1 //===- ProvenanceAnalysis.cpp - ObjC ARC Optimization ---------------------===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 /// \file
11 ///
12 /// This file defines a special form of Alias Analysis called ``Provenance
13 /// Analysis''. The word ``provenance'' refers to the history of the ownership
14 /// of an object. Thus ``Provenance Analysis'' is an analysis which attempts to
15 /// use various techniques to determine if locally
16 ///
17 /// WARNING: This file knows about certain library functions. It recognizes them
18 /// by name, and hardwires knowledge of their semantics.
19 ///
20 /// WARNING: This file knows about how certain Objective-C library functions are
21 /// used. Naive LLVM IR transformations which would otherwise be
22 /// behavior-preserving may break these assumptions.
23 //
24 //===----------------------------------------------------------------------===//
25
26 #include "ProvenanceAnalysis.h"
27 #include "llvm/ADT/SmallPtrSet.h"
28 #include "llvm/ADT/SmallVector.h"
29 #include "llvm/Analysis/AliasAnalysis.h"
30 #include "llvm/Analysis/ObjCARCAnalysisUtils.h"
31 #include "llvm/IR/Instructions.h"
32 #include "llvm/IR/Module.h"
33 #include "llvm/IR/Use.h"
34 #include "llvm/IR/User.h"
35 #include "llvm/IR/Value.h"
36 #include "llvm/Support/Casting.h"
37 #include <utility>
38
39 using namespace llvm;
40 using namespace llvm::objcarc;
41
relatedSelect(const SelectInst * A,const Value * B)42 bool ProvenanceAnalysis::relatedSelect(const SelectInst *A,
43 const Value *B) {
44 const DataLayout &DL = A->getModule()->getDataLayout();
45 // If the values are Selects with the same condition, we can do a more precise
46 // check: just check for relations between the values on corresponding arms.
47 if (const SelectInst *SB = dyn_cast<SelectInst>(B))
48 if (A->getCondition() == SB->getCondition())
49 return related(A->getTrueValue(), SB->getTrueValue(), DL) ||
50 related(A->getFalseValue(), SB->getFalseValue(), DL);
51
52 // Check both arms of the Select node individually.
53 return related(A->getTrueValue(), B, DL) ||
54 related(A->getFalseValue(), B, DL);
55 }
56
relatedPHI(const PHINode * A,const Value * B)57 bool ProvenanceAnalysis::relatedPHI(const PHINode *A,
58 const Value *B) {
59 const DataLayout &DL = A->getModule()->getDataLayout();
60 // If the values are PHIs in the same block, we can do a more precise as well
61 // as efficient check: just check for relations between the values on
62 // corresponding edges.
63 if (const PHINode *PNB = dyn_cast<PHINode>(B))
64 if (PNB->getParent() == A->getParent()) {
65 for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i)
66 if (related(A->getIncomingValue(i),
67 PNB->getIncomingValueForBlock(A->getIncomingBlock(i)), DL))
68 return true;
69 return false;
70 }
71
72 // Check each unique source of the PHI node against B.
73 SmallPtrSet<const Value *, 4> UniqueSrc;
74 for (Value *PV1 : A->incoming_values()) {
75 if (UniqueSrc.insert(PV1).second && related(PV1, B, DL))
76 return true;
77 }
78
79 // All of the arms checked out.
80 return false;
81 }
82
83 /// Test if the value of P, or any value covered by its provenance, is ever
84 /// stored within the function (not counting callees).
IsStoredObjCPointer(const Value * P)85 static bool IsStoredObjCPointer(const Value *P) {
86 SmallPtrSet<const Value *, 8> Visited;
87 SmallVector<const Value *, 8> Worklist;
88 Worklist.push_back(P);
89 Visited.insert(P);
90 do {
91 P = Worklist.pop_back_val();
92 for (const Use &U : P->uses()) {
93 const User *Ur = U.getUser();
94 if (isa<StoreInst>(Ur)) {
95 if (U.getOperandNo() == 0)
96 // The pointer is stored.
97 return true;
98 // The pointed is stored through.
99 continue;
100 }
101 if (isa<CallInst>(Ur))
102 // The pointer is passed as an argument, ignore this.
103 continue;
104 if (isa<PtrToIntInst>(P))
105 // Assume the worst.
106 return true;
107 if (Visited.insert(Ur).second)
108 Worklist.push_back(Ur);
109 }
110 } while (!Worklist.empty());
111
112 // Everything checked out.
113 return false;
114 }
115
relatedCheck(const Value * A,const Value * B,const DataLayout & DL)116 bool ProvenanceAnalysis::relatedCheck(const Value *A, const Value *B,
117 const DataLayout &DL) {
118 // Ask regular AliasAnalysis, for a first approximation.
119 switch (AA->alias(A, B)) {
120 case NoAlias:
121 return false;
122 case MustAlias:
123 case PartialAlias:
124 return true;
125 case MayAlias:
126 break;
127 }
128
129 bool AIsIdentified = IsObjCIdentifiedObject(A);
130 bool BIsIdentified = IsObjCIdentifiedObject(B);
131
132 // An ObjC-Identified object can't alias a load if it is never locally stored.
133 if (AIsIdentified) {
134 // Check for an obvious escape.
135 if (isa<LoadInst>(B))
136 return IsStoredObjCPointer(A);
137 if (BIsIdentified) {
138 // Check for an obvious escape.
139 if (isa<LoadInst>(A))
140 return IsStoredObjCPointer(B);
141 // Both pointers are identified and escapes aren't an evident problem.
142 return false;
143 }
144 } else if (BIsIdentified) {
145 // Check for an obvious escape.
146 if (isa<LoadInst>(A))
147 return IsStoredObjCPointer(B);
148 }
149
150 // Special handling for PHI and Select.
151 if (const PHINode *PN = dyn_cast<PHINode>(A))
152 return relatedPHI(PN, B);
153 if (const PHINode *PN = dyn_cast<PHINode>(B))
154 return relatedPHI(PN, A);
155 if (const SelectInst *S = dyn_cast<SelectInst>(A))
156 return relatedSelect(S, B);
157 if (const SelectInst *S = dyn_cast<SelectInst>(B))
158 return relatedSelect(S, A);
159
160 // Conservative.
161 return true;
162 }
163
related(const Value * A,const Value * B,const DataLayout & DL)164 bool ProvenanceAnalysis::related(const Value *A, const Value *B,
165 const DataLayout &DL) {
166 A = GetUnderlyingObjCPtrCached(A, DL, UnderlyingObjCPtrCache);
167 B = GetUnderlyingObjCPtrCached(B, DL, UnderlyingObjCPtrCache);
168
169 // Quick check.
170 if (A == B)
171 return true;
172
173 // Begin by inserting a conservative value into the map. If the insertion
174 // fails, we have the answer already. If it succeeds, leave it there until we
175 // compute the real answer to guard against recursive queries.
176 if (A > B) std::swap(A, B);
177 std::pair<CachedResultsTy::iterator, bool> Pair =
178 CachedResults.insert(std::make_pair(ValuePairTy(A, B), true));
179 if (!Pair.second)
180 return Pair.first->second;
181
182 bool Result = relatedCheck(A, B, DL);
183 CachedResults[ValuePairTy(A, B)] = Result;
184 return Result;
185 }
186