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_ANALYSIS_OBJCARCANALYSISUTILS_H 23 #define LLVM_ANALYSIS_OBJCARCANALYSISUTILS_H 24 25 #include "llvm/Analysis/ObjCARCInstKind.h" 26 #include "llvm/Analysis/ValueTracking.h" 27 #include "llvm/IR/Constants.h" 28 #include "llvm/IR/Module.h" 29 #include "llvm/IR/ValueHandle.h" 30 #include <optional> 31 32 namespace llvm { 33 34 class AAResults; 35 36 namespace objcarc { 37 38 /// A handy option to enable/disable all ARC Optimizations. 39 extern bool EnableARCOpts; 40 41 /// Test if the given module looks interesting to run ARC optimization 42 /// on. 43 inline bool ModuleHasARC(const Module &M) { 44 return 45 M.getNamedValue("llvm.objc.retain") || 46 M.getNamedValue("llvm.objc.release") || 47 M.getNamedValue("llvm.objc.autorelease") || 48 M.getNamedValue("llvm.objc.retainAutoreleasedReturnValue") || 49 M.getNamedValue("llvm.objc.unsafeClaimAutoreleasedReturnValue") || 50 M.getNamedValue("llvm.objc.retainBlock") || 51 M.getNamedValue("llvm.objc.autoreleaseReturnValue") || 52 M.getNamedValue("llvm.objc.autoreleasePoolPush") || 53 M.getNamedValue("llvm.objc.loadWeakRetained") || 54 M.getNamedValue("llvm.objc.loadWeak") || 55 M.getNamedValue("llvm.objc.destroyWeak") || 56 M.getNamedValue("llvm.objc.storeWeak") || 57 M.getNamedValue("llvm.objc.initWeak") || 58 M.getNamedValue("llvm.objc.moveWeak") || 59 M.getNamedValue("llvm.objc.copyWeak") || 60 M.getNamedValue("llvm.objc.retainedObject") || 61 M.getNamedValue("llvm.objc.unretainedObject") || 62 M.getNamedValue("llvm.objc.unretainedPointer") || 63 M.getNamedValue("llvm.objc.clang.arc.use"); 64 } 65 66 /// This is a wrapper around getUnderlyingObject which also knows how to 67 /// look through objc_retain and objc_autorelease calls, which we know to return 68 /// their argument verbatim. 69 inline const Value *GetUnderlyingObjCPtr(const Value *V) { 70 for (;;) { 71 V = getUnderlyingObject(V); 72 if (!IsForwarding(GetBasicARCInstKind(V))) 73 break; 74 V = cast<CallInst>(V)->getArgOperand(0); 75 } 76 77 return V; 78 } 79 80 /// A wrapper for GetUnderlyingObjCPtr used for results memoization. 81 inline const Value *GetUnderlyingObjCPtrCached( 82 const Value *V, 83 DenseMap<const Value *, std::pair<WeakVH, WeakTrackingVH>> &Cache) { 84 // The entry is invalid if either value handle is null. 85 auto InCache = Cache.lookup(V); 86 if (InCache.first && InCache.second) 87 return InCache.second; 88 89 const Value *Computed = GetUnderlyingObjCPtr(V); 90 Cache[V] = 91 std::make_pair(const_cast<Value *>(V), const_cast<Value *>(Computed)); 92 return Computed; 93 } 94 95 /// The RCIdentity root of a value \p V is a dominating value U for which 96 /// retaining or releasing U is equivalent to retaining or releasing V. In other 97 /// words, ARC operations on \p V are equivalent to ARC operations on \p U. 98 /// 99 /// We use this in the ARC optimizer to make it easier to match up ARC 100 /// operations by always mapping ARC operations to RCIdentityRoots instead of 101 /// pointers themselves. 102 /// 103 /// The two ways that we see RCIdentical values in ObjC are via: 104 /// 105 /// 1. PointerCasts 106 /// 2. Forwarding Calls that return their argument verbatim. 107 /// 108 /// Thus this function strips off pointer casts and forwarding calls. *NOTE* 109 /// This implies that two RCIdentical values must alias. 110 inline const Value *GetRCIdentityRoot(const Value *V) { 111 for (;;) { 112 V = V->stripPointerCasts(); 113 if (!IsForwarding(GetBasicARCInstKind(V))) 114 break; 115 V = cast<CallInst>(V)->getArgOperand(0); 116 } 117 return V; 118 } 119 120 /// Helper which calls const Value *GetRCIdentityRoot(const Value *V) and just 121 /// casts away the const of the result. For documentation about what an 122 /// RCIdentityRoot (and by extension GetRCIdentityRoot is) look at that 123 /// function. 124 inline Value *GetRCIdentityRoot(Value *V) { 125 return const_cast<Value *>(GetRCIdentityRoot((const Value *)V)); 126 } 127 128 /// Assuming the given instruction is one of the special calls such as 129 /// objc_retain or objc_release, return the RCIdentity root of the argument of 130 /// the call. 131 inline Value *GetArgRCIdentityRoot(Value *Inst) { 132 return GetRCIdentityRoot(cast<CallInst>(Inst)->getArgOperand(0)); 133 } 134 135 inline bool IsNullOrUndef(const Value *V) { 136 return isa<ConstantPointerNull>(V) || isa<UndefValue>(V); 137 } 138 139 inline bool IsNoopInstruction(const Instruction *I) { 140 return isa<BitCastInst>(I) || 141 (isa<GetElementPtrInst>(I) && 142 cast<GetElementPtrInst>(I)->hasAllZeroIndices()); 143 } 144 145 /// Test whether the given value is possible a retainable object pointer. 146 inline bool IsPotentialRetainableObjPtr(const Value *Op) { 147 // Pointers to static or stack storage are not valid retainable object 148 // pointers. 149 if (isa<Constant>(Op) || isa<AllocaInst>(Op)) 150 return false; 151 // Special arguments can not be a valid retainable object pointer. 152 if (const Argument *Arg = dyn_cast<Argument>(Op)) 153 if (Arg->hasPassPointeeByValueCopyAttr() || Arg->hasNestAttr() || 154 Arg->hasStructRetAttr()) 155 return false; 156 // Only consider values with pointer types. 157 // 158 // It seemes intuitive to exclude function pointer types as well, since 159 // functions are never retainable object pointers, however clang occasionally 160 // bitcasts retainable object pointers to function-pointer type temporarily. 161 PointerType *Ty = dyn_cast<PointerType>(Op->getType()); 162 if (!Ty) 163 return false; 164 // Conservatively assume anything else is a potential retainable object 165 // pointer. 166 return true; 167 } 168 169 bool IsPotentialRetainableObjPtr(const Value *Op, AAResults &AA); 170 171 /// Helper for GetARCInstKind. Determines what kind of construct CS 172 /// is. 173 inline ARCInstKind GetCallSiteClass(const CallBase &CB) { 174 for (const Use &U : CB.args()) 175 if (IsPotentialRetainableObjPtr(U)) 176 return CB.onlyReadsMemory() ? ARCInstKind::User : ARCInstKind::CallOrUser; 177 178 return CB.onlyReadsMemory() ? ARCInstKind::None : ARCInstKind::Call; 179 } 180 181 /// Return true if this value refers to a distinct and identifiable 182 /// object. 183 /// 184 /// This is similar to AliasAnalysis's isIdentifiedObject, except that it uses 185 /// special knowledge of ObjC conventions. 186 inline bool IsObjCIdentifiedObject(const Value *V) { 187 // Assume that call results and arguments have their own "provenance". 188 // Constants (including GlobalVariables) and Allocas are never 189 // reference-counted. 190 if (isa<CallInst>(V) || isa<InvokeInst>(V) || 191 isa<Argument>(V) || isa<Constant>(V) || 192 isa<AllocaInst>(V)) 193 return true; 194 195 if (const LoadInst *LI = dyn_cast<LoadInst>(V)) { 196 const Value *Pointer = 197 GetRCIdentityRoot(LI->getPointerOperand()); 198 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Pointer)) { 199 // A constant pointer can't be pointing to an object on the heap. It may 200 // be reference-counted, but it won't be deleted. 201 if (GV->isConstant()) 202 return true; 203 StringRef Name = GV->getName(); 204 // These special variables are known to hold values which are not 205 // reference-counted pointers. 206 if (Name.starts_with("\01l_objc_msgSend_fixup_")) 207 return true; 208 209 StringRef Section = GV->getSection(); 210 if (Section.contains("__message_refs") || 211 Section.contains("__objc_classrefs") || 212 Section.contains("__objc_superrefs") || 213 Section.contains("__objc_methname") || Section.contains("__cstring")) 214 return true; 215 } 216 } 217 218 return false; 219 } 220 221 enum class ARCMDKindID { 222 ImpreciseRelease, 223 CopyOnEscape, 224 NoObjCARCExceptions, 225 }; 226 227 /// A cache of MDKinds used by various ARC optimizations. 228 class ARCMDKindCache { 229 Module *M; 230 231 /// The Metadata Kind for clang.imprecise_release metadata. 232 std::optional<unsigned> ImpreciseReleaseMDKind; 233 234 /// The Metadata Kind for clang.arc.copy_on_escape metadata. 235 std::optional<unsigned> CopyOnEscapeMDKind; 236 237 /// The Metadata Kind for clang.arc.no_objc_arc_exceptions metadata. 238 std::optional<unsigned> NoObjCARCExceptionsMDKind; 239 240 public: 241 void init(Module *Mod) { 242 M = Mod; 243 ImpreciseReleaseMDKind = std::nullopt; 244 CopyOnEscapeMDKind = std::nullopt; 245 NoObjCARCExceptionsMDKind = std::nullopt; 246 } 247 248 unsigned get(ARCMDKindID ID) { 249 switch (ID) { 250 case ARCMDKindID::ImpreciseRelease: 251 if (!ImpreciseReleaseMDKind) 252 ImpreciseReleaseMDKind = 253 M->getContext().getMDKindID("clang.imprecise_release"); 254 return *ImpreciseReleaseMDKind; 255 case ARCMDKindID::CopyOnEscape: 256 if (!CopyOnEscapeMDKind) 257 CopyOnEscapeMDKind = 258 M->getContext().getMDKindID("clang.arc.copy_on_escape"); 259 return *CopyOnEscapeMDKind; 260 case ARCMDKindID::NoObjCARCExceptions: 261 if (!NoObjCARCExceptionsMDKind) 262 NoObjCARCExceptionsMDKind = 263 M->getContext().getMDKindID("clang.arc.no_objc_arc_exceptions"); 264 return *NoObjCARCExceptionsMDKind; 265 } 266 llvm_unreachable("Covered switch isn't covered?!"); 267 } 268 }; 269 270 } // end namespace objcarc 271 } // end namespace llvm 272 273 #endif 274