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//===- ObjCARCAnalysisUtils.h - ObjC ARC Analysis Utilities -----*- C++ -*-===//

//

// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.

// See https://llvm.org/LICENSE.txt for license information.

// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

//

//===----------------------------------------------------------------------===//

/// \file

/// This file defines common analysis utilities used by the ObjC ARC Optimizer.

/// ARC stands for Automatic Reference Counting and is a system for managing

/// reference counts for objects in Objective C.

///

/// WARNING: This file knows about certain library functions. It recognizes them

/// by name, and hardwires knowledge of their semantics.

///

/// WARNING: This file knows about how certain Objective-C library functions are

/// used. Naive LLVM IR transformations which would otherwise be

/// behavior-preserving may break these assumptions.

///

//===----------------------------------------------------------------------===//

#ifndef LLVM_ANALYSIS_OBJCARCANALYSISUTILS_H

#define LLVM_ANALYSIS_OBJCARCANALYSISUTILS_H

#include "llvm/Analysis/ObjCARCInstKind.h"

#include "llvm/Analysis/ValueTracking.h"

#include "llvm/IR/Constants.h"

#include "llvm/IR/Module.h"

#include "llvm/IR/ValueHandle.h"

#include <optional>

namespace llvm {

class AAResults;

namespace objcarc {

/// A handy option to enable/disable all ARC Optimizations.

extern bool EnableARCOpts;

/// Test if the given module looks interesting to run ARC optimization

/// on.

inline bool ModuleHasARC(const Module &M) {

  return

    M.getNamedValue("llvm.objc.retain") ||

    M.getNamedValue("llvm.objc.release") ||

    M.getNamedValue("llvm.objc.autorelease") ||

    M.getNamedValue("llvm.objc.retainAutoreleasedReturnValue") ||

    M.getNamedValue("llvm.objc.unsafeClaimAutoreleasedReturnValue") ||

    M.getNamedValue("llvm.objc.retainBlock") ||

    M.getNamedValue("llvm.objc.autoreleaseReturnValue") ||

    M.getNamedValue("llvm.objc.autoreleasePoolPush") ||

    M.getNamedValue("llvm.objc.loadWeakRetained") ||

    M.getNamedValue("llvm.objc.loadWeak") ||

    M.getNamedValue("llvm.objc.destroyWeak") ||

    M.getNamedValue("llvm.objc.storeWeak") ||

    M.getNamedValue("llvm.objc.initWeak") ||

    M.getNamedValue("llvm.objc.moveWeak") ||

    M.getNamedValue("llvm.objc.copyWeak") ||

    M.getNamedValue("llvm.objc.retainedObject") ||

    M.getNamedValue("llvm.objc.unretainedObject") ||

    M.getNamedValue("llvm.objc.unretainedPointer") ||

    M.getNamedValue("llvm.objc.clang.arc.use");

}

/// This is a wrapper around getUnderlyingObject which also knows how to

/// look through objc_retain and objc_autorelease calls, which we know to return

/// their argument verbatim.

inline const Value *GetUnderlyingObjCPtr(const Value *V) {

  for (;;) {

    V = getUnderlyingObject(V);

    if (!IsForwarding(GetBasicARCInstKind(V)))

      break;

    V = cast<CallInst>(V)->getArgOperand(0);

  }

  return V;

}

/// A wrapper for GetUnderlyingObjCPtr used for results memoization.

inline const Value *GetUnderlyingObjCPtrCached(

    const Value *V,

    DenseMap<const Value *, std::pair<WeakVH, WeakTrackingVH>> &Cache) {

  // The entry is invalid if either value handle is null.

  auto InCache = Cache.lookup(V);

  if (InCache.first && InCache.second)

    return InCache.second;

  const Value *Computed = GetUnderlyingObjCPtr(V);

  Cache[V] =

      std::make_pair(const_cast<Value *>(V), const_cast<Value *>(Computed));

  return Computed;

}

/// The RCIdentity root of a value \p V is a dominating value U for which

/// retaining or releasing U is equivalent to retaining or releasing V. In other

/// words, ARC operations on \p V are equivalent to ARC operations on \p U.

///

/// We use this in the ARC optimizer to make it easier to match up ARC

/// operations by always mapping ARC operations to RCIdentityRoots instead of

/// pointers themselves.

///

/// The two ways that we see RCIdentical values in ObjC are via:

///

///   1. PointerCasts

///   2. Forwarding Calls that return their argument verbatim.

///

/// Thus this function strips off pointer casts and forwarding calls. *NOTE*

/// This implies that two RCIdentical values must alias.

inline const Value *GetRCIdentityRoot(const Value *V) {

  for (;;) {

    V = V->stripPointerCasts();

    if (!IsForwarding(GetBasicARCInstKind(V)))

      break;

    V = cast<CallInst>(V)->getArgOperand(0);

  }

  return V;

}

/// Helper which calls const Value *GetRCIdentityRoot(const Value *V) and just

/// casts away the const of the result. For documentation about what an

/// RCIdentityRoot (and by extension GetRCIdentityRoot is) look at that

/// function.

inline Value *GetRCIdentityRoot(Value *V) {

  return const_cast<Value *>(GetRCIdentityRoot((const Value *)V));

}

/// Assuming the given instruction is one of the special calls such as

/// objc_retain or objc_release, return the RCIdentity root of the argument of

/// the call.

inline Value *GetArgRCIdentityRoot(Value *Inst) {

  return GetRCIdentityRoot(cast<CallInst>(Inst)->getArgOperand(0));

}

inline bool IsNullOrUndef(const Value *V) {

  return isa<ConstantPointerNull>(V) || isa<UndefValue>(V);

}

inline bool IsNoopInstruction(const Instruction *I) {

  return isa<BitCastInst>(I) ||

    (isa<GetElementPtrInst>(I) &&

     cast<GetElementPtrInst>(I)->hasAllZeroIndices());

}

/// Test whether the given value is possible a retainable object pointer.

inline bool IsPotentialRetainableObjPtr(const Value *Op) {

  // Pointers to static or stack storage are not valid retainable object

  // pointers.

  if (isa<Constant>(Op) || isa<AllocaInst>(Op))

    return false;

  // Special arguments can not be a valid retainable object pointer.

  if (const Argument *Arg = dyn_cast<Argument>(Op))

    if (Arg->hasPassPointeeByValueCopyAttr() || Arg->hasNestAttr() ||

        Arg->hasStructRetAttr())

      return false;

  // Only consider values with pointer types.

  //

  // It seemes intuitive to exclude function pointer types as well, since

  // functions are never retainable object pointers, however clang occasionally

  // bitcasts retainable object pointers to function-pointer type temporarily.

  PointerType *Ty = dyn_cast<PointerType>(Op->getType());

  if (!Ty)

    return false;

  // Conservatively assume anything else is a potential retainable object

  // pointer.

  return true;

}

bool IsPotentialRetainableObjPtr(const Value *Op, AAResults &AA);

/// Helper for GetARCInstKind. Determines what kind of construct CS

/// is.

inline ARCInstKind GetCallSiteClass(const CallBase &CB) {

  for (const Use &U : CB.args())

    if (IsPotentialRetainableObjPtr(U))

      return CB.onlyReadsMemory() ? ARCInstKind::User : ARCInstKind::CallOrUser;

  return CB.onlyReadsMemory() ? ARCInstKind::None : ARCInstKind::Call;

}

/// Return true if this value refers to a distinct and identifiable

/// object.

///

/// This is similar to AliasAnalysis's isIdentifiedObject, except that it uses

/// special knowledge of ObjC conventions.

inline bool IsObjCIdentifiedObject(const Value *V) {

  // Assume that call results and arguments have their own "provenance".

  // Constants (including GlobalVariables) and Allocas are never

  // reference-counted.

  if (isa<CallInst>(V) || isa<InvokeInst>(V) ||

      isa<Argument>(V) || isa<Constant>(V) ||

      isa<AllocaInst>(V))

    return true;

  if (const LoadInst *LI = dyn_cast<LoadInst>(V)) {

    const Value *Pointer =

      GetRCIdentityRoot(LI->getPointerOperand());

    if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Pointer)) {

      // A constant pointer can't be pointing to an object on the heap. It may

      // be reference-counted, but it won't be deleted.

      if (GV->isConstant())

        return true;

      StringRef Name = GV->getName();

      // These special variables are known to hold values which are not

      // reference-counted pointers.

      if (Name.startswith("\01l_objc_msgSend_fixup_"))

        return true;

      StringRef Section = GV->getSection();

      if (Section.contains("__message_refs") ||

          Section.contains("__objc_classrefs") ||

          Section.contains("__objc_superrefs") ||

          Section.contains("__objc_methname") || Section.contains("__cstring"))

        return true;

    }

  }

  return false;

}

enum class ARCMDKindID {

  ImpreciseRelease,

  CopyOnEscape,

  NoObjCARCExceptions,

};

/// A cache of MDKinds used by various ARC optimizations.

class ARCMDKindCache {

  Module *M;

  /// The Metadata Kind for clang.imprecise_release metadata.

  std::optional<unsigned> ImpreciseReleaseMDKind;

  /// The Metadata Kind for clang.arc.copy_on_escape metadata.

  std::optional<unsigned> CopyOnEscapeMDKind;

  /// The Metadata Kind for clang.arc.no_objc_arc_exceptions metadata.

  std::optional<unsigned> NoObjCARCExceptionsMDKind;

public:

  void init(Module *Mod) {

    M = Mod;

    ImpreciseReleaseMDKind = std::nullopt;

    CopyOnEscapeMDKind = std::nullopt;

    NoObjCARCExceptionsMDKind = std::nullopt;

  }

  unsigned get(ARCMDKindID ID) {

    switch (ID) {

    case ARCMDKindID::ImpreciseRelease:

      if (!ImpreciseReleaseMDKind)

        ImpreciseReleaseMDKind =

            M->getContext().getMDKindID("clang.imprecise_release");

      return *ImpreciseReleaseMDKind;

    case ARCMDKindID::CopyOnEscape:

      if (!CopyOnEscapeMDKind)

        CopyOnEscapeMDKind =

            M->getContext().getMDKindID("clang.arc.copy_on_escape");

      return *CopyOnEscapeMDKind;

    case ARCMDKindID::NoObjCARCExceptions:

      if (!NoObjCARCExceptionsMDKind)

        NoObjCARCExceptionsMDKind =

            M->getContext().getMDKindID("clang.arc.no_objc_arc_exceptions");

      return *NoObjCARCExceptionsMDKind;

    }

    llvm_unreachable("Covered switch isn't covered?!");

  }

};

} // end namespace objcarc

} // end namespace llvm

#endif