1 //===- ObjCARCAnalysisUtils.h - ObjC ARC Analysis Utilities -----*- C++ -*-===// 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 /// \file 9 /// This file defines common analysis utilities used by the ObjC ARC Optimizer. 10 /// ARC stands for Automatic Reference Counting and is a system for managing 11 /// reference counts for objects in Objective C. 12 /// 13 /// WARNING: This file knows about certain library functions. It recognizes them 14 /// by name, and hardwires knowledge of their semantics. 15 /// 16 /// WARNING: This file knows about how certain Objective-C library functions are 17 /// used. Naive LLVM IR transformations which would otherwise be 18 /// behavior-preserving may break these assumptions. 19 /// 20 //===----------------------------------------------------------------------===// 21 22 #ifndef LLVM_LIB_ANALYSIS_OBJCARCANALYSISUTILS_H 23 #define LLVM_LIB_ANALYSIS_OBJCARCANALYSISUTILS_H 24 25 #include "llvm/ADT/Optional.h" 26 #include "llvm/ADT/StringSwitch.h" 27 #include "llvm/Analysis/AliasAnalysis.h" 28 #include "llvm/Analysis/ObjCARCInstKind.h" 29 #include "llvm/Analysis/Passes.h" 30 #include "llvm/Analysis/ValueTracking.h" 31 #include "llvm/IR/CallSite.h" 32 #include "llvm/IR/Constants.h" 33 #include "llvm/IR/InstIterator.h" 34 #include "llvm/IR/LLVMContext.h" 35 #include "llvm/IR/Module.h" 36 #include "llvm/IR/ValueHandle.h" 37 #include "llvm/Pass.h" 38 39 namespace llvm { 40 class raw_ostream; 41 } 42 43 namespace llvm { 44 namespace objcarc { 45 46 /// A handy option to enable/disable all ARC Optimizations. 47 extern bool EnableARCOpts; 48 49 /// Test if the given module looks interesting to run ARC optimization 50 /// on. 51 inline bool ModuleHasARC(const Module &M) { 52 return 53 M.getNamedValue("llvm.objc.retain") || 54 M.getNamedValue("llvm.objc.release") || 55 M.getNamedValue("llvm.objc.autorelease") || 56 M.getNamedValue("llvm.objc.retainAutoreleasedReturnValue") || 57 M.getNamedValue("llvm.objc.unsafeClaimAutoreleasedReturnValue") || 58 M.getNamedValue("llvm.objc.retainBlock") || 59 M.getNamedValue("llvm.objc.autoreleaseReturnValue") || 60 M.getNamedValue("llvm.objc.autoreleasePoolPush") || 61 M.getNamedValue("llvm.objc.loadWeakRetained") || 62 M.getNamedValue("llvm.objc.loadWeak") || 63 M.getNamedValue("llvm.objc.destroyWeak") || 64 M.getNamedValue("llvm.objc.storeWeak") || 65 M.getNamedValue("llvm.objc.initWeak") || 66 M.getNamedValue("llvm.objc.moveWeak") || 67 M.getNamedValue("llvm.objc.copyWeak") || 68 M.getNamedValue("llvm.objc.retainedObject") || 69 M.getNamedValue("llvm.objc.unretainedObject") || 70 M.getNamedValue("llvm.objc.unretainedPointer") || 71 M.getNamedValue("llvm.objc.clang.arc.use"); 72 } 73 74 /// 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. 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 /// A wrapper for GetUnderlyingObjCPtr used for results memoization. 90 inline const Value * 91 GetUnderlyingObjCPtrCached(const Value *V, const DataLayout &DL, 92 DenseMap<const Value *, WeakTrackingVH> &Cache) { 93 if (auto InCache = Cache.lookup(V)) 94 return InCache; 95 96 const Value *Computed = GetUnderlyingObjCPtr(V, DL); 97 Cache[V] = const_cast<Value *>(Computed); 98 return Computed; 99 } 100 101 /// The RCIdentity root of a value \p V is a dominating value U for which 102 /// retaining or releasing U is equivalent to retaining or releasing V. In other 103 /// words, ARC operations on \p V are equivalent to ARC operations on \p U. 104 /// 105 /// We use this in the ARC optimizer to make it easier to match up ARC 106 /// operations by always mapping ARC operations to RCIdentityRoots instead of 107 /// pointers themselves. 108 /// 109 /// The two ways that we see RCIdentical values in ObjC are via: 110 /// 111 /// 1. PointerCasts 112 /// 2. Forwarding Calls that return their argument verbatim. 113 /// 114 /// Thus this function strips off pointer casts and forwarding calls. *NOTE* 115 /// This implies that two RCIdentical values must alias. 116 inline const Value *GetRCIdentityRoot(const Value *V) { 117 for (;;) { 118 V = V->stripPointerCasts(); 119 if (!IsForwarding(GetBasicARCInstKind(V))) 120 break; 121 V = cast<CallInst>(V)->getArgOperand(0); 122 } 123 return V; 124 } 125 126 /// Helper which calls const Value *GetRCIdentityRoot(const Value *V) and just 127 /// casts away the const of the result. For documentation about what an 128 /// RCIdentityRoot (and by extension GetRCIdentityRoot is) look at that 129 /// function. 130 inline Value *GetRCIdentityRoot(Value *V) { 131 return const_cast<Value *>(GetRCIdentityRoot((const Value *)V)); 132 } 133 134 /// Assuming the given instruction is one of the special calls such as 135 /// objc_retain or objc_release, return the RCIdentity root of the argument of 136 /// the call. 137 inline Value *GetArgRCIdentityRoot(Value *Inst) { 138 return GetRCIdentityRoot(cast<CallInst>(Inst)->getArgOperand(0)); 139 } 140 141 inline bool IsNullOrUndef(const Value *V) { 142 return isa<ConstantPointerNull>(V) || isa<UndefValue>(V); 143 } 144 145 inline bool IsNoopInstruction(const Instruction *I) { 146 return isa<BitCastInst>(I) || 147 (isa<GetElementPtrInst>(I) && 148 cast<GetElementPtrInst>(I)->hasAllZeroIndices()); 149 } 150 151 /// Test whether the given value is possible a retainable object pointer. 152 inline bool IsPotentialRetainableObjPtr(const Value *Op) { 153 // Pointers to static or stack storage are not valid retainable object 154 // pointers. 155 if (isa<Constant>(Op) || isa<AllocaInst>(Op)) 156 return false; 157 // Special arguments can not be a valid retainable object pointer. 158 if (const Argument *Arg = dyn_cast<Argument>(Op)) 159 if (Arg->hasByValAttr() || 160 Arg->hasInAllocaAttr() || 161 Arg->hasNestAttr() || 162 Arg->hasStructRetAttr()) 163 return false; 164 // Only consider values with pointer types. 165 // 166 // It seemes intuitive to exclude function pointer types as well, since 167 // functions are never retainable object pointers, however clang occasionally 168 // bitcasts retainable object pointers to function-pointer type temporarily. 169 PointerType *Ty = dyn_cast<PointerType>(Op->getType()); 170 if (!Ty) 171 return false; 172 // Conservatively assume anything else is a potential retainable object 173 // pointer. 174 return true; 175 } 176 177 inline bool IsPotentialRetainableObjPtr(const Value *Op, 178 AliasAnalysis &AA) { 179 // First make the rudimentary check. 180 if (!IsPotentialRetainableObjPtr(Op)) 181 return false; 182 183 // Objects in constant memory are not reference-counted. 184 if (AA.pointsToConstantMemory(Op)) 185 return false; 186 187 // Pointers in constant memory are not pointing to reference-counted objects. 188 if (const LoadInst *LI = dyn_cast<LoadInst>(Op)) 189 if (AA.pointsToConstantMemory(LI->getPointerOperand())) 190 return false; 191 192 // Otherwise assume the worst. 193 return true; 194 } 195 196 /// Helper for GetARCInstKind. Determines what kind of construct CS 197 /// is. 198 inline ARCInstKind GetCallSiteClass(ImmutableCallSite CS) { 199 for (ImmutableCallSite::arg_iterator I = CS.arg_begin(), E = CS.arg_end(); 200 I != E; ++I) 201 if (IsPotentialRetainableObjPtr(*I)) 202 return CS.onlyReadsMemory() ? ARCInstKind::User : ARCInstKind::CallOrUser; 203 204 return CS.onlyReadsMemory() ? ARCInstKind::None : ARCInstKind::Call; 205 } 206 207 /// Return true if this value refers to a distinct and identifiable 208 /// object. 209 /// 210 /// This is similar to AliasAnalysis's isIdentifiedObject, except that it uses 211 /// special knowledge of ObjC conventions. 212 inline bool IsObjCIdentifiedObject(const Value *V) { 213 // Assume that call results and arguments have their own "provenance". 214 // Constants (including GlobalVariables) and Allocas are never 215 // reference-counted. 216 if (isa<CallInst>(V) || isa<InvokeInst>(V) || 217 isa<Argument>(V) || isa<Constant>(V) || 218 isa<AllocaInst>(V)) 219 return true; 220 221 if (const LoadInst *LI = dyn_cast<LoadInst>(V)) { 222 const Value *Pointer = 223 GetRCIdentityRoot(LI->getPointerOperand()); 224 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Pointer)) { 225 // A constant pointer can't be pointing to an object on the heap. It may 226 // be reference-counted, but it won't be deleted. 227 if (GV->isConstant()) 228 return true; 229 StringRef Name = GV->getName(); 230 // These special variables are known to hold values which are not 231 // reference-counted pointers. 232 if (Name.startswith("\01l_objc_msgSend_fixup_")) 233 return true; 234 235 StringRef Section = GV->getSection(); 236 if (Section.find("__message_refs") != StringRef::npos || 237 Section.find("__objc_classrefs") != StringRef::npos || 238 Section.find("__objc_superrefs") != StringRef::npos || 239 Section.find("__objc_methname") != StringRef::npos || 240 Section.find("__cstring") != StringRef::npos) 241 return true; 242 } 243 } 244 245 return false; 246 } 247 248 enum class ARCMDKindID { 249 ImpreciseRelease, 250 CopyOnEscape, 251 NoObjCARCExceptions, 252 }; 253 254 /// A cache of MDKinds used by various ARC optimizations. 255 class ARCMDKindCache { 256 Module *M; 257 258 /// The Metadata Kind for clang.imprecise_release metadata. 259 llvm::Optional<unsigned> ImpreciseReleaseMDKind; 260 261 /// The Metadata Kind for clang.arc.copy_on_escape metadata. 262 llvm::Optional<unsigned> CopyOnEscapeMDKind; 263 264 /// The Metadata Kind for clang.arc.no_objc_arc_exceptions metadata. 265 llvm::Optional<unsigned> NoObjCARCExceptionsMDKind; 266 267 public: 268 void init(Module *Mod) { 269 M = Mod; 270 ImpreciseReleaseMDKind = NoneType::None; 271 CopyOnEscapeMDKind = NoneType::None; 272 NoObjCARCExceptionsMDKind = NoneType::None; 273 } 274 275 unsigned get(ARCMDKindID ID) { 276 switch (ID) { 277 case ARCMDKindID::ImpreciseRelease: 278 if (!ImpreciseReleaseMDKind) 279 ImpreciseReleaseMDKind = 280 M->getContext().getMDKindID("clang.imprecise_release"); 281 return *ImpreciseReleaseMDKind; 282 case ARCMDKindID::CopyOnEscape: 283 if (!CopyOnEscapeMDKind) 284 CopyOnEscapeMDKind = 285 M->getContext().getMDKindID("clang.arc.copy_on_escape"); 286 return *CopyOnEscapeMDKind; 287 case ARCMDKindID::NoObjCARCExceptions: 288 if (!NoObjCARCExceptionsMDKind) 289 NoObjCARCExceptionsMDKind = 290 M->getContext().getMDKindID("clang.arc.no_objc_arc_exceptions"); 291 return *NoObjCARCExceptionsMDKind; 292 } 293 llvm_unreachable("Covered switch isn't covered?!"); 294 } 295 }; 296 297 } // end namespace objcarc 298 } // end namespace llvm 299 300 #endif 301