LLVM  6.0.0svn
ProvenanceAnalysis.cpp
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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 "ProvenanceAnalysis.h"
26 #include "ObjCARC.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.
65  for (Value *PV1 : A->incoming_values()) {
66  if (UniqueSrc.insert(PV1).second && related(PV1, B, DL))
67  return true;
68  }
69 
70  // All of the arms checked out.
71  return false;
72 }
73 
74 /// Test if the value of P, or any value covered by its provenance, is ever
75 /// stored within the function (not counting callees).
76 static bool IsStoredObjCPointer(const Value *P) {
79  Worklist.push_back(P);
80  Visited.insert(P);
81  do {
82  P = Worklist.pop_back_val();
83  for (const Use &U : P->uses()) {
84  const User *Ur = U.getUser();
85  if (isa<StoreInst>(Ur)) {
86  if (U.getOperandNo() == 0)
87  // The pointer is stored.
88  return true;
89  // The pointed is stored through.
90  continue;
91  }
92  if (isa<CallInst>(Ur))
93  // The pointer is passed as an argument, ignore this.
94  continue;
95  if (isa<PtrToIntInst>(P))
96  // Assume the worst.
97  return true;
98  if (Visited.insert(Ur).second)
99  Worklist.push_back(Ur);
100  }
101  } while (!Worklist.empty());
102 
103  // Everything checked out.
104  return false;
105 }
106 
107 bool ProvenanceAnalysis::relatedCheck(const Value *A, const Value *B,
108  const DataLayout &DL) {
109  // Skip past provenance pass-throughs.
110  A = GetUnderlyingObjCPtr(A, DL);
111  B = GetUnderlyingObjCPtr(B, DL);
112 
113  // Quick check.
114  if (A == B)
115  return true;
116 
117  // Ask regular AliasAnalysis, for a first approximation.
118  switch (AA->alias(A, B)) {
119  case NoAlias:
120  return false;
121  case MustAlias:
122  case PartialAlias:
123  return true;
124  case MayAlias:
125  break;
126  }
127 
128  bool AIsIdentified = IsObjCIdentifiedObject(A);
129  bool BIsIdentified = IsObjCIdentifiedObject(B);
130 
131  // An ObjC-Identified object can't alias a load if it is never locally stored.
132  if (AIsIdentified) {
133  // Check for an obvious escape.
134  if (isa<LoadInst>(B))
135  return IsStoredObjCPointer(A);
136  if (BIsIdentified) {
137  // Check for an obvious escape.
138  if (isa<LoadInst>(A))
139  return IsStoredObjCPointer(B);
140  // Both pointers are identified and escapes aren't an evident problem.
141  return false;
142  }
143  } else if (BIsIdentified) {
144  // Check for an obvious escape.
145  if (isa<LoadInst>(A))
146  return IsStoredObjCPointer(B);
147  }
148 
149  // Special handling for PHI and Select.
150  if (const PHINode *PN = dyn_cast<PHINode>(A))
151  return relatedPHI(PN, B);
152  if (const PHINode *PN = dyn_cast<PHINode>(B))
153  return relatedPHI(PN, A);
154  if (const SelectInst *S = dyn_cast<SelectInst>(A))
155  return relatedSelect(S, B);
156  if (const SelectInst *S = dyn_cast<SelectInst>(B))
157  return relatedSelect(S, A);
158 
159  // Conservative.
160  return true;
161 }
162 
163 bool ProvenanceAnalysis::related(const Value *A, const Value *B,
164  const DataLayout &DL) {
165  // Begin by inserting a conservative value into the map. If the insertion
166  // fails, we have the answer already. If it succeeds, leave it there until we
167  // compute the real answer to guard against recursive queries.
168  if (A > B) std::swap(A, B);
169  std::pair<CachedResultsTy::iterator, bool> Pair =
170  CachedResults.insert(std::make_pair(ValuePairTy(A, B), true));
171  if (!Pair.second)
172  return Pair.first->second;
173 
174  bool Result = relatedCheck(A, B, DL);
175  CachedResults[ValuePairTy(A, B)] = Result;
176  return Result;
177 }
The two locations precisely alias each other.
Definition: AliasAnalysis.h:85
void push_back(const T &Elt)
Definition: SmallVector.h:212
A parsed version of the target data layout string in and methods for querying it. ...
Definition: DataLayout.h:109
iterator_range< use_iterator > uses()
Definition: Value.h:350
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
The two locations alias, but only due to a partial overlap.
Definition: AliasAnalysis.h:83
const Value * getTrueValue() const
The two locations do not alias at all.
Definition: AliasAnalysis.h:79
bool IsObjCIdentifiedObject(const Value *V)
Return true if this value refers to a distinct and identifiable object.
The two locations may or may not alias. This is the least precise result.
Definition: AliasAnalysis.h:81
std::pair< iterator, bool > insert(const std::pair< KeyT, ValueT > &KV)
Definition: DenseMap.h:176
AliasResult alias(const MemoryLocation &LocA, const MemoryLocation &LocB)
The main low level interface to the alias analysis implementation.
This class represents the LLVM &#39;select&#39; instruction.
const DataLayout & getDataLayout() const
Get the data layout for the module&#39;s target platform.
Definition: Module.cpp:361
A Use represents the edge between a Value definition and its users.
Definition: Use.h:56
This file defines common definitions/declarations used by the ObjC ARC Optimizer. ...
#define P(N)
static bool IsStoredObjCPointer(const Value *P)
Test if the value of P, or any value covered by its provenance, is ever stored within the function (n...
#define A
Definition: LargeTest.cpp:12
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:372
const Value * getCondition() const
Value * getIncomingValue(unsigned i) const
Return incoming value number x.
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:423
#define B
Definition: LargeTest.cpp:24
This file declares a special form of Alias Analysis called ``Provenance Analysis&#39;&#39;.
LLVM_NODISCARD T pop_back_val()
Definition: SmallVector.h:385
unsigned getNumIncomingValues() const
Return the number of incoming edges.
void swap(llvm::BitVector &LHS, llvm::BitVector &RHS)
Implement std::swap in terms of BitVector swap.
Definition: BitVector.h:923
bool related(const Value *A, const Value *B, const DataLayout &DL)
const Module * getModule() const
Return the module owning the function this instruction belongs to or nullptr it the function does not...
Definition: Instruction.cpp:57
const Value * getFalseValue() const
LLVM_NODISCARD bool empty() const
Definition: SmallVector.h:61
BasicBlock * getIncomingBlock(unsigned i) const
Return incoming basic block number i.
const Value * GetUnderlyingObjCPtr(const Value *V, const DataLayout &DL)
This is a wrapper around getUnderlyingObject which also knows how to look through objc_retain and obj...
LLVM Value Representation.
Definition: Value.h:73
op_range incoming_values()
const BasicBlock * getParent() const
Definition: Instruction.h:66