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 //===----------------------------------------------------------------------===//
24 
25 #include "llvm/Analysis/ObjCARCAliasAnalysis.h"
26 #include "llvm/Analysis/ObjCARCAnalysisUtils.h"
27 #include "llvm/Analysis/Passes.h"
28 #include "llvm/IR/Function.h"
29 #include "llvm/IR/Instruction.h"
30 #include "llvm/IR/Value.h"
31 #include "llvm/InitializePasses.h"
32 #include "llvm/Pass.h"
33 
34 #define DEBUG_TYPE "objc-arc-aa"
35 
36 using namespace llvm;
37 using namespace llvm::objcarc;
38 
39 AliasResult ObjCARCAAResult::alias(const MemoryLocation &LocA,
40                                    const MemoryLocation &LocB,
41                                    AAQueryInfo &AAQI) {
42   if (!EnableARCOpts)
43     return AAResultBase::alias(LocA, LocB, AAQI);
44 
45   // First, strip off no-ops, including ObjC-specific no-ops, and try making a
46   // precise alias query.
47   const Value *SA = GetRCIdentityRoot(LocA.Ptr);
48   const Value *SB = GetRCIdentityRoot(LocB.Ptr);
49   AliasResult Result =
50       AAResultBase::alias(MemoryLocation(SA, LocA.Size, LocA.AATags),
51                           MemoryLocation(SB, LocB.Size, LocB.AATags), AAQI);
52   if (Result != AliasResult::MayAlias)
53     return Result;
54 
55   // If that failed, climb to the underlying object, including climbing through
56   // ObjC-specific no-ops, and try making an imprecise alias query.
57   const Value *UA = GetUnderlyingObjCPtr(SA);
58   const Value *UB = GetUnderlyingObjCPtr(SB);
59   if (UA != SA || UB != SB) {
60     Result = AAResultBase::alias(MemoryLocation::getBeforeOrAfter(UA),
61                                  MemoryLocation::getBeforeOrAfter(UB), AAQI);
62     // We can't use MustAlias or PartialAlias results here because
63     // GetUnderlyingObjCPtr may return an offsetted pointer value.
64     if (Result == AliasResult::NoAlias)
65       return AliasResult::NoAlias;
66   }
67 
68   // If that failed, fail. We don't need to chain here, since that's covered
69   // by the earlier precise query.
70   return AliasResult::MayAlias;
71 }
72 
73 bool ObjCARCAAResult::pointsToConstantMemory(const MemoryLocation &Loc,
74                                              AAQueryInfo &AAQI, bool OrLocal) {
75   if (!EnableARCOpts)
76     return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
77 
78   // First, strip off no-ops, including ObjC-specific no-ops, and try making
79   // a precise alias query.
80   const Value *S = GetRCIdentityRoot(Loc.Ptr);
81   if (AAResultBase::pointsToConstantMemory(
82           MemoryLocation(S, Loc.Size, Loc.AATags), AAQI, OrLocal))
83     return true;
84 
85   // If that failed, climb to the underlying object, including climbing through
86   // ObjC-specific no-ops, and try making an imprecise alias query.
87   const Value *U = GetUnderlyingObjCPtr(S);
88   if (U != S)
89     return AAResultBase::pointsToConstantMemory(
90         MemoryLocation::getBeforeOrAfter(U), AAQI, OrLocal);
91 
92   // If that failed, fail. We don't need to chain here, since that's covered
93   // by the earlier precise query.
94   return false;
95 }
96 
97 FunctionModRefBehavior ObjCARCAAResult::getModRefBehavior(const Function *F) {
98   if (!EnableARCOpts)
99     return AAResultBase::getModRefBehavior(F);
100 
101   switch (GetFunctionClass(F)) {
102   case ARCInstKind::NoopCast:
103     return FMRB_DoesNotAccessMemory;
104   default:
105     break;
106   }
107 
108   return AAResultBase::getModRefBehavior(F);
109 }
110 
111 ModRefInfo ObjCARCAAResult::getModRefInfo(const CallBase *Call,
112                                           const MemoryLocation &Loc,
113                                           AAQueryInfo &AAQI) {
114   if (!EnableARCOpts)
115     return AAResultBase::getModRefInfo(Call, Loc, AAQI);
116 
117   switch (GetBasicARCInstKind(Call)) {
118   case ARCInstKind::Retain:
119   case ARCInstKind::RetainRV:
120   case ARCInstKind::Autorelease:
121   case ARCInstKind::AutoreleaseRV:
122   case ARCInstKind::NoopCast:
123   case ARCInstKind::AutoreleasepoolPush:
124   case ARCInstKind::FusedRetainAutorelease:
125   case ARCInstKind::FusedRetainAutoreleaseRV:
126     // These functions don't access any memory visible to the compiler.
127     // Note that this doesn't include objc_retainBlock, because it updates
128     // pointers when it copies block data.
129     return ModRefInfo::NoModRef;
130   default:
131     break;
132   }
133 
134   return AAResultBase::getModRefInfo(Call, Loc, AAQI);
135 }
136 
137 AnalysisKey ObjCARCAA::Key;
138 
139 ObjCARCAAResult ObjCARCAA::run(Function &F, FunctionAnalysisManager &AM) {
140   return ObjCARCAAResult(F.getParent()->getDataLayout());
141 }
142 
143 char ObjCARCAAWrapperPass::ID = 0;
144 INITIALIZE_PASS(ObjCARCAAWrapperPass, "objc-arc-aa",
145                 "ObjC-ARC-Based Alias Analysis", false, true)
146 
147 ImmutablePass *llvm::createObjCARCAAWrapperPass() {
148   return new ObjCARCAAWrapperPass();
149 }
150 
151 ObjCARCAAWrapperPass::ObjCARCAAWrapperPass() : ImmutablePass(ID) {
152   initializeObjCARCAAWrapperPassPass(*PassRegistry::getPassRegistry());
153 }
154 
155 bool ObjCARCAAWrapperPass::doInitialization(Module &M) {
156   Result.reset(new ObjCARCAAResult(M.getDataLayout()));
157   return false;
158 }
159 
160 bool ObjCARCAAWrapperPass::doFinalization(Module &M) {
161   Result.reset();
162   return false;
163 }
164 
165 void ObjCARCAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
166   AU.setPreservesAll();
167 }
168