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      1 //===- ObjCARC.h - ObjC ARC Optimization --------------*- C++ -*-----------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 /// \file
     10 /// This file defines common definitions/declarations used by the ObjC ARC
     11 /// Optimizer. ARC stands for Automatic Reference Counting and is a system for
     12 /// managing reference counts for objects in Objective C.
     13 ///
     14 /// WARNING: This file knows about certain library functions. It recognizes them
     15 /// by name, and hardwires knowledge of their semantics.
     16 ///
     17 /// WARNING: This file knows about how certain Objective-C library functions are
     18 /// used. Naive LLVM IR transformations which would otherwise be
     19 /// behavior-preserving may break these assumptions.
     20 ///
     21 //===----------------------------------------------------------------------===//
     22 
     23 #ifndef LLVM_TRANSFORMS_SCALAR_OBJCARC_H
     24 #define LLVM_TRANSFORMS_SCALAR_OBJCARC_H
     25 
     26 #include "llvm/ADT/StringSwitch.h"
     27 #include "llvm/Analysis/AliasAnalysis.h"
     28 #include "llvm/Analysis/Passes.h"
     29 #include "llvm/Analysis/ValueTracking.h"
     30 #include "llvm/IR/CallSite.h"
     31 #include "llvm/IR/InstIterator.h"
     32 #include "llvm/IR/Module.h"
     33 #include "llvm/Pass.h"
     34 #include "llvm/Transforms/ObjCARC.h"
     35 #include "llvm/Transforms/Utils/Local.h"
     36 
     37 namespace llvm {
     38 class raw_ostream;
     39 }
     40 
     41 namespace llvm {
     42 namespace objcarc {
     43 
     44 /// \brief A handy option to enable/disable all ARC Optimizations.
     45 extern bool EnableARCOpts;
     46 
     47 /// \brief Test if the given module looks interesting to run ARC optimization
     48 /// on.
     49 static inline bool ModuleHasARC(const Module &M) {
     50   return
     51     M.getNamedValue("objc_retain") ||
     52     M.getNamedValue("objc_release") ||
     53     M.getNamedValue("objc_autorelease") ||
     54     M.getNamedValue("objc_retainAutoreleasedReturnValue") ||
     55     M.getNamedValue("objc_retainBlock") ||
     56     M.getNamedValue("objc_autoreleaseReturnValue") ||
     57     M.getNamedValue("objc_autoreleasePoolPush") ||
     58     M.getNamedValue("objc_loadWeakRetained") ||
     59     M.getNamedValue("objc_loadWeak") ||
     60     M.getNamedValue("objc_destroyWeak") ||
     61     M.getNamedValue("objc_storeWeak") ||
     62     M.getNamedValue("objc_initWeak") ||
     63     M.getNamedValue("objc_moveWeak") ||
     64     M.getNamedValue("objc_copyWeak") ||
     65     M.getNamedValue("objc_retainedObject") ||
     66     M.getNamedValue("objc_unretainedObject") ||
     67     M.getNamedValue("objc_unretainedPointer") ||
     68     M.getNamedValue("clang.arc.use");
     69 }
     70 
     71 /// \enum InstructionClass
     72 /// \brief A simple classification for instructions.
     73 enum InstructionClass {
     74   IC_Retain,              ///< objc_retain
     75   IC_RetainRV,            ///< objc_retainAutoreleasedReturnValue
     76   IC_RetainBlock,         ///< objc_retainBlock
     77   IC_Release,             ///< objc_release
     78   IC_Autorelease,         ///< objc_autorelease
     79   IC_AutoreleaseRV,       ///< objc_autoreleaseReturnValue
     80   IC_AutoreleasepoolPush, ///< objc_autoreleasePoolPush
     81   IC_AutoreleasepoolPop,  ///< objc_autoreleasePoolPop
     82   IC_NoopCast,            ///< objc_retainedObject, etc.
     83   IC_FusedRetainAutorelease, ///< objc_retainAutorelease
     84   IC_FusedRetainAutoreleaseRV, ///< objc_retainAutoreleaseReturnValue
     85   IC_LoadWeakRetained,    ///< objc_loadWeakRetained (primitive)
     86   IC_StoreWeak,           ///< objc_storeWeak (primitive)
     87   IC_InitWeak,            ///< objc_initWeak (derived)
     88   IC_LoadWeak,            ///< objc_loadWeak (derived)
     89   IC_MoveWeak,            ///< objc_moveWeak (derived)
     90   IC_CopyWeak,            ///< objc_copyWeak (derived)
     91   IC_DestroyWeak,         ///< objc_destroyWeak (derived)
     92   IC_StoreStrong,         ///< objc_storeStrong (derived)
     93   IC_IntrinsicUser,       ///< clang.arc.use
     94   IC_CallOrUser,          ///< could call objc_release and/or "use" pointers
     95   IC_Call,                ///< could call objc_release
     96   IC_User,                ///< could "use" a pointer
     97   IC_None                 ///< anything else
     98 };
     99 
    100 raw_ostream &operator<<(raw_ostream &OS, const InstructionClass Class);
    101 
    102 /// \brief Test if the given class is a kind of user.
    103 inline static bool IsUser(InstructionClass Class) {
    104   return Class == IC_User ||
    105          Class == IC_CallOrUser ||
    106          Class == IC_IntrinsicUser;
    107 }
    108 
    109 /// \brief Test if the given class is objc_retain or equivalent.
    110 static inline bool IsRetain(InstructionClass Class) {
    111   return Class == IC_Retain ||
    112          Class == IC_RetainRV;
    113 }
    114 
    115 /// \brief Test if the given class is objc_autorelease or equivalent.
    116 static inline bool IsAutorelease(InstructionClass Class) {
    117   return Class == IC_Autorelease ||
    118          Class == IC_AutoreleaseRV;
    119 }
    120 
    121 /// \brief Test if the given class represents instructions which return their
    122 /// argument verbatim.
    123 static inline bool IsForwarding(InstructionClass Class) {
    124   return Class == IC_Retain ||
    125          Class == IC_RetainRV ||
    126          Class == IC_Autorelease ||
    127          Class == IC_AutoreleaseRV ||
    128          Class == IC_NoopCast;
    129 }
    130 
    131 /// \brief Test if the given class represents instructions which do nothing if
    132 /// passed a null pointer.
    133 static inline bool IsNoopOnNull(InstructionClass Class) {
    134   return Class == IC_Retain ||
    135          Class == IC_RetainRV ||
    136          Class == IC_Release ||
    137          Class == IC_Autorelease ||
    138          Class == IC_AutoreleaseRV ||
    139          Class == IC_RetainBlock;
    140 }
    141 
    142 /// \brief Test if the given class represents instructions which are always safe
    143 /// to mark with the "tail" keyword.
    144 static inline bool IsAlwaysTail(InstructionClass Class) {
    145   // IC_RetainBlock may be given a stack argument.
    146   return Class == IC_Retain ||
    147          Class == IC_RetainRV ||
    148          Class == IC_AutoreleaseRV;
    149 }
    150 
    151 /// \brief Test if the given class represents instructions which are never safe
    152 /// to mark with the "tail" keyword.
    153 static inline bool IsNeverTail(InstructionClass Class) {
    154   /// It is never safe to tail call objc_autorelease since by tail calling
    155   /// objc_autorelease, we also tail call -[NSObject autorelease] which supports
    156   /// fast autoreleasing causing our object to be potentially reclaimed from the
    157   /// autorelease pool which violates the semantics of __autoreleasing types in
    158   /// ARC.
    159   return Class == IC_Autorelease;
    160 }
    161 
    162 /// \brief Test if the given class represents instructions which are always safe
    163 /// to mark with the nounwind attribute.
    164 static inline bool IsNoThrow(InstructionClass Class) {
    165   // objc_retainBlock is not nounwind because it calls user copy constructors
    166   // which could theoretically throw.
    167   return Class == IC_Retain ||
    168          Class == IC_RetainRV ||
    169          Class == IC_Release ||
    170          Class == IC_Autorelease ||
    171          Class == IC_AutoreleaseRV ||
    172          Class == IC_AutoreleasepoolPush ||
    173          Class == IC_AutoreleasepoolPop;
    174 }
    175 
    176 /// Test whether the given instruction can autorelease any pointer or cause an
    177 /// autoreleasepool pop.
    178 static inline bool
    179 CanInterruptRV(InstructionClass Class) {
    180   switch (Class) {
    181   case IC_AutoreleasepoolPop:
    182   case IC_CallOrUser:
    183   case IC_Call:
    184   case IC_Autorelease:
    185   case IC_AutoreleaseRV:
    186   case IC_FusedRetainAutorelease:
    187   case IC_FusedRetainAutoreleaseRV:
    188     return true;
    189   default:
    190     return false;
    191   }
    192 }
    193 
    194 /// \brief Determine if F is one of the special known Functions.  If it isn't,
    195 /// return IC_CallOrUser.
    196 InstructionClass GetFunctionClass(const Function *F);
    197 
    198 /// \brief Determine which objc runtime call instruction class V belongs to.
    199 ///
    200 /// This is similar to GetInstructionClass except that it only detects objc
    201 /// runtime calls. This allows it to be faster.
    202 ///
    203 static inline InstructionClass GetBasicInstructionClass(const Value *V) {
    204   if (const CallInst *CI = dyn_cast<CallInst>(V)) {
    205     if (const Function *F = CI->getCalledFunction())
    206       return GetFunctionClass(F);
    207     // Otherwise, be conservative.
    208     return IC_CallOrUser;
    209   }
    210 
    211   // Otherwise, be conservative.
    212   return isa<InvokeInst>(V) ? IC_CallOrUser : IC_User;
    213 }
    214 
    215 /// \brief Determine what kind of construct V is.
    216 InstructionClass GetInstructionClass(const Value *V);
    217 
    218 /// \brief This is a wrapper around getUnderlyingObject which also knows how to
    219 /// look through objc_retain and objc_autorelease calls, which we know to return
    220 /// their argument verbatim.
    221 static inline const Value *GetUnderlyingObjCPtr(const Value *V) {
    222   for (;;) {
    223     V = GetUnderlyingObject(V);
    224     if (!IsForwarding(GetBasicInstructionClass(V)))
    225       break;
    226     V = cast<CallInst>(V)->getArgOperand(0);
    227   }
    228 
    229   return V;
    230 }
    231 
    232 /// \brief This is a wrapper around Value::stripPointerCasts which also knows
    233 /// how to look through objc_retain and objc_autorelease calls, which we know to
    234 /// return their argument verbatim.
    235 static inline const Value *StripPointerCastsAndObjCCalls(const Value *V) {
    236   for (;;) {
    237     V = V->stripPointerCasts();
    238     if (!IsForwarding(GetBasicInstructionClass(V)))
    239       break;
    240     V = cast<CallInst>(V)->getArgOperand(0);
    241   }
    242   return V;
    243 }
    244 
    245 /// \brief This is a wrapper around Value::stripPointerCasts which also knows
    246 /// how to look through objc_retain and objc_autorelease calls, which we know to
    247 /// return their argument verbatim.
    248 static inline Value *StripPointerCastsAndObjCCalls(Value *V) {
    249   for (;;) {
    250     V = V->stripPointerCasts();
    251     if (!IsForwarding(GetBasicInstructionClass(V)))
    252       break;
    253     V = cast<CallInst>(V)->getArgOperand(0);
    254   }
    255   return V;
    256 }
    257 
    258 /// \brief Assuming the given instruction is one of the special calls such as
    259 /// objc_retain or objc_release, return the argument value, stripped of no-op
    260 /// casts and forwarding calls.
    261 static inline Value *GetObjCArg(Value *Inst) {
    262   return StripPointerCastsAndObjCCalls(cast<CallInst>(Inst)->getArgOperand(0));
    263 }
    264 
    265 static inline bool IsNullOrUndef(const Value *V) {
    266   return isa<ConstantPointerNull>(V) || isa<UndefValue>(V);
    267 }
    268 
    269 static inline bool IsNoopInstruction(const Instruction *I) {
    270   return isa<BitCastInst>(I) ||
    271     (isa<GetElementPtrInst>(I) &&
    272      cast<GetElementPtrInst>(I)->hasAllZeroIndices());
    273 }
    274 
    275 
    276 /// \brief Erase the given instruction.
    277 ///
    278 /// Many ObjC calls return their argument verbatim,
    279 /// so if it's such a call and the return value has users, replace them with the
    280 /// argument value.
    281 ///
    282 static inline void EraseInstruction(Instruction *CI) {
    283   Value *OldArg = cast<CallInst>(CI)->getArgOperand(0);
    284 
    285   bool Unused = CI->use_empty();
    286 
    287   if (!Unused) {
    288     // Replace the return value with the argument.
    289     assert((IsForwarding(GetBasicInstructionClass(CI)) ||
    290             (IsNoopOnNull(GetBasicInstructionClass(CI)) &&
    291              isa<ConstantPointerNull>(OldArg))) &&
    292            "Can't delete non-forwarding instruction with users!");
    293     CI->replaceAllUsesWith(OldArg);
    294   }
    295 
    296   CI->eraseFromParent();
    297 
    298   if (Unused)
    299     RecursivelyDeleteTriviallyDeadInstructions(OldArg);
    300 }
    301 
    302 /// \brief Test whether the given value is possible a retainable object pointer.
    303 static inline bool IsPotentialRetainableObjPtr(const Value *Op) {
    304   // Pointers to static or stack storage are not valid retainable object
    305   // pointers.
    306   if (isa<Constant>(Op) || isa<AllocaInst>(Op))
    307     return false;
    308   // Special arguments can not be a valid retainable object pointer.
    309   if (const Argument *Arg = dyn_cast<Argument>(Op))
    310     if (Arg->hasByValAttr() ||
    311         Arg->hasInAllocaAttr() ||
    312         Arg->hasNestAttr() ||
    313         Arg->hasStructRetAttr())
    314       return false;
    315   // Only consider values with pointer types.
    316   //
    317   // It seemes intuitive to exclude function pointer types as well, since
    318   // functions are never retainable object pointers, however clang occasionally
    319   // bitcasts retainable object pointers to function-pointer type temporarily.
    320   PointerType *Ty = dyn_cast<PointerType>(Op->getType());
    321   if (!Ty)
    322     return false;
    323   // Conservatively assume anything else is a potential retainable object
    324   // pointer.
    325   return true;
    326 }
    327 
    328 static inline bool IsPotentialRetainableObjPtr(const Value *Op,
    329                                                AliasAnalysis &AA) {
    330   // First make the rudimentary check.
    331   if (!IsPotentialRetainableObjPtr(Op))
    332     return false;
    333 
    334   // Objects in constant memory are not reference-counted.
    335   if (AA.pointsToConstantMemory(Op))
    336     return false;
    337 
    338   // Pointers in constant memory are not pointing to reference-counted objects.
    339   if (const LoadInst *LI = dyn_cast<LoadInst>(Op))
    340     if (AA.pointsToConstantMemory(LI->getPointerOperand()))
    341       return false;
    342 
    343   // Otherwise assume the worst.
    344   return true;
    345 }
    346 
    347 /// \brief Helper for GetInstructionClass. Determines what kind of construct CS
    348 /// is.
    349 static inline InstructionClass GetCallSiteClass(ImmutableCallSite CS) {
    350   for (ImmutableCallSite::arg_iterator I = CS.arg_begin(), E = CS.arg_end();
    351        I != E; ++I)
    352     if (IsPotentialRetainableObjPtr(*I))
    353       return CS.onlyReadsMemory() ? IC_User : IC_CallOrUser;
    354 
    355   return CS.onlyReadsMemory() ? IC_None : IC_Call;
    356 }
    357 
    358 /// \brief Return true if this value refers to a distinct and identifiable
    359 /// object.
    360 ///
    361 /// This is similar to AliasAnalysis's isIdentifiedObject, except that it uses
    362 /// special knowledge of ObjC conventions.
    363 static inline bool IsObjCIdentifiedObject(const Value *V) {
    364   // Assume that call results and arguments have their own "provenance".
    365   // Constants (including GlobalVariables) and Allocas are never
    366   // reference-counted.
    367   if (isa<CallInst>(V) || isa<InvokeInst>(V) ||
    368       isa<Argument>(V) || isa<Constant>(V) ||
    369       isa<AllocaInst>(V))
    370     return true;
    371 
    372   if (const LoadInst *LI = dyn_cast<LoadInst>(V)) {
    373     const Value *Pointer =
    374       StripPointerCastsAndObjCCalls(LI->getPointerOperand());
    375     if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Pointer)) {
    376       // A constant pointer can't be pointing to an object on the heap. It may
    377       // be reference-counted, but it won't be deleted.
    378       if (GV->isConstant())
    379         return true;
    380       StringRef Name = GV->getName();
    381       // These special variables are known to hold values which are not
    382       // reference-counted pointers.
    383       if (Name.startswith("\01L_OBJC_SELECTOR_REFERENCES_") ||
    384           Name.startswith("\01L_OBJC_CLASSLIST_REFERENCES_") ||
    385           Name.startswith("\01L_OBJC_CLASSLIST_SUP_REFS_$_") ||
    386           Name.startswith("\01L_OBJC_METH_VAR_NAME_") ||
    387           Name.startswith("\01l_objc_msgSend_fixup_"))
    388         return true;
    389     }
    390   }
    391 
    392   return false;
    393 }
    394 
    395 } // end namespace objcarc
    396 } // end namespace llvm
    397 
    398 #endif // LLVM_TRANSFORMS_SCALAR_OBJCARC_H
    399