1 //===- ProvenanceAnalysis.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 //
9 /// \file
10 ///
11 /// This file defines a special form of Alias Analysis called ``Provenance
12 /// Analysis''. The word ``provenance'' refers to the history of the ownership
13 /// of an object. Thus ``Provenance Analysis'' is an analysis which attempts to
14 /// use various techniques to determine if locally
15 ///
16 /// WARNING: This file knows about certain library functions. It recognizes them
17 /// by name, and hardwires knowledge of their semantics.
18 ///
19 /// WARNING: This file knows about how certain Objective-C library functions are
20 /// used. Naive LLVM IR transformations which would otherwise be
21 /// behavior-preserving may break these assumptions.
22 //
23 //===----------------------------------------------------------------------===//
24
25 #include "ProvenanceAnalysis.h"
26 #include "llvm/ADT/SmallPtrSet.h"
27 #include "llvm/ADT/SmallVector.h"
28 #include "llvm/Analysis/AliasAnalysis.h"
29 #include "llvm/Analysis/ObjCARCAnalysisUtils.h"
30 #include "llvm/IR/Instructions.h"
31 #include "llvm/IR/Module.h"
32 #include "llvm/IR/Use.h"
33 #include "llvm/IR/User.h"
34 #include "llvm/IR/Value.h"
35 #include "llvm/Support/Casting.h"
36 #include <utility>
37
38 using namespace llvm;
39 using namespace llvm::objcarc;
40
relatedSelect(const SelectInst * A,const Value * B)41 bool ProvenanceAnalysis::relatedSelect(const SelectInst *A,
42 const Value *B) {
43 // If the values are Selects with the same condition, we can do a more precise
44 // check: just check for relations between the values on corresponding arms.
45 if (const SelectInst *SB = dyn_cast<SelectInst>(B))
46 if (A->getCondition() == SB->getCondition())
47 return related(A->getTrueValue(), SB->getTrueValue()) ||
48 related(A->getFalseValue(), SB->getFalseValue());
49
50 // Check both arms of the Select node individually.
51 return related(A->getTrueValue(), B) || related(A->getFalseValue(), B);
52 }
53
relatedPHI(const PHINode * A,const Value * B)54 bool ProvenanceAnalysis::relatedPHI(const PHINode *A,
55 const Value *B) {
56 // If the values are PHIs in the same block, we can do a more precise as well
57 // as efficient check: just check for relations between the values on
58 // corresponding edges.
59 if (const PHINode *PNB = dyn_cast<PHINode>(B))
60 if (PNB->getParent() == A->getParent()) {
61 for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i)
62 if (related(A->getIncomingValue(i),
63 PNB->getIncomingValueForBlock(A->getIncomingBlock(i))))
64 return true;
65 return false;
66 }
67
68 // Check each unique source of the PHI node against B.
69 SmallPtrSet<const Value *, 4> UniqueSrc;
70 for (Value *PV1 : A->incoming_values()) {
71 if (UniqueSrc.insert(PV1).second && related(PV1, B))
72 return true;
73 }
74
75 // All of the arms checked out.
76 return false;
77 }
78
79 /// Test if the value of P, or any value covered by its provenance, is ever
80 /// stored within the function (not counting callees).
IsStoredObjCPointer(const Value * P)81 static bool IsStoredObjCPointer(const Value *P) {
82 SmallPtrSet<const Value *, 8> Visited;
83 SmallVector<const Value *, 8> Worklist;
84 Worklist.push_back(P);
85 Visited.insert(P);
86 do {
87 P = Worklist.pop_back_val();
88 for (const Use &U : P->uses()) {
89 const User *Ur = U.getUser();
90 if (isa<StoreInst>(Ur)) {
91 if (U.getOperandNo() == 0)
92 // The pointer is stored.
93 return true;
94 // The pointed is stored through.
95 continue;
96 }
97 if (isa<CallInst>(Ur))
98 // The pointer is passed as an argument, ignore this.
99 continue;
100 if (isa<PtrToIntInst>(P))
101 // Assume the worst.
102 return true;
103 if (Visited.insert(Ur).second)
104 Worklist.push_back(Ur);
105 }
106 } while (!Worklist.empty());
107
108 // Everything checked out.
109 return false;
110 }
111
relatedCheck(const Value * A,const Value * B)112 bool ProvenanceAnalysis::relatedCheck(const Value *A, const Value *B) {
113 // Ask regular AliasAnalysis, for a first approximation.
114 switch (AA->alias(A, B)) {
115 case AliasResult::NoAlias:
116 return false;
117 case AliasResult::MustAlias:
118 case AliasResult::PartialAlias:
119 return true;
120 case AliasResult::MayAlias:
121 break;
122 }
123
124 bool AIsIdentified = IsObjCIdentifiedObject(A);
125 bool BIsIdentified = IsObjCIdentifiedObject(B);
126
127 // An ObjC-Identified object can't alias a load if it is never locally stored.
128 if (AIsIdentified) {
129 // Check for an obvious escape.
130 if (isa<LoadInst>(B))
131 return IsStoredObjCPointer(A);
132 if (BIsIdentified) {
133 // Check for an obvious escape.
134 if (isa<LoadInst>(A))
135 return IsStoredObjCPointer(B);
136 // Both pointers are identified and escapes aren't an evident problem.
137 return false;
138 }
139 } else if (BIsIdentified) {
140 // Check for an obvious escape.
141 if (isa<LoadInst>(A))
142 return IsStoredObjCPointer(B);
143 }
144
145 // Special handling for PHI and Select.
146 if (const PHINode *PN = dyn_cast<PHINode>(A))
147 return relatedPHI(PN, B);
148 if (const PHINode *PN = dyn_cast<PHINode>(B))
149 return relatedPHI(PN, A);
150 if (const SelectInst *S = dyn_cast<SelectInst>(A))
151 return relatedSelect(S, B);
152 if (const SelectInst *S = dyn_cast<SelectInst>(B))
153 return relatedSelect(S, A);
154
155 // Conservative.
156 return true;
157 }
158
related(const Value * A,const Value * B)159 bool ProvenanceAnalysis::related(const Value *A, const Value *B) {
160 A = GetUnderlyingObjCPtrCached(A, UnderlyingObjCPtrCache);
161 B = GetUnderlyingObjCPtrCached(B, UnderlyingObjCPtrCache);
162
163 // Quick check.
164 if (A == B)
165 return true;
166
167 // Begin by inserting a conservative value into the map. If the insertion
168 // fails, we have the answer already. If it succeeds, leave it there until we
169 // compute the real answer to guard against recursive queries.
170 std::pair<CachedResultsTy::iterator, bool> Pair =
171 CachedResults.insert(std::make_pair(ValuePairTy(A, B), true));
172 if (!Pair.second)
173 return Pair.first->second;
174
175 bool Result = relatedCheck(A, B);
176 CachedResults[ValuePairTy(A, B)] = Result;
177 return Result;
178 }
179