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/Analysis/Passes.h"
33#include "llvm/IR/Function.h"
34#include "llvm/InitializePasses.h"
35#include "llvm/Pass.h"
36
37#define DEBUG_TYPE "objc-arc-aa"
38
39using namespace llvm;
40using namespace llvm::objcarc;
41
42AliasResult ObjCARCAAResult::alias(const MemoryLocation &LocA,
43                                   const MemoryLocation &LocB,
44                                   AAQueryInfo &AAQI, const Instruction *) {
45  if (!EnableARCOpts)
46    return AAResultBase::alias(LocA, LocB, AAQI, nullptr);
47
48  // First, strip off no-ops, including ObjC-specific no-ops, and try making a
49  // precise alias query.
50  const Value *SA = GetRCIdentityRoot(LocA.Ptr);
51  const Value *SB = GetRCIdentityRoot(LocB.Ptr);
52  AliasResult Result = AAResultBase::alias(
53      MemoryLocation(SA, LocA.Size, LocA.AATags),
54      MemoryLocation(SB, LocB.Size, LocB.AATags), AAQI, nullptr);
55  if (Result != AliasResult::MayAlias)
56    return Result;
57
58  // If that failed, climb to the underlying object, including climbing through
59  // ObjC-specific no-ops, and try making an imprecise alias query.
60  const Value *UA = GetUnderlyingObjCPtr(SA);
61  const Value *UB = GetUnderlyingObjCPtr(SB);
62  if (UA != SA || UB != SB) {
63    Result = AAResultBase::alias(MemoryLocation::getBeforeOrAfter(UA),
64                                 MemoryLocation::getBeforeOrAfter(UB), AAQI,
65                                 nullptr);
66    // We can't use MustAlias or PartialAlias results here because
67    // GetUnderlyingObjCPtr may return an offsetted pointer value.
68    if (Result == AliasResult::NoAlias)
69      return AliasResult::NoAlias;
70  }
71
72  // If that failed, fail. We don't need to chain here, since that's covered
73  // by the earlier precise query.
74  return AliasResult::MayAlias;
75}
76
77ModRefInfo ObjCARCAAResult::getModRefInfoMask(const MemoryLocation &Loc,
78                                              AAQueryInfo &AAQI,
79                                              bool IgnoreLocals) {
80  if (!EnableARCOpts)
81    return AAResultBase::getModRefInfoMask(Loc, AAQI, IgnoreLocals);
82
83  // First, strip off no-ops, including ObjC-specific no-ops, and try making
84  // a precise alias query.
85  const Value *S = GetRCIdentityRoot(Loc.Ptr);
86  if (isNoModRef(AAResultBase::getModRefInfoMask(
87          MemoryLocation(S, Loc.Size, Loc.AATags), AAQI, IgnoreLocals)))
88    return ModRefInfo::NoModRef;
89
90  // If that failed, climb to the underlying object, including climbing through
91  // ObjC-specific no-ops, and try making an imprecise alias query.
92  const Value *U = GetUnderlyingObjCPtr(S);
93  if (U != S)
94    return AAResultBase::getModRefInfoMask(MemoryLocation::getBeforeOrAfter(U),
95                                           AAQI, IgnoreLocals);
96
97  // If that failed, fail. We don't need to chain here, since that's covered
98  // by the earlier precise query.
99  return ModRefInfo::ModRef;
100}
101
102MemoryEffects ObjCARCAAResult::getMemoryEffects(const Function *F) {
103  if (!EnableARCOpts)
104    return AAResultBase::getMemoryEffects(F);
105
106  switch (GetFunctionClass(F)) {
107  case ARCInstKind::NoopCast:
108    return MemoryEffects::none();
109  default:
110    break;
111  }
112
113  return AAResultBase::getMemoryEffects(F);
114}
115
116ModRefInfo ObjCARCAAResult::getModRefInfo(const CallBase *Call,
117                                          const MemoryLocation &Loc,
118                                          AAQueryInfo &AAQI) {
119  if (!EnableARCOpts)
120    return AAResultBase::getModRefInfo(Call, Loc, AAQI);
121
122  switch (GetBasicARCInstKind(Call)) {
123  case ARCInstKind::Retain:
124  case ARCInstKind::RetainRV:
125  case ARCInstKind::Autorelease:
126  case ARCInstKind::AutoreleaseRV:
127  case ARCInstKind::NoopCast:
128  case ARCInstKind::AutoreleasepoolPush:
129  case ARCInstKind::FusedRetainAutorelease:
130  case ARCInstKind::FusedRetainAutoreleaseRV:
131    // These functions don't access any memory visible to the compiler.
132    // Note that this doesn't include objc_retainBlock, because it updates
133    // pointers when it copies block data.
134    return ModRefInfo::NoModRef;
135  default:
136    break;
137  }
138
139  return AAResultBase::getModRefInfo(Call, Loc, AAQI);
140}
141
142AnalysisKey ObjCARCAA::Key;
143
144ObjCARCAAResult ObjCARCAA::run(Function &F, FunctionAnalysisManager &AM) {
145  return ObjCARCAAResult(F.getParent()->getDataLayout());
146}
147