1 //===- ObjCARCAnalysisUtils.h - ObjC ARC Analysis Utilities -----*- 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 analysis utilities used by the ObjC ARC Optimizer.
11 /// ARC stands for Automatic Reference Counting and is a system for managing
12 /// 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_LIB_ANALYSIS_OBJCARCANALYSISUTILS_H
24 #define LLVM_LIB_ANALYSIS_OBJCARCANALYSISUTILS_H
25 
26 #include "llvm/ADT/StringSwitch.h"
27 #include "llvm/ADT/Optional.h"
28 #include "llvm/Analysis/AliasAnalysis.h"
29 #include "llvm/Analysis/ObjCARCInstKind.h"
30 #include "llvm/Analysis/Passes.h"
31 #include "llvm/Analysis/ValueTracking.h"
32 #include "llvm/IR/CallSite.h"
33 #include "llvm/IR/Constants.h"
34 #include "llvm/IR/InstIterator.h"
35 #include "llvm/IR/LLVMContext.h"
36 #include "llvm/IR/Module.h"
37 #include "llvm/Pass.h"
38 
39 namespace llvm {
40 class raw_ostream;
41 }
42 
43 namespace llvm {
44 namespace objcarc {
45 
46 /// \brief A handy option to enable/disable all ARC Optimizations.
47 extern bool EnableARCOpts;
48 
49 /// \brief Test if the given module looks interesting to run ARC optimization
50 /// on.
ModuleHasARC(const Module & M)51 inline bool ModuleHasARC(const Module &M) {
52   return
53     M.getNamedValue("objc_retain") ||
54     M.getNamedValue("objc_release") ||
55     M.getNamedValue("objc_autorelease") ||
56     M.getNamedValue("objc_retainAutoreleasedReturnValue") ||
57     M.getNamedValue("objc_retainBlock") ||
58     M.getNamedValue("objc_autoreleaseReturnValue") ||
59     M.getNamedValue("objc_autoreleasePoolPush") ||
60     M.getNamedValue("objc_loadWeakRetained") ||
61     M.getNamedValue("objc_loadWeak") ||
62     M.getNamedValue("objc_destroyWeak") ||
63     M.getNamedValue("objc_storeWeak") ||
64     M.getNamedValue("objc_initWeak") ||
65     M.getNamedValue("objc_moveWeak") ||
66     M.getNamedValue("objc_copyWeak") ||
67     M.getNamedValue("objc_retainedObject") ||
68     M.getNamedValue("objc_unretainedObject") ||
69     M.getNamedValue("objc_unretainedPointer") ||
70     M.getNamedValue("clang.arc.use");
71 }
72 
73 /// \brief This is a wrapper around getUnderlyingObject which also knows how to
74 /// look through objc_retain and objc_autorelease calls, which we know to return
75 /// their argument verbatim.
GetUnderlyingObjCPtr(const Value * V,const DataLayout & DL)76 inline const Value *GetUnderlyingObjCPtr(const Value *V,
77                                                 const DataLayout &DL) {
78   for (;;) {
79     V = GetUnderlyingObject(V, DL);
80     if (!IsForwarding(GetBasicARCInstKind(V)))
81       break;
82     V = cast<CallInst>(V)->getArgOperand(0);
83   }
84 
85   return V;
86 }
87 
88 /// The RCIdentity root of a value \p V is a dominating value U for which
89 /// retaining or releasing U is equivalent to retaining or releasing V. In other
90 /// words, ARC operations on \p V are equivalent to ARC operations on \p U.
91 ///
92 /// We use this in the ARC optimizer to make it easier to match up ARC
93 /// operations by always mapping ARC operations to RCIdentityRoots instead of
94 /// pointers themselves.
95 ///
96 /// The two ways that we see RCIdentical values in ObjC are via:
97 ///
98 ///   1. PointerCasts
99 ///   2. Forwarding Calls that return their argument verbatim.
100 ///
101 /// Thus this function strips off pointer casts and forwarding calls. *NOTE*
102 /// This implies that two RCIdentical values must alias.
GetRCIdentityRoot(const Value * V)103 inline const Value *GetRCIdentityRoot(const Value *V) {
104   for (;;) {
105     V = V->stripPointerCasts();
106     if (!IsForwarding(GetBasicARCInstKind(V)))
107       break;
108     V = cast<CallInst>(V)->getArgOperand(0);
109   }
110   return V;
111 }
112 
113 /// Helper which calls const Value *GetRCIdentityRoot(const Value *V) and just
114 /// casts away the const of the result. For documentation about what an
115 /// RCIdentityRoot (and by extension GetRCIdentityRoot is) look at that
116 /// function.
GetRCIdentityRoot(Value * V)117 inline Value *GetRCIdentityRoot(Value *V) {
118   return const_cast<Value *>(GetRCIdentityRoot((const Value *)V));
119 }
120 
121 /// \brief Assuming the given instruction is one of the special calls such as
122 /// objc_retain or objc_release, return the RCIdentity root of the argument of
123 /// the call.
GetArgRCIdentityRoot(Value * Inst)124 inline Value *GetArgRCIdentityRoot(Value *Inst) {
125   return GetRCIdentityRoot(cast<CallInst>(Inst)->getArgOperand(0));
126 }
127 
IsNullOrUndef(const Value * V)128 inline bool IsNullOrUndef(const Value *V) {
129   return isa<ConstantPointerNull>(V) || isa<UndefValue>(V);
130 }
131 
IsNoopInstruction(const Instruction * I)132 inline bool IsNoopInstruction(const Instruction *I) {
133   return isa<BitCastInst>(I) ||
134     (isa<GetElementPtrInst>(I) &&
135      cast<GetElementPtrInst>(I)->hasAllZeroIndices());
136 }
137 
138 /// \brief Test whether the given value is possible a retainable object pointer.
IsPotentialRetainableObjPtr(const Value * Op)139 inline bool IsPotentialRetainableObjPtr(const Value *Op) {
140   // Pointers to static or stack storage are not valid retainable object
141   // pointers.
142   if (isa<Constant>(Op) || isa<AllocaInst>(Op))
143     return false;
144   // Special arguments can not be a valid retainable object pointer.
145   if (const Argument *Arg = dyn_cast<Argument>(Op))
146     if (Arg->hasByValAttr() ||
147         Arg->hasInAllocaAttr() ||
148         Arg->hasNestAttr() ||
149         Arg->hasStructRetAttr())
150       return false;
151   // Only consider values with pointer types.
152   //
153   // It seemes intuitive to exclude function pointer types as well, since
154   // functions are never retainable object pointers, however clang occasionally
155   // bitcasts retainable object pointers to function-pointer type temporarily.
156   PointerType *Ty = dyn_cast<PointerType>(Op->getType());
157   if (!Ty)
158     return false;
159   // Conservatively assume anything else is a potential retainable object
160   // pointer.
161   return true;
162 }
163 
IsPotentialRetainableObjPtr(const Value * Op,AliasAnalysis & AA)164 inline bool IsPotentialRetainableObjPtr(const Value *Op,
165                                                AliasAnalysis &AA) {
166   // First make the rudimentary check.
167   if (!IsPotentialRetainableObjPtr(Op))
168     return false;
169 
170   // Objects in constant memory are not reference-counted.
171   if (AA.pointsToConstantMemory(Op))
172     return false;
173 
174   // Pointers in constant memory are not pointing to reference-counted objects.
175   if (const LoadInst *LI = dyn_cast<LoadInst>(Op))
176     if (AA.pointsToConstantMemory(LI->getPointerOperand()))
177       return false;
178 
179   // Otherwise assume the worst.
180   return true;
181 }
182 
183 /// \brief Helper for GetARCInstKind. Determines what kind of construct CS
184 /// is.
GetCallSiteClass(ImmutableCallSite CS)185 inline ARCInstKind GetCallSiteClass(ImmutableCallSite CS) {
186   for (ImmutableCallSite::arg_iterator I = CS.arg_begin(), E = CS.arg_end();
187        I != E; ++I)
188     if (IsPotentialRetainableObjPtr(*I))
189       return CS.onlyReadsMemory() ? ARCInstKind::User : ARCInstKind::CallOrUser;
190 
191   return CS.onlyReadsMemory() ? ARCInstKind::None : ARCInstKind::Call;
192 }
193 
194 /// \brief Return true if this value refers to a distinct and identifiable
195 /// object.
196 ///
197 /// This is similar to AliasAnalysis's isIdentifiedObject, except that it uses
198 /// special knowledge of ObjC conventions.
IsObjCIdentifiedObject(const Value * V)199 inline bool IsObjCIdentifiedObject(const Value *V) {
200   // Assume that call results and arguments have their own "provenance".
201   // Constants (including GlobalVariables) and Allocas are never
202   // reference-counted.
203   if (isa<CallInst>(V) || isa<InvokeInst>(V) ||
204       isa<Argument>(V) || isa<Constant>(V) ||
205       isa<AllocaInst>(V))
206     return true;
207 
208   if (const LoadInst *LI = dyn_cast<LoadInst>(V)) {
209     const Value *Pointer =
210       GetRCIdentityRoot(LI->getPointerOperand());
211     if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Pointer)) {
212       // A constant pointer can't be pointing to an object on the heap. It may
213       // be reference-counted, but it won't be deleted.
214       if (GV->isConstant())
215         return true;
216       StringRef Name = GV->getName();
217       // These special variables are known to hold values which are not
218       // reference-counted pointers.
219       if (Name.startswith("\01l_objc_msgSend_fixup_"))
220         return true;
221 
222       StringRef Section = GV->getSection();
223       if (Section.find("__message_refs") != StringRef::npos ||
224           Section.find("__objc_classrefs") != StringRef::npos ||
225           Section.find("__objc_superrefs") != StringRef::npos ||
226           Section.find("__objc_methname") != StringRef::npos ||
227           Section.find("__cstring") != StringRef::npos)
228         return true;
229     }
230   }
231 
232   return false;
233 }
234 
235 enum class ARCMDKindID {
236   ImpreciseRelease,
237   CopyOnEscape,
238   NoObjCARCExceptions,
239 };
240 
241 /// A cache of MDKinds used by various ARC optimizations.
242 class ARCMDKindCache {
243   Module *M;
244 
245   /// The Metadata Kind for clang.imprecise_release metadata.
246   llvm::Optional<unsigned> ImpreciseReleaseMDKind;
247 
248   /// The Metadata Kind for clang.arc.copy_on_escape metadata.
249   llvm::Optional<unsigned> CopyOnEscapeMDKind;
250 
251   /// The Metadata Kind for clang.arc.no_objc_arc_exceptions metadata.
252   llvm::Optional<unsigned> NoObjCARCExceptionsMDKind;
253 
254 public:
init(Module * Mod)255   void init(Module *Mod) {
256     M = Mod;
257     ImpreciseReleaseMDKind = NoneType::None;
258     CopyOnEscapeMDKind = NoneType::None;
259     NoObjCARCExceptionsMDKind = NoneType::None;
260   }
261 
get(ARCMDKindID ID)262   unsigned get(ARCMDKindID ID) {
263     switch (ID) {
264     case ARCMDKindID::ImpreciseRelease:
265       if (!ImpreciseReleaseMDKind)
266         ImpreciseReleaseMDKind =
267             M->getContext().getMDKindID("clang.imprecise_release");
268       return *ImpreciseReleaseMDKind;
269     case ARCMDKindID::CopyOnEscape:
270       if (!CopyOnEscapeMDKind)
271         CopyOnEscapeMDKind =
272             M->getContext().getMDKindID("clang.arc.copy_on_escape");
273       return *CopyOnEscapeMDKind;
274     case ARCMDKindID::NoObjCARCExceptions:
275       if (!NoObjCARCExceptionsMDKind)
276         NoObjCARCExceptionsMDKind =
277             M->getContext().getMDKindID("clang.arc.no_objc_arc_exceptions");
278       return *NoObjCARCExceptionsMDKind;
279     }
280     llvm_unreachable("Covered switch isn't covered?!");
281   }
282 };
283 
284 } // end namespace objcarc
285 } // end namespace llvm
286 
287 #endif
288