
1870                        "OverflowOccurredValue.");
1871                 NewDelta -= PathCount;
1872               }
1873             }
1874           NewReleases.push_back(NewRetainRelease);
1875         }
1876       }
1877     }
1878     NewRetains.clear();
1879     if (NewReleases.empty()) break;
1880 
1881     // Back the other way.
1882     for (Instruction *NewRelease : NewReleases) {
1883       auto It = Releases.find(NewRelease);
1884       assert(It != Releases.end());
1885       const RRInfo &NewReleaseRRI = It->second;
1886       KnownSafeBU &= NewReleaseRRI.KnownSafe;
1887       CFGHazardAfflicted |= NewReleaseRRI.CFGHazardAfflicted;
1888       for (Instruction *NewReleaseRetain : NewReleaseRRI.Calls) {
1889         auto Jt = Retains.find(NewReleaseRetain);
1890         if (Jt == Retains.end())
1891           return false;
1892         const RRInfo &NewReleaseRetainRRI = Jt->second;
1893 
1894         // If the retain does not have a reference to the release as well,
1895         // something happened which is unaccounted for. Do not do anything.
1896         //
1897         // This can happen if we catch an additive overflow during path count
1898         // merging.
1899         if (!NewReleaseRetainRRI.Calls.count(NewRelease))
1900           return false;
1901 
1902         if (RetainsToMove.Calls.insert(NewReleaseRetain).second) {
1903           // If we overflow when we compute the path count, don't remove/move
1904           // anything.
1905           const BBState &NRRBBState = BBStates[NewReleaseRetain->getParent()];
1906           unsigned PathCount = BBState::OverflowOccurredValue;
1907           if (NRRBBState.GetAllPathCountWithOverflow(PathCount))
1908             return false;
1909           assert(PathCount != BBState::OverflowOccurredValue &&
1910                  "PathCount at this point can not be "
1911                  "OverflowOccurredValue.");
1912           OldDelta += PathCount;
1913           OldCount += PathCount;
1914 
1915           // Collect the optimal insertion points.
1916           if (!KnownSafe)
1917             for (Instruction *RIP : NewReleaseRetainRRI.ReverseInsertPts) {
1918               if (RetainsToMove.ReverseInsertPts.insert(RIP).second) {
1919                 // If we overflow when we compute the path count, don't
1920                 // remove/move anything.
1921                 const BBState &RIPBBState = BBStates[RIP->getParent()];
1922 
1923                 PathCount = BBState::OverflowOccurredValue;
1924                 if (RIPBBState.GetAllPathCountWithOverflow(PathCount))
1925                   return false;
1926                 assert(PathCount != BBState::OverflowOccurredValue &&
1927                        "PathCount at this point can not be "
1928                        "OverflowOccurredValue.");
1929                 NewDelta += PathCount;
1930                 NewCount += PathCount;
1931               }
1932             }
1933           NewRetains.push_back(NewReleaseRetain);
1934         }
1935       }
1936     }
1937     if (NewRetains.empty()) break;
1938   }
1939 
1940   // We can only remove pointers if we are known safe in both directions.
1941   bool UnconditionallySafe = KnownSafeTD && KnownSafeBU;
1942   if (UnconditionallySafe) {
1943     RetainsToMove.ReverseInsertPts.clear();
1944     ReleasesToMove.ReverseInsertPts.clear();
1945     NewCount = 0;
1946   } else {
1947     // Determine whether the new insertion points we computed preserve the
1948     // balance of retain and release calls through the program.
1949     // TODO: If the fully aggressive solution isn't valid, try to find a
1950     // less aggressive solution which is.
1951     if (NewDelta != 0)
1952       return false;
1953 
1954     // At this point, we are not going to remove any RR pairs, but we still are
1955     // able to move RR pairs. If one of our pointers is afflicted with
1956     // CFGHazards, we cannot perform such code motion so exit early.
1957     const bool WillPerformCodeMotion =
1958         !RetainsToMove.ReverseInsertPts.empty() ||
1959         !ReleasesToMove.ReverseInsertPts.empty();
1960     if (CFGHazardAfflicted && WillPerformCodeMotion)
1961       return false;
1962   }
1963 
1964   // Determine whether the original call points are balanced in the retain and
1965   // release calls through the program. If not, conservatively don't touch
1966   // them.
1967   // TODO: It's theoretically possible to do code motion in this case, as
1968   // long as the existing imbalances are maintained.
1969   if (OldDelta != 0)
1970     return false;
1971 
1972   Changed = true;
1973   assert(OldCount != 0 && "Unreachable code?");
1974   NumRRs += OldCount - NewCount;
1975   // Set to true if we completely removed any RR pairs.
1976   AnyPairsCompletelyEliminated = NewCount == 0;
1977 
1978   // We can move calls!
1979   return true;
1980 }
1981 
1982 /// Identify pairings between the retains and releases, and delete and/or move
1983 /// them.
1984 bool ObjCARCOpt::PerformCodePlacement(
1985     DenseMap<const BasicBlock *, BBState> &BBStates,
1986     BlotMapVector<Value *, RRInfo> &Retains,
1987     DenseMap<Value *, RRInfo> &Releases, Module *M) {
1988   LLVM_DEBUG(dbgs() << "\n== ObjCARCOpt::PerformCodePlacement ==\n");
1989 
1990   bool AnyPairsCompletelyEliminated = false;
1991   SmallVector<Instruction *, 8> DeadInsts;
1992 
1993   // Visit each retain.
1994   for (BlotMapVector<Value *, RRInfo>::const_iterator I = Retains.begin(),
1995                                                       E = Retains.end();
1996        I != E; ++I) {
1997     Value *V = I->first;
1998     if (!V) continue; // blotted
1999 
2000     Instruction *Retain = cast<Instruction>(V);
2001 
2002     LLVM_DEBUG(dbgs() << "Visiting: " << *Retain << "\n");
2003 
2004     Value *Arg = GetArgRCIdentityRoot(Retain);
2005 
2006     // If the object being released is in static or stack storage, we know it's
2007     // not being managed by ObjC reference counting, so we can delete pairs
2008     // regardless of what possible decrements or uses lie between them.
2009     bool KnownSafe = isa<Constant>(Arg) || isa<AllocaInst>(Arg);
2010 
2011     // A constant pointer can't be pointing to an object on the heap. It may
2012     // be reference-counted, but it won't be deleted.
2013     if (const LoadInst *LI = dyn_cast<LoadInst>(Arg))
2014       if (const GlobalVariable *GV =
2015             dyn_cast<GlobalVariable>(
2016               GetRCIdentityRoot(LI->getPointerOperand())))
2017         if (GV->isConstant())
2018           KnownSafe = true;
2019 
2020     // Connect the dots between the top-down-collected RetainsToMove and
2021     // bottom-up-collected ReleasesToMove to form sets of related calls.
2022     RRInfo RetainsToMove, ReleasesToMove;
2023 
2024     bool PerformMoveCalls = PairUpRetainsAndReleases(
2025         BBStates, Retains, Releases, M, Retain, DeadInsts,
2026         RetainsToMove, ReleasesToMove, Arg, KnownSafe,
2027         AnyPairsCompletelyEliminated);
2028 
2029     if (PerformMoveCalls) {
2030       // Ok, everything checks out and we're all set. Let's move/delete some
2031       // code!
2032       MoveCalls(Arg, RetainsToMove, ReleasesToMove,
2033                 Retains, Releases, DeadInsts, M);
2034     }
2035   }
2036 
2037   // Now that we're done moving everything, we can delete the newly dead
2038   // instructions, as we no longer need them as insert points.
2039   while (!DeadInsts.empty())
2040     EraseInstruction(DeadInsts.pop_back_val());
2041 
2042   return AnyPairsCompletelyEliminated;
2043 }
2044 
2045 /// Weak pointer optimizations.
2046 void ObjCARCOpt::OptimizeWeakCalls(Function &F) {
2047   LLVM_DEBUG(dbgs() << "\n== ObjCARCOpt::OptimizeWeakCalls ==\n");
2048 
2049   // First, do memdep-style RLE and S2L optimizations. We can't use memdep
2050   // itself because it uses AliasAnalysis and we need to do provenance
2051   // queries instead.
2052   for (inst_iterator I = inst_begin(&F), E = inst_end(&F); I != E; ) {
2053     Instruction *Inst = &*I++;
2054 
2055     LLVM_DEBUG(dbgs() << "Visiting: " << *Inst << "\n");
2056 
2057     ARCInstKind Class = GetBasicARCInstKind(Inst);
2058     if (Class != ARCInstKind::LoadWeak &&
2059         Class != ARCInstKind::LoadWeakRetained)
2060       continue;
2061 
2062     // Delete objc_loadWeak calls with no users.
2063     if (Class == ARCInstKind::LoadWeak && Inst->use_empty()) {
2064       Inst->eraseFromParent();
2065       Changed = true;
2066       continue;
2067     }
2068 
2069     // TODO: For now, just look for an earlier available version of this value
2070     // within the same block. Theoretically, we could do memdep-style non-local
2071     // analysis too, but that would want caching. A better approach would be to
2072     // use the technique that EarlyCSE uses.
2073     inst_iterator Current = std::prev(I);
2074     BasicBlock *CurrentBB = &*Current.getBasicBlockIterator();
2075     for (BasicBlock::iterator B = CurrentBB->begin(),
2076                               J = Current.getInstructionIterator();
2077          J != B; --J) {
2078       Instruction *EarlierInst = &*std::prev(J);
2079       ARCInstKind EarlierClass = GetARCInstKind(EarlierInst);
2080       switch (EarlierClass) {
2081       case ARCInstKind::LoadWeak:
2082       case ARCInstKind::LoadWeakRetained: {
2083         // If this is loading from the same pointer, replace this load's value
2084         // with that one.
2085         CallInst *Call = cast<CallInst>(Inst);
2086         CallInst *EarlierCall = cast<CallInst>(EarlierInst);
2087         Value *Arg = Call->getArgOperand(0);
2088         Value *EarlierArg = EarlierCall->getArgOperand(0);
2089         switch (PA.getAA()->alias(Arg, EarlierArg)) {
2090         case MustAlias:
2091           Changed = true;
2092           // If the load has a builtin retain, insert a plain retain for it.
2093           if (Class == ARCInstKind::LoadWeakRetained) {
2094             Function *Decl = EP.get(ARCRuntimeEntryPointKind::Retain);
2095             CallInst *CI = CallInst::Create(Decl, EarlierCall, "", Call);
2096             CI->setTailCall();
2097           }
2098           // Zap the fully redundant load.
2099           Call->replaceAllUsesWith(EarlierCall);
2100           Call->eraseFromParent();
2101           goto clobbered;
2102         case MayAlias:
2103         case PartialAlias:
2104           goto clobbered;
2105         case NoAlias:
2106           break;
2107         }
2108         break;
2109       }
2110       case ARCInstKind::StoreWeak:
2111       case ARCInstKind::InitWeak: {
2112         // If this is storing to the same pointer and has the same size etc.
2113         // replace this load's value with the stored value.
2114         CallInst *Call = cast<CallInst>(Inst);
2115         CallInst *EarlierCall = cast<CallInst>(EarlierInst);
2116         Value *Arg = Call->getArgOperand(0);
2117         Value *EarlierArg = EarlierCall->getArgOperand(0);
2118         switch (PA.getAA()->alias(Arg, EarlierArg)) {
2119         case MustAlias:
2120           Changed = true;
2121           // If the load has a builtin retain, insert a plain retain for it.
2122           if (Class == ARCInstKind::LoadWeakRetained) {
2123             Function *Decl = EP.get(ARCRuntimeEntryPointKind::Retain);
2124             CallInst *CI = CallInst::Create(Decl, EarlierCall, "", Call);
2125             CI->setTailCall();
2126           }
2127           // Zap the fully redundant load.
2128           Call->replaceAllUsesWith(EarlierCall->getArgOperand(1));
2129           Call->eraseFromParent();
2130           goto clobbered;
2131         case MayAlias:
2132         case PartialAlias:
2133           goto clobbered;
2134         case NoAlias:
2135           break;
2136         }
2137         break;
2138       }
2139       case ARCInstKind::MoveWeak:
2140       case ARCInstKind::CopyWeak:
2141         // TOOD: Grab the copied value.
2142         goto clobbered;
2143       case ARCInstKind::AutoreleasepoolPush:
2144       case ARCInstKind::None:
2145       case ARCInstKind::IntrinsicUser:
2146       case ARCInstKind::User:
2147         // Weak pointers are only modified through the weak entry points
2148         // (and arbitrary calls, which could call the weak entry points).
2149         break;
2150       default:
2151         // Anything else could modify the weak pointer.
2152         goto clobbered;
2153       }
2154     }
2155   clobbered:;
2156   }
2157 
2158   // Then, for each destroyWeak with an alloca operand, check to see if
2159   // the alloca and all its users can be zapped.
2160   for (inst_iterator I = inst_begin(&F), E = inst_end(&F); I != E; ) {
2161     Instruction *Inst = &*I++;
2162     ARCInstKind Class = GetBasicARCInstKind(Inst);
2163     if (Class != ARCInstKind::DestroyWeak)
2164       continue;
2165 
2166     CallInst *Call = cast<CallInst>(Inst);
2167     Value *Arg = Call->getArgOperand(0);
2168     if (AllocaInst *Alloca = dyn_cast<AllocaInst>(Arg)) {
2169       for (User *U : Alloca->users()) {
2170         const Instruction *UserInst = cast<Instruction>(U);
2171         switch (GetBasicARCInstKind(UserInst)) {
2172         case ARCInstKind::InitWeak:
2173         case ARCInstKind::StoreWeak:
2174         case ARCInstKind::DestroyWeak:
2175           continue;
2176         default:
2177           goto done;
2178         }
2179       }
2180       Changed = true;
2181       for (auto UI = Alloca->user_begin(), UE = Alloca->user_end(); UI != UE;) {
2182         CallInst *UserInst = cast<CallInst>(*UI++);
2183         switch (GetBasicARCInstKind(UserInst)) {
2184         case ARCInstKind::InitWeak:
2185         case ARCInstKind::StoreWeak:
2186           // These functions return their second argument.
2187           UserInst->replaceAllUsesWith(UserInst->getArgOperand(1));
2188           break;
2189         case ARCInstKind::DestroyWeak:
2190           // No return value.
2191           break;
2192         default:
2193           llvm_unreachable("alloca really is used!");
2194         }
2195         UserInst->eraseFromParent();
2196       }
2197       Alloca->eraseFromParent();
2198     done:;
2199     }
2200   }
2201 }
2202 
2203 /// Identify program paths which execute sequences of retains and releases which
2204 /// can be eliminated.
2205 bool ObjCARCOpt::OptimizeSequences(Function &F) {
2206   // Releases, Retains - These are used to store the results of the main flow
2207   // analysis. These use Value* as the key instead of Instruction* so that the
2208   // map stays valid when we get around to rewriting code and calls get
2209   // replaced by arguments.
2210   DenseMap<Value *, RRInfo> Releases;
2211   BlotMapVector<Value *, RRInfo> Retains;
2212 
2213   // This is used during the traversal of the function to track the
2214   // states for each identified object at each block.
2215   DenseMap<const BasicBlock *, BBState> BBStates;
2216 
2217   // Analyze the CFG of the function, and all instructions.
2218   bool NestingDetected = Visit(F, BBStates, Retains, Releases);
2219 
2220   if (DisableRetainReleasePairing)
2221     return false;
2222 
2223   // Transform.
2224   bool AnyPairsCompletelyEliminated = PerformCodePlacement(BBStates, Retains,
2225                                                            Releases,
2226                                                            F.getParent());
2227 
2228   return AnyPairsCompletelyEliminated && NestingDetected;
2229 }
2230 
2231 /// Check if there is a dependent call earlier that does not have anything in
2232 /// between the Retain and the call that can affect the reference count of their
2233 /// shared pointer argument. Note that Retain need not be in BB.
2234 static CallInst *HasSafePathToPredecessorCall(const Value *Arg,
2235                                               Instruction *Retain,
2236                                               ProvenanceAnalysis &PA) {
2237   auto *Call = dyn_cast_or_null<CallInst>(findSingleDependency(
2238       CanChangeRetainCount, Arg, Retain->getParent(), Retain, PA));
2239 
2240   // Check that the pointer is the return value of the call.
2241   if (!Call || Arg != Call)
2242     return nullptr;
2243 
2244   // Check that the call is a regular call.
2245   ARCInstKind Class = GetBasicARCInstKind(Call);
2246   return Class == ARCInstKind::CallOrUser || Class == ARCInstKind::Call
2247              ? Call
2248              : nullptr;
2249 }
2250 
2251 /// Find a dependent retain that precedes the given autorelease for which there
2252 /// is nothing in between the two instructions that can affect the ref count of
2253 /// Arg.
2254 static CallInst *
2255 FindPredecessorRetainWithSafePath(const Value *Arg, BasicBlock *BB,
2256                                   Instruction *Autorelease,
2257                                   ProvenanceAnalysis &PA) {
2258   auto *Retain = dyn_cast_or_null<CallInst>(
2259       findSingleDependency(CanChangeRetainCount, Arg, BB, Autorelease, PA));
2260 
2261   // Check that we found a retain with the same argument.
2262   if (!Retain || !IsRetain(GetBasicARCInstKind(Retain)) ||
2263       GetArgRCIdentityRoot(Retain) != Arg) {
2264     return nullptr;
2265   }
2266 
2267   return Retain;
2268 }
2269 
2270 /// Look for an ``autorelease'' instruction dependent on Arg such that there are
2271 /// no instructions dependent on Arg that need a positive ref count in between
2272 /// the autorelease and the ret.
2273 static CallInst *
2274 FindPredecessorAutoreleaseWithSafePath(const Value *Arg, BasicBlock *BB,
2275                                        ReturnInst *Ret,
2276                                        ProvenanceAnalysis &PA) {
2277   SmallPtrSet<Instruction *, 4> DepInsts;
2278   auto *Autorelease = dyn_cast_or_null<CallInst>(
2279       findSingleDependency(NeedsPositiveRetainCount, Arg, BB, Ret, PA));
2280 
2281   if (!Autorelease)
2282     return nullptr;
2283   ARCInstKind AutoreleaseClass = GetBasicARCInstKind(Autorelease);
2284   if (!IsAutorelease(AutoreleaseClass))
2285     return nullptr;
2286   if (GetArgRCIdentityRoot(Autorelease) != Arg)
2287     return nullptr;
2288 
2289   return Autorelease;
2290 }
2291 
2292 /// Look for this pattern:
2293 /// \code
2294 ///    %call = call i8* @something(...)
2295 ///    %2 = call i8* @objc_retain(i8* %call)
2296 ///    %3 = call i8* @objc_autorelease(i8* %2)
2297 ///    ret i8* %3
2298 /// \endcode
2299 /// And delete the retain and autorelease.
2300 void ObjCARCOpt::OptimizeReturns(Function &F) {
2301   if (!F.getReturnType()->isPointerTy())
2302     return;
2303 
2304   LLVM_DEBUG(dbgs() << "\n== ObjCARCOpt::OptimizeReturns ==\n");
2305 
2306   for (BasicBlock &BB: F) {
2307     ReturnInst *Ret = dyn_cast<ReturnInst>(&BB.back());
2308     if (!Ret)
2309       continue;
2310 
2311     LLVM_DEBUG(dbgs() << "Visiting: " << *Ret << "\n");
2312 
2313     const Value *Arg = GetRCIdentityRoot(Ret->getOperand(0));
2314 
2315     // Look for an ``autorelease'' instruction that is a predecessor of Ret and
2316     // dependent on Arg such that there are no instructions dependent on Arg
2317     // that need a positive ref count in between the autorelease and Ret.
2318     CallInst *Autorelease =
2319         FindPredecessorAutoreleaseWithSafePath(Arg, &BB, Ret, PA);
2320 
2321     if (!Autorelease)
2322       continue;
2323 
2324     CallInst *Retain = FindPredecessorRetainWithSafePath(
2325         Arg, Autorelease->getParent(), Autorelease, PA);
2326 
2327     if (!Retain)
2328       continue;
2329 
2330     // Check that there is nothing that can affect the reference count
2331     // between the retain and the call.  Note that Retain need not be in BB.
2332     CallInst *Call = HasSafePathToPredecessorCall(Arg, Retain, PA);
2333 
2334     // Don't remove retainRV/autoreleaseRV pairs if the call isn't a tail call.
2335     if (!Call ||
2336         (!Call->isTailCall() &&
2337          GetBasicARCInstKind(Retain) == ARCInstKind::RetainRV &&
2338          GetBasicARCInstKind(Autorelease) == ARCInstKind::AutoreleaseRV))
2339       continue;
2340 
2341     // If so, we can zap the retain and autorelease.
2342     Changed = true;
2343     ++NumRets;
2344     LLVM_DEBUG(dbgs() << "Erasing: " << *Retain << "\nErasing: " << *Autorelease
2345                       << "\n");
2346     EraseInstruction(Retain);
2347     EraseInstruction(Autorelease);
2348   }
2349 }
2350 
2351 #ifndef NDEBUG
2352 void
2353 ObjCARCOpt::GatherStatistics(Function &F, bool AfterOptimization) {
2354   Statistic &NumRetains =
2355       AfterOptimization ? NumRetainsAfterOpt : NumRetainsBeforeOpt;
2356   Statistic &NumReleases =
2357       AfterOptimization ? NumReleasesAfterOpt : NumReleasesBeforeOpt;
2358 
2359   for (inst_iterator I = inst_begin(&F), E = inst_end(&F); I != E; ) {
2360     Instruction *Inst = &*I++;
2361     switch (GetBasicARCInstKind(Inst)) {
2362     default:
2363       break;
2364     case ARCInstKind::Retain:
2365       ++NumRetains;
2366       break;
2367     case ARCInstKind::Release:
2368       ++NumReleases;
2369       break;
2370     }
2371   }
2372 }
2373 #endif
2374 
2375 void ObjCARCOpt::init(Module &M) {
2376   if (!EnableARCOpts)
2377     return;
2378 
2379   // If nothing in the Module uses ARC, don't do anything.
2380   Run = ModuleHasARC(M);
2381   if (!Run)
2382     return;
2383 
2384   // Intuitively, objc_retain and others are nocapture, however in practice
2385   // they are not, because they return their argument value. And objc_release
2386   // calls finalizers which can have arbitrary side effects.
2387   MDKindCache.init(&M);
2388 
2389   // Initialize our runtime entry point cache.
2390   EP.init(&M);
2391 }
2392 
2393 bool ObjCARCOpt::run(Function &F, AAResults &AA) {
2394   if (!EnableARCOpts)
2395     return false;
2396 
2397   // If nothing in the Module uses ARC, don't do anything.
2398   if (!Run)
2399     return false;
2400 
2401   Changed = false;
2402 
2403   LLVM_DEBUG(dbgs() << "<<< ObjCARCOpt: Visiting Function: " << F.getName()
2404                     << " >>>"
2405                        "\n");
2406 
2407   PA.setAA(&AA);
2408 
2409 #ifndef NDEBUG
2410   if (AreStatisticsEnabled()) {
2411     GatherStatistics(F, false);
2412   }
2413 #endif
2414 
2415   // This pass performs several distinct transformations. As a compile-time aid
2416   // when compiling code that isn't ObjC, skip these if the relevant ObjC
2417   // library functions aren't declared.
2418 
2419   // Preliminary optimizations. This also computes UsedInThisFunction.
2420   OptimizeIndividualCalls(F);
2421 
2422   // Optimizations for weak pointers.
2423   if (UsedInThisFunction & ((1 << unsigned(ARCInstKind::LoadWeak)) |
2424                             (1 << unsigned(ARCInstKind::LoadWeakRetained)) |
2425                             (1 << unsigned(ARCInstKind::StoreWeak)) |
2426                             (1 << unsigned(ARCInstKind::InitWeak)) |
2427                             (1 << unsigned(ARCInstKind::CopyWeak)) |
2428                             (1 << unsigned(ARCInstKind::MoveWeak)) |
2429                             (1 << unsigned(ARCInstKind::DestroyWeak))))
2430     OptimizeWeakCalls(F);
2431 
2432   // Optimizations for retain+release pairs.
2433   if (UsedInThisFunction & ((1 << unsigned(ARCInstKind::Retain)) |
2434                             (1 << unsigned(ARCInstKind::RetainRV)) |
2435                             (1 << unsigned(ARCInstKind::RetainBlock))))
2436     if (UsedInThisFunction & (1 << unsigned(ARCInstKind::Release)))
2437       // Run OptimizeSequences until it either stops making changes or
2438       // no retain+release pair nesting is detected.
2439       while (OptimizeSequences(F)) {}
2440 
2441   // Optimizations if objc_autorelease is used.
2442   if (UsedInThisFunction & ((1 << unsigned(ARCInstKind::Autorelease)) |
2443                             (1 << unsigned(ARCInstKind::AutoreleaseRV))))
2444     OptimizeReturns(F);
2445 
2446   // Gather statistics after optimization.
2447 #ifndef NDEBUG
2448   if (AreStatisticsEnabled()) {
2449     GatherStatistics(F, true);
2450   }
2451 #endif
2452 
2453   LLVM_DEBUG(dbgs() << "\n");
2454 
2455   return Changed;
2456 }
2457 
2458 void ObjCARCOpt::releaseMemory() {
2459   PA.clear();
2460 }
2461 
2462 /// @}
2463 ///
2464 
2465 PreservedAnalyses ObjCARCOptPass::run(Function &F,
2466                                       FunctionAnalysisManager &AM) {
2467   ObjCARCOpt OCAO;
2468   OCAO.init(*F.getParent());
2469 
2470   bool Changed = OCAO.run(F, AM.getResult<AAManager>(F));
2471   if (Changed) {
2472     PreservedAnalyses PA;
2473     PA.preserveSet<CFGAnalyses>();
2474     return PA;
2475   }
2476   return PreservedAnalyses::all();
2477 }
2478