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
38using namespace llvm;
39using namespace llvm::objcarc;
40
41bool ProvenanceAnalysis::relatedSelect(const SelectInst *A,
42                                       const Value *B) {
43  const DataLayout &DL = A->getModule()->getDataLayout();
44  // If the values are Selects with the same condition, we can do a more precise
45  // check: just check for relations between the values on corresponding arms.
46  if (const SelectInst *SB = dyn_cast<SelectInst>(B))
47    if (A->getCondition() == SB->getCondition())
48      return related(A->getTrueValue(), SB->getTrueValue(), DL) ||
49             related(A->getFalseValue(), SB->getFalseValue(), DL);
50
51  // Check both arms of the Select node individually.
52  return related(A->getTrueValue(), B, DL) ||
53         related(A->getFalseValue(), B, DL);
54}
55
56bool ProvenanceAnalysis::relatedPHI(const PHINode *A,
57                                    const Value *B) {
58  const DataLayout &DL = A->getModule()->getDataLayout();
59  // If the values are PHIs in the same block, we can do a more precise as well
60  // as efficient check: just check for relations between the values on
61  // corresponding edges.
62  if (const PHINode *PNB = dyn_cast<PHINode>(B))
63    if (PNB->getParent() == A->getParent()) {
64      for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i)
65        if (related(A->getIncomingValue(i),
66                    PNB->getIncomingValueForBlock(A->getIncomingBlock(i)), DL))
67          return true;
68      return false;
69    }
70
71  // Check each unique source of the PHI node against B.
72  SmallPtrSet<const Value *, 4> UniqueSrc;
73  for (Value *PV1 : A->incoming_values()) {
74    if (UniqueSrc.insert(PV1).second && related(PV1, B, DL))
75      return true;
76  }
77
78  // All of the arms checked out.
79  return false;
80}
81
82/// Test if the value of P, or any value covered by its provenance, is ever
83/// stored within the function (not counting callees).
84static bool IsStoredObjCPointer(const Value *P) {
85  SmallPtrSet<const Value *, 8> Visited;
86  SmallVector<const Value *, 8> Worklist;
87  Worklist.push_back(P);
88  Visited.insert(P);
89  do {
90    P = Worklist.pop_back_val();
91    for (const Use &U : P->uses()) {
92      const User *Ur = U.getUser();
93      if (isa<StoreInst>(Ur)) {
94        if (U.getOperandNo() == 0)
95          // The pointer is stored.
96          return true;
97        // The pointed is stored through.
98        continue;
99      }
100      if (isa<CallInst>(Ur))
101        // The pointer is passed as an argument, ignore this.
102        continue;
103      if (isa<PtrToIntInst>(P))
104        // Assume the worst.
105        return true;
106      if (Visited.insert(Ur).second)
107        Worklist.push_back(Ur);
108    }
109  } while (!Worklist.empty());
110
111  // Everything checked out.
112  return false;
113}
114
115bool ProvenanceAnalysis::relatedCheck(const Value *A, const Value *B,
116                                      const DataLayout &DL) {
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
163bool ProvenanceAnalysis::related(const Value *A, const Value *B,
164                                 const DataLayout &DL) {
165  A = GetUnderlyingObjCPtrCached(A, DL, UnderlyingObjCPtrCache);
166  B = GetUnderlyingObjCPtrCached(B, DL, UnderlyingObjCPtrCache);
167
168  // Quick check.
169  if (A == B)
170    return true;
171
172  // Begin by inserting a conservative value into the map. If the insertion
173  // fails, we have the answer already. If it succeeds, leave it there until we
174  // compute the real answer to guard against recursive queries.
175  if (A > B) std::swap(A, B);
176  std::pair<CachedResultsTy::iterator, bool> Pair =
177    CachedResults.insert(std::make_pair(ValuePairTy(A, B), true));
178  if (!Pair.second)
179    return Pair.first->second;
180
181  bool Result = relatedCheck(A, B, DL);
182  CachedResults[ValuePairTy(A, B)] = Result;
183  return Result;
184}
185