llvm/lib/Analysis/ObjCARCAliasAnalysis.cpp

   1 //===- ObjCARCAliasAnalysis.cpp - ObjC ARC Optimization -------------------===//
   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 a simple ARC-aware AliasAnalysis using special knowledge
  10 /// of Objective C to enhance other optimization passes which rely on the Alias
  11 /// Analysis infrastructure.
  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 /// TODO: Theoretically we could check for dependencies between objc_* calls
  21 /// and FMRB_OnlyAccessesArgumentPointees calls or other well-behaved calls.
  22 ///
  23 /// TODO: The calls here to AAResultBase member functions are all effectively
  24 /// no-ops that just return a conservative result. The original intent was to
  25 /// chain to another analysis for a recursive query, but this was lost in a
  26 /// refactor. These should instead be rephrased in terms of queries to AAQI.AAR.
  27 ///
  28 //===----------------------------------------------------------------------===//
  29
  30 #include "llvm/Analysis/ObjCARCAliasAnalysis.h"
  31 #include "llvm/Analysis/ObjCARCAnalysisUtils.h"
  32 #include "llvm/IR/Function.h"
  33 #include "llvm/Pass.h"
  34
  35 #define DEBUG_TYPE "objc-arc-aa"
  36
  37 using namespace llvm;
  38 using namespace llvm::objcarc;
  39
  40 AliasResult ObjCARCAAResult::alias(const MemoryLocation &LocA,
  41                                    const MemoryLocation &LocB,
  42                                    AAQueryInfo &AAQI, const Instruction *) {
  43   if (!EnableARCOpts)
  44     return AAResultBase::alias(LocA, LocB, AAQI, nullptr);
  45
  46   // First, strip off no-ops, including ObjC-specific no-ops, and try making a
  47   // precise alias query.
  48   const Value *SA = GetRCIdentityRoot(LocA.Ptr);
  49   const Value *SB = GetRCIdentityRoot(LocB.Ptr);
  50   AliasResult Result = AAResultBase::alias(
  51       MemoryLocation(SA, LocA.Size, LocA.AATags),
  52       MemoryLocation(SB, LocB.Size, LocB.AATags), AAQI, nullptr);
  53   if (Result != AliasResult::MayAlias)
  54     return Result;
  55
  56   // If that failed, climb to the underlying object, including climbing through
  57   // ObjC-specific no-ops, and try making an imprecise alias query.
  58   const Value *UA = GetUnderlyingObjCPtr(SA);
  59   const Value *UB = GetUnderlyingObjCPtr(SB);
  60   if (UA != SA || UB != SB) {
  61     Result = AAResultBase::alias(MemoryLocation::getBeforeOrAfter(UA),
  62                                  MemoryLocation::getBeforeOrAfter(UB), AAQI,
  63                                  nullptr);
  64     // We can't use MustAlias or PartialAlias results here because
  65     // GetUnderlyingObjCPtr may return an offsetted pointer value.
  66     if (Result == AliasResult::NoAlias)
  67       return AliasResult::NoAlias;
  68   }
  69
  70   // If that failed, fail. We don't need to chain here, since that's covered
  71   // by the earlier precise query.
  72   return AliasResult::MayAlias;
  73 }
  74
  75 ModRefInfo ObjCARCAAResult::getModRefInfoMask(const MemoryLocation &Loc,
  76                                               AAQueryInfo &AAQI,
  77                                               bool IgnoreLocals) {
  78   if (!EnableARCOpts)
  79     return AAResultBase::getModRefInfoMask(Loc, AAQI, IgnoreLocals);
  80
  81   // First, strip off no-ops, including ObjC-specific no-ops, and try making
  82   // a precise alias query.
  83   const Value *S = GetRCIdentityRoot(Loc.Ptr);
  84   if (isNoModRef(AAResultBase::getModRefInfoMask(
  85           MemoryLocation(S, Loc.Size, Loc.AATags), AAQI, IgnoreLocals)))
  86     return ModRefInfo::NoModRef;
  87
  88   // If that failed, climb to the underlying object, including climbing through
  89   // ObjC-specific no-ops, and try making an imprecise alias query.
  90   const Value *U = GetUnderlyingObjCPtr(S);
  91   if (U != S)
  92     return AAResultBase::getModRefInfoMask(MemoryLocation::getBeforeOrAfter(U),
  93                                            AAQI, IgnoreLocals);
  94
  95   // If that failed, fail. We don't need to chain here, since that's covered
  96   // by the earlier precise query.
  97   return ModRefInfo::ModRef;
  98 }
  99
 100 MemoryEffects ObjCARCAAResult::getMemoryEffects(const Function *F) {
 101   if (!EnableARCOpts)
 102     return AAResultBase::getMemoryEffects(F);
 103
 104   switch (GetFunctionClass(F)) {
 105   case ARCInstKind::NoopCast:
 106     return MemoryEffects::none();
 107   default:
 108     break;
 109   }
 110
 111   return AAResultBase::getMemoryEffects(F);
 112 }
 113
 114 ModRefInfo ObjCARCAAResult::getModRefInfo(const CallBase *Call,
 115                                           const MemoryLocation &Loc,
 116                                           AAQueryInfo &AAQI) {
 117   if (!EnableARCOpts)
 118     return AAResultBase::getModRefInfo(Call, Loc, AAQI);
 119
 120   switch (GetBasicARCInstKind(Call)) {
 121   case ARCInstKind::Retain:
 122   case ARCInstKind::RetainRV:
 123   case ARCInstKind::Autorelease:
 124   case ARCInstKind::AutoreleaseRV:
 125   case ARCInstKind::NoopCast:
 126   case ARCInstKind::AutoreleasepoolPush:
 127   case ARCInstKind::FusedRetainAutorelease:
 128   case ARCInstKind::FusedRetainAutoreleaseRV:
 129     // These functions don't access any memory visible to the compiler.
 130     // Note that this doesn't include objc_retainBlock, because it updates
 131     // pointers when it copies block data.
 132     return ModRefInfo::NoModRef;
 133   default:
 134     break;
 135   }
 136
 137   return AAResultBase::getModRefInfo(Call, Loc, AAQI);
 138 }
 139
 140 AnalysisKey ObjCARCAA::Key;
 141
 142 ObjCARCAAResult ObjCARCAA::run(Function &F, FunctionAnalysisManager &AM) {
 143   return ObjCARCAAResult(F.getDataLayout());
 144 }