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 /// \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 "ObjCARC.h"
26 #include "ProvenanceAnalysis.h"
27 #include "llvm/ADT/STLExtras.h"
28 #include "llvm/ADT/SmallPtrSet.h"
29 
30 using namespace llvm;
31 using namespace llvm::objcarc;
32 
33 bool ProvenanceAnalysis::relatedSelect(const SelectInst *A,
34                                        const Value *B) {
35   const DataLayout &DL = A->getModule()->getDataLayout();
36   // If the values are Selects with the same condition, we can do a more precise
37   // check: just check for relations between the values on corresponding arms.
38   if (const SelectInst *SB = dyn_cast<SelectInst>(B))
39     if (A->getCondition() == SB->getCondition())
40       return related(A->getTrueValue(), SB->getTrueValue(), DL) ||
41              related(A->getFalseValue(), SB->getFalseValue(), DL);
42 
43   // Check both arms of the Select node individually.
44   return related(A->getTrueValue(), B, DL) ||
45          related(A->getFalseValue(), B, DL);
46 }
47 
48 bool ProvenanceAnalysis::relatedPHI(const PHINode *A,
49                                     const Value *B) {
50   const DataLayout &DL = A->getModule()->getDataLayout();
51   // If the values are PHIs in the same block, we can do a more precise as well
52   // as efficient check: just check for relations between the values on
53   // corresponding edges.
54   if (const PHINode *PNB = dyn_cast<PHINode>(B))
55     if (PNB->getParent() == A->getParent()) {
56       for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i)
57         if (related(A->getIncomingValue(i),
58                     PNB->getIncomingValueForBlock(A->getIncomingBlock(i)), DL))
59           return true;
60       return false;
61     }
62 
63   // Check each unique source of the PHI node against B.
64   SmallPtrSet<const Value *, 4> UniqueSrc;
65   for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i) {
66     const Value *PV1 = A->getIncomingValue(i);
67     if (UniqueSrc.insert(PV1).second && related(PV1, B, DL))
68       return true;
69   }
70 
71   // All of the arms checked out.
72   return false;
73 }
74 
75 /// Test if the value of P, or any value covered by its provenance, is ever
76 /// stored within the function (not counting callees).
77 static bool IsStoredObjCPointer(const Value *P) {
78   SmallPtrSet<const Value *, 8> Visited;
79   SmallVector<const Value *, 8> Worklist;
80   Worklist.push_back(P);
81   Visited.insert(P);
82   do {
83     P = Worklist.pop_back_val();
84     for (const Use &U : P->uses()) {
85       const User *Ur = U.getUser();
86       if (isa<StoreInst>(Ur)) {
87         if (U.getOperandNo() == 0)
88           // The pointer is stored.
89           return true;
90         // The pointed is stored through.
91         continue;
92       }
93       if (isa<CallInst>(Ur))
94         // The pointer is passed as an argument, ignore this.
95         continue;
96       if (isa<PtrToIntInst>(P))
97         // Assume the worst.
98         return true;
99       if (Visited.insert(Ur).second)
100         Worklist.push_back(Ur);
101     }
102   } while (!Worklist.empty());
103 
104   // Everything checked out.
105   return false;
106 }
107 
108 bool ProvenanceAnalysis::relatedCheck(const Value *A, const Value *B,
109                                       const DataLayout &DL) {
110   // Skip past provenance pass-throughs.
111   A = GetUnderlyingObjCPtr(A, DL);
112   B = GetUnderlyingObjCPtr(B, DL);
113 
114   // Quick check.
115   if (A == B)
116     return true;
117 
118   // Ask regular AliasAnalysis, for a first approximation.
119   switch (AA->alias(A, B)) {
120   case AliasAnalysis::NoAlias:
121     return false;
122   case AliasAnalysis::MustAlias:
123   case AliasAnalysis::PartialAlias:
124     return true;
125   case AliasAnalysis::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 
164 bool ProvenanceAnalysis::related(const Value *A, const Value *B,
165                                  const DataLayout &DL) {
166   // Begin by inserting a conservative value into the map. If the insertion
167   // fails, we have the answer already. If it succeeds, leave it there until we
168   // compute the real answer to guard against recursive queries.
169   if (A > B) std::swap(A, B);
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, DL);
176   CachedResults[ValuePairTy(A, B)] = Result;
177   return Result;
178 }
179