include/llvm/Analysis/ObjCARCAnalysisUtils.h

   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