1//===- ObjCARCAnalysisUtils.h - ObjC ARC Analysis Utilities -----*- C++ -*-===//
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 common analysis utilities used by the ObjC ARC Optimizer.
10/// ARC stands for Automatic Reference Counting and is a system for managing
11/// reference counts for objects in Objective C.
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//===----------------------------------------------------------------------===//
21
22#ifndef LLVM_ANALYSIS_OBJCARCANALYSISUTILS_H
23#define LLVM_ANALYSIS_OBJCARCANALYSISUTILS_H
24
25#include "llvm/Analysis/ObjCARCInstKind.h"
26#include "llvm/Analysis/ValueTracking.h"
27#include "llvm/IR/Constants.h"
28#include "llvm/IR/Module.h"
29#include "llvm/IR/ValueHandle.h"
30#include <optional>
31
32namespace llvm {
33
34class AAResults;
35
36namespace objcarc {
37
38/// A handy option to enable/disable all ARC Optimizations.
39extern bool EnableARCOpts;
40
41/// Test if the given module looks interesting to run ARC optimization
42/// on.
43inline bool ModuleHasARC(const Module &M) {
44 return M.getNamedValue(Name: "llvm.objc.retain") ||
45 M.getNamedValue(Name: "llvm.objc.release") ||
46 M.getNamedValue(Name: "llvm.objc.autorelease") ||
47 M.getNamedValue(Name: "llvm.objc.retainAutoreleasedReturnValue") ||
48 M.getNamedValue(Name: "llvm.objc.unsafeClaimAutoreleasedReturnValue") ||
49 M.getNamedValue(Name: "llvm.objc.retainBlock") ||
50 M.getNamedValue(Name: "llvm.objc.autoreleaseReturnValue") ||
51 M.getNamedValue(Name: "llvm.objc.autoreleasePoolPush") ||
52 M.getNamedValue(Name: "llvm.objc.loadWeakRetained") ||
53 M.getNamedValue(Name: "llvm.objc.loadWeak") ||
54 M.getNamedValue(Name: "llvm.objc.destroyWeak") ||
55 M.getNamedValue(Name: "llvm.objc.storeWeak") ||
56 M.getNamedValue(Name: "llvm.objc.initWeak") ||
57 M.getNamedValue(Name: "llvm.objc.moveWeak") ||
58 M.getNamedValue(Name: "llvm.objc.copyWeak") ||
59 M.getNamedValue(Name: "llvm.objc.retainedObject") ||
60 M.getNamedValue(Name: "llvm.objc.unretainedObject") ||
61 M.getNamedValue(Name: "llvm.objc.unretainedPointer") ||
62 M.getNamedValue(Name: "llvm.objc.clang.arc.noop.use") ||
63 M.getNamedValue(Name: "llvm.objc.clang.arc.use");
64}
65
66/// This is a wrapper around getUnderlyingObject which also knows how to
67/// look through objc_retain and objc_autorelease calls, which we know to return
68/// their argument verbatim.
69inline const Value *GetUnderlyingObjCPtr(const Value *V) {
70 for (;;) {
71 V = getUnderlyingObject(V);
72 if (!IsForwarding(Class: GetBasicARCInstKind(V)))
73 break;
74 V = cast<CallInst>(Val: V)->getArgOperand(i: 0);
75 }
76
77 return V;
78}
79
80/// A wrapper for GetUnderlyingObjCPtr used for results memoization.
81inline const Value *GetUnderlyingObjCPtrCached(
82 const Value *V,
83 DenseMap<const Value *, std::pair<WeakVH, WeakTrackingVH>> &Cache) {
84 // The entry is invalid if either value handle is null.
85 auto InCache = Cache.lookup(Val: V);
86 if (InCache.first && InCache.second)
87 return InCache.second;
88
89 const Value *Computed = GetUnderlyingObjCPtr(V);
90 Cache[V] =
91 std::make_pair(x: const_cast<Value *>(V), y: const_cast<Value *>(Computed));
92 return Computed;
93}
94
95/// The RCIdentity root of a value \p V is a dominating value U for which
96/// retaining or releasing U is equivalent to retaining or releasing V. In other
97/// words, ARC operations on \p V are equivalent to ARC operations on \p U.
98///
99/// We use this in the ARC optimizer to make it easier to match up ARC
100/// operations by always mapping ARC operations to RCIdentityRoots instead of
101/// pointers themselves.
102///
103/// The two ways that we see RCIdentical values in ObjC are via:
104///
105/// 1. PointerCasts
106/// 2. Forwarding Calls that return their argument verbatim.
107///
108/// Thus this function strips off pointer casts and forwarding calls. *NOTE*
109/// This implies that two RCIdentical values must alias.
110inline const Value *GetRCIdentityRoot(const Value *V) {
111 for (;;) {
112 V = V->stripPointerCasts();
113 if (!IsForwarding(Class: GetBasicARCInstKind(V)))
114 break;
115 V = cast<CallInst>(Val: V)->getArgOperand(i: 0);
116 }
117 return V;
118}
119
120/// Helper which calls const Value *GetRCIdentityRoot(const Value *V) and just
121/// casts away the const of the result. For documentation about what an
122/// RCIdentityRoot (and by extension GetRCIdentityRoot is) look at that
123/// function.
124inline Value *GetRCIdentityRoot(Value *V) {
125 return const_cast<Value *>(GetRCIdentityRoot(V: (const Value *)V));
126}
127
128/// Assuming the given instruction is one of the special calls such as
129/// objc_retain or objc_release, return the RCIdentity root of the argument of
130/// the call.
131inline Value *GetArgRCIdentityRoot(Value *Inst) {
132 return GetRCIdentityRoot(V: cast<CallInst>(Val: Inst)->getArgOperand(i: 0));
133}
134
135inline bool IsNullOrUndef(const Value *V) {
136 return isa<ConstantPointerNull>(Val: V) || isa<UndefValue>(Val: V);
137}
138
139inline bool IsNoopInstruction(const Instruction *I) {
140 return isa<BitCastInst>(Val: I) ||
141 (isa<GetElementPtrInst>(Val: I) &&
142 cast<GetElementPtrInst>(Val: I)->hasAllZeroIndices());
143}
144
145/// Test whether the given value is possible a retainable object pointer.
146inline bool IsPotentialRetainableObjPtr(const Value *Op) {
147 // Pointers to static or stack storage are not valid retainable object
148 // pointers.
149 if (isa<Constant>(Val: Op) || isa<AllocaInst>(Val: Op))
150 return false;
151 // Special arguments can not be a valid retainable object pointer.
152 if (const Argument *Arg = dyn_cast<Argument>(Val: Op))
153 if (Arg->hasPassPointeeByValueCopyAttr() || Arg->hasNestAttr() ||
154 Arg->hasStructRetAttr())
155 return false;
156 // Only consider values with pointer types.
157 //
158 // It seemes intuitive to exclude function pointer types as well, since
159 // functions are never retainable object pointers, however clang occasionally
160 // bitcasts retainable object pointers to function-pointer type temporarily.
161 PointerType *Ty = dyn_cast<PointerType>(Val: Op->getType());
162 if (!Ty)
163 return false;
164 // Conservatively assume anything else is a potential retainable object
165 // pointer.
166 return true;
167}
168
169bool IsPotentialRetainableObjPtr(const Value *Op, AAResults &AA);
170
171/// Helper for GetARCInstKind. Determines what kind of construct CS
172/// is.
173inline ARCInstKind GetCallSiteClass(const CallBase &CB) {
174 for (const Use &U : CB.args())
175 if (IsPotentialRetainableObjPtr(Op: U))
176 return CB.onlyReadsMemory() ? ARCInstKind::User : ARCInstKind::CallOrUser;
177
178 return CB.onlyReadsMemory() ? ARCInstKind::None : ARCInstKind::Call;
179}
180
181/// Return true if this value refers to a distinct and identifiable
182/// object.
183///
184/// This is similar to AliasAnalysis's isIdentifiedObject, except that it uses
185/// special knowledge of ObjC conventions.
186inline bool IsObjCIdentifiedObject(const Value *V) {
187 // Assume that call results and arguments have their own "provenance".
188 // Constants (including GlobalVariables) and Allocas are never
189 // reference-counted.
190 if (isa<CallInst>(Val: V) || isa<InvokeInst>(Val: V) ||
191 isa<Argument>(Val: V) || isa<Constant>(Val: V) ||
192 isa<AllocaInst>(Val: V))
193 return true;
194
195 if (const LoadInst *LI = dyn_cast<LoadInst>(Val: V)) {
196 const Value *Pointer =
197 GetRCIdentityRoot(V: LI->getPointerOperand());
198 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Val: Pointer)) {
199 // A constant pointer can't be pointing to an object on the heap. It may
200 // be reference-counted, but it won't be deleted.
201 if (GV->isConstant())
202 return true;
203 StringRef Name = GV->getName();
204 // These special variables are known to hold values which are not
205 // reference-counted pointers.
206 if (Name.starts_with(Prefix: "\01l_objc_msgSend_fixup_"))
207 return true;
208
209 StringRef Section = GV->getSection();
210 if (Section.contains(Other: "__message_refs") ||
211 Section.contains(Other: "__objc_classrefs") ||
212 Section.contains(Other: "__objc_superrefs") ||
213 Section.contains(Other: "__objc_methname") || Section.contains(Other: "__cstring"))
214 return true;
215 }
216 }
217
218 return false;
219}
220
221enum class ARCMDKindID {
222 ImpreciseRelease,
223 CopyOnEscape,
224 NoObjCARCExceptions,
225};
226
227/// A cache of MDKinds used by various ARC optimizations.
228class ARCMDKindCache {
229 Module *M;
230
231 /// The Metadata Kind for clang.imprecise_release metadata.
232 std::optional<unsigned> ImpreciseReleaseMDKind;
233
234 /// The Metadata Kind for clang.arc.copy_on_escape metadata.
235 std::optional<unsigned> CopyOnEscapeMDKind;
236
237 /// The Metadata Kind for clang.arc.no_objc_arc_exceptions metadata.
238 std::optional<unsigned> NoObjCARCExceptionsMDKind;
239
240public:
241 void init(Module *Mod) {
242 M = Mod;
243 ImpreciseReleaseMDKind = std::nullopt;
244 CopyOnEscapeMDKind = std::nullopt;
245 NoObjCARCExceptionsMDKind = std::nullopt;
246 }
247
248 unsigned get(ARCMDKindID ID) {
249 switch (ID) {
250 case ARCMDKindID::ImpreciseRelease:
251 if (!ImpreciseReleaseMDKind)
252 ImpreciseReleaseMDKind =
253 M->getContext().getMDKindID(Name: "clang.imprecise_release");
254 return *ImpreciseReleaseMDKind;
255 case ARCMDKindID::CopyOnEscape:
256 if (!CopyOnEscapeMDKind)
257 CopyOnEscapeMDKind =
258 M->getContext().getMDKindID(Name: "clang.arc.copy_on_escape");
259 return *CopyOnEscapeMDKind;
260 case ARCMDKindID::NoObjCARCExceptions:
261 if (!NoObjCARCExceptionsMDKind)
262 NoObjCARCExceptionsMDKind =
263 M->getContext().getMDKindID(Name: "clang.arc.no_objc_arc_exceptions");
264 return *NoObjCARCExceptionsMDKind;
265 }
266 llvm_unreachable("Covered switch isn't covered?!");
267 }
268};
269
270} // end namespace objcarc
271} // end namespace llvm
272
273#endif
274