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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.starts_with("\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
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