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_LIB_TRANSFORMS_OBJCARC_OBJCARC_H
24 #define LLVM_LIB_TRANSFORMS_OBJCARC_OBJCARC_H
25
26 #include "llvm/ADT/StringSwitch.h"
27 #include "llvm/ADT/Optional.h"
28 #include "llvm/Analysis/AliasAnalysis.h"
29 #include "llvm/Analysis/Passes.h"
30 #include "llvm/Analysis/ValueTracking.h"
31 #include "llvm/IR/CallSite.h"
32 #include "llvm/IR/InstIterator.h"
33 #include "llvm/IR/Module.h"
34 #include "llvm/Pass.h"
35 #include "llvm/Transforms/ObjCARC.h"
36 #include "llvm/Transforms/Utils/Local.h"
37 #include "ARCInstKind.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 static 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 static 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 static 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 static 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 static inline Value *GetArgRCIdentityRoot(Value *Inst) {
125 return GetRCIdentityRoot(cast<CallInst>(Inst)->getArgOperand(0));
126 }
127
IsNullOrUndef(const Value * V)128 static inline bool IsNullOrUndef(const Value *V) {
129 return isa<ConstantPointerNull>(V) || isa<UndefValue>(V);
130 }
131
IsNoopInstruction(const Instruction * I)132 static inline bool IsNoopInstruction(const Instruction *I) {
133 return isa<BitCastInst>(I) ||
134 (isa<GetElementPtrInst>(I) &&
135 cast<GetElementPtrInst>(I)->hasAllZeroIndices());
136 }
137
138
139 /// \brief Erase the given instruction.
140 ///
141 /// Many ObjC calls return their argument verbatim,
142 /// so if it's such a call and the return value has users, replace them with the
143 /// argument value.
144 ///
EraseInstruction(Instruction * CI)145 static inline void EraseInstruction(Instruction *CI) {
146 Value *OldArg = cast<CallInst>(CI)->getArgOperand(0);
147
148 bool Unused = CI->use_empty();
149
150 if (!Unused) {
151 // Replace the return value with the argument.
152 assert((IsForwarding(GetBasicARCInstKind(CI)) ||
153 (IsNoopOnNull(GetBasicARCInstKind(CI)) &&
154 isa<ConstantPointerNull>(OldArg))) &&
155 "Can't delete non-forwarding instruction with users!");
156 CI->replaceAllUsesWith(OldArg);
157 }
158
159 CI->eraseFromParent();
160
161 if (Unused)
162 RecursivelyDeleteTriviallyDeadInstructions(OldArg);
163 }
164
165 /// \brief Test whether the given value is possible a retainable object pointer.
IsPotentialRetainableObjPtr(const Value * Op)166 static inline bool IsPotentialRetainableObjPtr(const Value *Op) {
167 // Pointers to static or stack storage are not valid retainable object
168 // pointers.
169 if (isa<Constant>(Op) || isa<AllocaInst>(Op))
170 return false;
171 // Special arguments can not be a valid retainable object pointer.
172 if (const Argument *Arg = dyn_cast<Argument>(Op))
173 if (Arg->hasByValAttr() ||
174 Arg->hasInAllocaAttr() ||
175 Arg->hasNestAttr() ||
176 Arg->hasStructRetAttr())
177 return false;
178 // Only consider values with pointer types.
179 //
180 // It seemes intuitive to exclude function pointer types as well, since
181 // functions are never retainable object pointers, however clang occasionally
182 // bitcasts retainable object pointers to function-pointer type temporarily.
183 PointerType *Ty = dyn_cast<PointerType>(Op->getType());
184 if (!Ty)
185 return false;
186 // Conservatively assume anything else is a potential retainable object
187 // pointer.
188 return true;
189 }
190
IsPotentialRetainableObjPtr(const Value * Op,AliasAnalysis & AA)191 static inline bool IsPotentialRetainableObjPtr(const Value *Op,
192 AliasAnalysis &AA) {
193 // First make the rudimentary check.
194 if (!IsPotentialRetainableObjPtr(Op))
195 return false;
196
197 // Objects in constant memory are not reference-counted.
198 if (AA.pointsToConstantMemory(Op))
199 return false;
200
201 // Pointers in constant memory are not pointing to reference-counted objects.
202 if (const LoadInst *LI = dyn_cast<LoadInst>(Op))
203 if (AA.pointsToConstantMemory(LI->getPointerOperand()))
204 return false;
205
206 // Otherwise assume the worst.
207 return true;
208 }
209
210 /// \brief Helper for GetARCInstKind. Determines what kind of construct CS
211 /// is.
GetCallSiteClass(ImmutableCallSite CS)212 static inline ARCInstKind GetCallSiteClass(ImmutableCallSite CS) {
213 for (ImmutableCallSite::arg_iterator I = CS.arg_begin(), E = CS.arg_end();
214 I != E; ++I)
215 if (IsPotentialRetainableObjPtr(*I))
216 return CS.onlyReadsMemory() ? ARCInstKind::User : ARCInstKind::CallOrUser;
217
218 return CS.onlyReadsMemory() ? ARCInstKind::None : ARCInstKind::Call;
219 }
220
221 /// \brief Return true if this value refers to a distinct and identifiable
222 /// object.
223 ///
224 /// This is similar to AliasAnalysis's isIdentifiedObject, except that it uses
225 /// special knowledge of ObjC conventions.
IsObjCIdentifiedObject(const Value * V)226 static inline bool IsObjCIdentifiedObject(const Value *V) {
227 // Assume that call results and arguments have their own "provenance".
228 // Constants (including GlobalVariables) and Allocas are never
229 // reference-counted.
230 if (isa<CallInst>(V) || isa<InvokeInst>(V) ||
231 isa<Argument>(V) || isa<Constant>(V) ||
232 isa<AllocaInst>(V))
233 return true;
234
235 if (const LoadInst *LI = dyn_cast<LoadInst>(V)) {
236 const Value *Pointer =
237 GetRCIdentityRoot(LI->getPointerOperand());
238 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Pointer)) {
239 // A constant pointer can't be pointing to an object on the heap. It may
240 // be reference-counted, but it won't be deleted.
241 if (GV->isConstant())
242 return true;
243 StringRef Name = GV->getName();
244 // These special variables are known to hold values which are not
245 // reference-counted pointers.
246 if (Name.startswith("\01l_objc_msgSend_fixup_"))
247 return true;
248
249 StringRef Section = GV->getSection();
250 if (Section.find("__message_refs") != StringRef::npos ||
251 Section.find("__objc_classrefs") != StringRef::npos ||
252 Section.find("__objc_superrefs") != StringRef::npos ||
253 Section.find("__objc_methname") != StringRef::npos ||
254 Section.find("__cstring") != StringRef::npos)
255 return true;
256 }
257 }
258
259 return false;
260 }
261
262 enum class ARCMDKindID {
263 ImpreciseRelease,
264 CopyOnEscape,
265 NoObjCARCExceptions,
266 };
267
268 /// A cache of MDKinds used by various ARC optimizations.
269 class ARCMDKindCache {
270 Module *M;
271
272 /// The Metadata Kind for clang.imprecise_release metadata.
273 llvm::Optional<unsigned> ImpreciseReleaseMDKind;
274
275 /// The Metadata Kind for clang.arc.copy_on_escape metadata.
276 llvm::Optional<unsigned> CopyOnEscapeMDKind;
277
278 /// The Metadata Kind for clang.arc.no_objc_arc_exceptions metadata.
279 llvm::Optional<unsigned> NoObjCARCExceptionsMDKind;
280
281 public:
init(Module * Mod)282 void init(Module *Mod) {
283 M = Mod;
284 ImpreciseReleaseMDKind = NoneType::None;
285 CopyOnEscapeMDKind = NoneType::None;
286 NoObjCARCExceptionsMDKind = NoneType::None;
287 }
288
get(ARCMDKindID ID)289 unsigned get(ARCMDKindID ID) {
290 switch (ID) {
291 case ARCMDKindID::ImpreciseRelease:
292 if (!ImpreciseReleaseMDKind)
293 ImpreciseReleaseMDKind =
294 M->getContext().getMDKindID("clang.imprecise_release");
295 return *ImpreciseReleaseMDKind;
296 case ARCMDKindID::CopyOnEscape:
297 if (!CopyOnEscapeMDKind)
298 CopyOnEscapeMDKind =
299 M->getContext().getMDKindID("clang.arc.copy_on_escape");
300 return *CopyOnEscapeMDKind;
301 case ARCMDKindID::NoObjCARCExceptions:
302 if (!NoObjCARCExceptionsMDKind)
303 NoObjCARCExceptionsMDKind =
304 M->getContext().getMDKindID("clang.arc.no_objc_arc_exceptions");
305 return *NoObjCARCExceptionsMDKind;
306 }
307 llvm_unreachable("Covered switch isn't covered?!");
308 }
309 };
310
311 } // end namespace objcarc
312 } // end namespace llvm
313
314 #endif
315