• Home
  • Line#
  • Scopes#
  • Navigate#
  • Raw
  • Download
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_unsafeClaimAutoreleasedReturnValue") ||
58     M.getNamedValue("objc_retainBlock") ||
59     M.getNamedValue("objc_autoreleaseReturnValue") ||
60     M.getNamedValue("objc_autoreleasePoolPush") ||
61     M.getNamedValue("objc_loadWeakRetained") ||
62     M.getNamedValue("objc_loadWeak") ||
63     M.getNamedValue("objc_destroyWeak") ||
64     M.getNamedValue("objc_storeWeak") ||
65     M.getNamedValue("objc_initWeak") ||
66     M.getNamedValue("objc_moveWeak") ||
67     M.getNamedValue("objc_copyWeak") ||
68     M.getNamedValue("objc_retainedObject") ||
69     M.getNamedValue("objc_unretainedObject") ||
70     M.getNamedValue("objc_unretainedPointer") ||
71     M.getNamedValue("clang.arc.use");
72 }
73 
74 /// \brief This is a wrapper around getUnderlyingObject which also knows how to
75 /// look through objc_retain and objc_autorelease calls, which we know to return
76 /// their argument verbatim.
GetUnderlyingObjCPtr(const Value * V,const DataLayout & DL)77 inline const Value *GetUnderlyingObjCPtr(const Value *V,
78                                                 const DataLayout &DL) {
79   for (;;) {
80     V = GetUnderlyingObject(V, DL);
81     if (!IsForwarding(GetBasicARCInstKind(V)))
82       break;
83     V = cast<CallInst>(V)->getArgOperand(0);
84   }
85 
86   return V;
87 }
88 
89 /// The RCIdentity root of a value \p V is a dominating value U for which
90 /// retaining or releasing U is equivalent to retaining or releasing V. In other
91 /// words, ARC operations on \p V are equivalent to ARC operations on \p U.
92 ///
93 /// We use this in the ARC optimizer to make it easier to match up ARC
94 /// operations by always mapping ARC operations to RCIdentityRoots instead of
95 /// pointers themselves.
96 ///
97 /// The two ways that we see RCIdentical values in ObjC are via:
98 ///
99 ///   1. PointerCasts
100 ///   2. Forwarding Calls that return their argument verbatim.
101 ///
102 /// Thus this function strips off pointer casts and forwarding calls. *NOTE*
103 /// This implies that two RCIdentical values must alias.
GetRCIdentityRoot(const Value * V)104 inline const Value *GetRCIdentityRoot(const Value *V) {
105   for (;;) {
106     V = V->stripPointerCasts();
107     if (!IsForwarding(GetBasicARCInstKind(V)))
108       break;
109     V = cast<CallInst>(V)->getArgOperand(0);
110   }
111   return V;
112 }
113 
114 /// Helper which calls const Value *GetRCIdentityRoot(const Value *V) and just
115 /// casts away the const of the result. For documentation about what an
116 /// RCIdentityRoot (and by extension GetRCIdentityRoot is) look at that
117 /// function.
GetRCIdentityRoot(Value * V)118 inline Value *GetRCIdentityRoot(Value *V) {
119   return const_cast<Value *>(GetRCIdentityRoot((const Value *)V));
120 }
121 
122 /// \brief Assuming the given instruction is one of the special calls such as
123 /// objc_retain or objc_release, return the RCIdentity root of the argument of
124 /// the call.
GetArgRCIdentityRoot(Value * Inst)125 inline Value *GetArgRCIdentityRoot(Value *Inst) {
126   return GetRCIdentityRoot(cast<CallInst>(Inst)->getArgOperand(0));
127 }
128 
IsNullOrUndef(const Value * V)129 inline bool IsNullOrUndef(const Value *V) {
130   return isa<ConstantPointerNull>(V) || isa<UndefValue>(V);
131 }
132 
IsNoopInstruction(const Instruction * I)133 inline bool IsNoopInstruction(const Instruction *I) {
134   return isa<BitCastInst>(I) ||
135     (isa<GetElementPtrInst>(I) &&
136      cast<GetElementPtrInst>(I)->hasAllZeroIndices());
137 }
138 
139 /// \brief Test whether the given value is possible a retainable object pointer.
IsPotentialRetainableObjPtr(const Value * Op)140 inline bool IsPotentialRetainableObjPtr(const Value *Op) {
141   // Pointers to static or stack storage are not valid retainable object
142   // pointers.
143   if (isa<Constant>(Op) || isa<AllocaInst>(Op))
144     return false;
145   // Special arguments can not be a valid retainable object pointer.
146   if (const Argument *Arg = dyn_cast<Argument>(Op))
147     if (Arg->hasByValAttr() ||
148         Arg->hasInAllocaAttr() ||
149         Arg->hasNestAttr() ||
150         Arg->hasStructRetAttr())
151       return false;
152   // Only consider values with pointer types.
153   //
154   // It seemes intuitive to exclude function pointer types as well, since
155   // functions are never retainable object pointers, however clang occasionally
156   // bitcasts retainable object pointers to function-pointer type temporarily.
157   PointerType *Ty = dyn_cast<PointerType>(Op->getType());
158   if (!Ty)
159     return false;
160   // Conservatively assume anything else is a potential retainable object
161   // pointer.
162   return true;
163 }
164 
IsPotentialRetainableObjPtr(const Value * Op,AliasAnalysis & AA)165 inline bool IsPotentialRetainableObjPtr(const Value *Op,
166                                                AliasAnalysis &AA) {
167   // First make the rudimentary check.
168   if (!IsPotentialRetainableObjPtr(Op))
169     return false;
170 
171   // Objects in constant memory are not reference-counted.
172   if (AA.pointsToConstantMemory(Op))
173     return false;
174 
175   // Pointers in constant memory are not pointing to reference-counted objects.
176   if (const LoadInst *LI = dyn_cast<LoadInst>(Op))
177     if (AA.pointsToConstantMemory(LI->getPointerOperand()))
178       return false;
179 
180   // Otherwise assume the worst.
181   return true;
182 }
183 
184 /// \brief Helper for GetARCInstKind. Determines what kind of construct CS
185 /// is.
GetCallSiteClass(ImmutableCallSite CS)186 inline ARCInstKind GetCallSiteClass(ImmutableCallSite CS) {
187   for (ImmutableCallSite::arg_iterator I = CS.arg_begin(), E = CS.arg_end();
188        I != E; ++I)
189     if (IsPotentialRetainableObjPtr(*I))
190       return CS.onlyReadsMemory() ? ARCInstKind::User : ARCInstKind::CallOrUser;
191 
192   return CS.onlyReadsMemory() ? ARCInstKind::None : ARCInstKind::Call;
193 }
194 
195 /// \brief Return true if this value refers to a distinct and identifiable
196 /// object.
197 ///
198 /// This is similar to AliasAnalysis's isIdentifiedObject, except that it uses
199 /// special knowledge of ObjC conventions.
IsObjCIdentifiedObject(const Value * V)200 inline bool IsObjCIdentifiedObject(const Value *V) {
201   // Assume that call results and arguments have their own "provenance".
202   // Constants (including GlobalVariables) and Allocas are never
203   // reference-counted.
204   if (isa<CallInst>(V) || isa<InvokeInst>(V) ||
205       isa<Argument>(V) || isa<Constant>(V) ||
206       isa<AllocaInst>(V))
207     return true;
208 
209   if (const LoadInst *LI = dyn_cast<LoadInst>(V)) {
210     const Value *Pointer =
211       GetRCIdentityRoot(LI->getPointerOperand());
212     if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Pointer)) {
213       // A constant pointer can't be pointing to an object on the heap. It may
214       // be reference-counted, but it won't be deleted.
215       if (GV->isConstant())
216         return true;
217       StringRef Name = GV->getName();
218       // These special variables are known to hold values which are not
219       // reference-counted pointers.
220       if (Name.startswith("\01l_objc_msgSend_fixup_"))
221         return true;
222 
223       StringRef Section = GV->getSection();
224       if (Section.find("__message_refs") != StringRef::npos ||
225           Section.find("__objc_classrefs") != StringRef::npos ||
226           Section.find("__objc_superrefs") != StringRef::npos ||
227           Section.find("__objc_methname") != StringRef::npos ||
228           Section.find("__cstring") != StringRef::npos)
229         return true;
230     }
231   }
232 
233   return false;
234 }
235 
236 enum class ARCMDKindID {
237   ImpreciseRelease,
238   CopyOnEscape,
239   NoObjCARCExceptions,
240 };
241 
242 /// A cache of MDKinds used by various ARC optimizations.
243 class ARCMDKindCache {
244   Module *M;
245 
246   /// The Metadata Kind for clang.imprecise_release metadata.
247   llvm::Optional<unsigned> ImpreciseReleaseMDKind;
248 
249   /// The Metadata Kind for clang.arc.copy_on_escape metadata.
250   llvm::Optional<unsigned> CopyOnEscapeMDKind;
251 
252   /// The Metadata Kind for clang.arc.no_objc_arc_exceptions metadata.
253   llvm::Optional<unsigned> NoObjCARCExceptionsMDKind;
254 
255 public:
init(Module * Mod)256   void init(Module *Mod) {
257     M = Mod;
258     ImpreciseReleaseMDKind = NoneType::None;
259     CopyOnEscapeMDKind = NoneType::None;
260     NoObjCARCExceptionsMDKind = NoneType::None;
261   }
262 
get(ARCMDKindID ID)263   unsigned get(ARCMDKindID ID) {
264     switch (ID) {
265     case ARCMDKindID::ImpreciseRelease:
266       if (!ImpreciseReleaseMDKind)
267         ImpreciseReleaseMDKind =
268             M->getContext().getMDKindID("clang.imprecise_release");
269       return *ImpreciseReleaseMDKind;
270     case ARCMDKindID::CopyOnEscape:
271       if (!CopyOnEscapeMDKind)
272         CopyOnEscapeMDKind =
273             M->getContext().getMDKindID("clang.arc.copy_on_escape");
274       return *CopyOnEscapeMDKind;
275     case ARCMDKindID::NoObjCARCExceptions:
276       if (!NoObjCARCExceptionsMDKind)
277         NoObjCARCExceptionsMDKind =
278             M->getContext().getMDKindID("clang.arc.no_objc_arc_exceptions");
279       return *NoObjCARCExceptionsMDKind;
280     }
281     llvm_unreachable("Covered switch isn't covered?!");
282   }
283 };
284 
285 } // end namespace objcarc
286 } // end namespace llvm
287 
288 #endif
289