llvm/lib/Transforms/ObjCARC/ProvenanceAnalysis.cpp

   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/Use.h"
  32 #include "llvm/IR/User.h"
  33 #include "llvm/IR/Value.h"
  34 #include "llvm/Support/Casting.h"
  35 #include <utility>
  36
  37 using namespace llvm;
  38 using namespace llvm::objcarc;
  39
  40 bool ProvenanceAnalysis::relatedSelect(const SelectInst *A,
  41                                        const Value *B) {
  42   // If the values are Selects with the same condition, we can do a more precise
  43   // check: just check for relations between the values on corresponding arms.
  44   if (const SelectInst *SB = dyn_cast<SelectInst>(B))
  45     if (A->getCondition() == SB->getCondition())
  46       return related(A->getTrueValue(), SB->getTrueValue()) ||
  47              related(A->getFalseValue(), SB->getFalseValue());
  48
  49   // Check both arms of the Select node individually.
  50   return related(A->getTrueValue(), B) || related(A->getFalseValue(), B);
  51 }
  52
  53 bool ProvenanceAnalysis::relatedPHI(const PHINode *A,
  54                                     const Value *B) {
  55   // If the values are PHIs in the same block, we can do a more precise as well
  56   // as efficient check: just check for relations between the values on
  57   // corresponding edges.
  58   if (const PHINode *PNB = dyn_cast<PHINode>(B))
  59     if (PNB->getParent() == A->getParent()) {
  60       for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i)
  61         if (related(A->getIncomingValue(i),
  62                     PNB->getIncomingValueForBlock(A->getIncomingBlock(i))))
  63           return true;
  64       return false;
  65     }
  66
  67   // Check each unique source of the PHI node against B.
  68   SmallPtrSet<const Value *, 4> UniqueSrc;
  69   for (Value *PV1 : A->incoming_values()) {
  70     if (UniqueSrc.insert(PV1).second && related(PV1, B))
  71       return true;
  72   }
  73
  74   // All of the arms checked out.
  75   return false;
  76 }
  77
  78 /// Test if the value of P, or any value covered by its provenance, is ever
  79 /// stored within the function (not counting callees).
  80 static bool IsStoredObjCPointer(const Value *P) {
  81   SmallPtrSet<const Value *, 8> Visited;
  82   SmallVector<const Value *, 8> Worklist;
  83   Worklist.push_back(P);
  84   Visited.insert(P);
  85   do {
  86     P = Worklist.pop_back_val();
  87     for (const Use &U : P->uses()) {
  88       const User *Ur = U.getUser();
  89       if (isa<StoreInst>(Ur)) {
  90         if (U.getOperandNo() == 0)
  91           // The pointer is stored.
  92           return true;
  93         // The pointed is stored through.
  94         continue;
  95       }
  96       if (isa<CallInst>(Ur))
  97         // The pointer is passed as an argument, ignore this.
  98         continue;
  99       if (isa<PtrToIntInst>(P))
 100         // Assume the worst.
 101         return true;
 102       if (Visited.insert(Ur).second)
 103         Worklist.push_back(Ur);
 104     }
 105   } while (!Worklist.empty());
 106
 107   // Everything checked out.
 108   return false;
 109 }
 110
 111 bool ProvenanceAnalysis::relatedCheck(const Value *A, const Value *B) {
 112   // Ask regular AliasAnalysis, for a first approximation.
 113   switch (AA->alias(A, B)) {
 114   case AliasResult::NoAlias:
 115     return false;
 116   case AliasResult::MustAlias:
 117   case AliasResult::PartialAlias:
 118     return true;
 119   case AliasResult::MayAlias:
 120     break;
 121   }
 122
 123   bool AIsIdentified = IsObjCIdentifiedObject(A);
 124   bool BIsIdentified = IsObjCIdentifiedObject(B);
 125
 126   // An ObjC-Identified object can't alias a load if it is never locally stored.
 127   if (AIsIdentified) {
 128     // Check for an obvious escape.
 129     if (isa<LoadInst>(B))
 130       return IsStoredObjCPointer(A);
 131     if (BIsIdentified) {
 132       // Check for an obvious escape.
 133       if (isa<LoadInst>(A))
 134         return IsStoredObjCPointer(B);
 135       // Both pointers are identified and escapes aren't an evident problem.
 136       return false;
 137     }
 138   } else if (BIsIdentified) {
 139     // Check for an obvious escape.
 140     if (isa<LoadInst>(A))
 141       return IsStoredObjCPointer(B);
 142   }
 143
 144    // Special handling for PHI and Select.
 145   if (const PHINode *PN = dyn_cast<PHINode>(A))
 146     return relatedPHI(PN, B);
 147   if (const PHINode *PN = dyn_cast<PHINode>(B))
 148     return relatedPHI(PN, A);
 149   if (const SelectInst *S = dyn_cast<SelectInst>(A))
 150     return relatedSelect(S, B);
 151   if (const SelectInst *S = dyn_cast<SelectInst>(B))
 152     return relatedSelect(S, A);
 153
 154   // Conservative.
 155   return true;
 156 }
 157
 158 bool ProvenanceAnalysis::related(const Value *A, const Value *B) {
 159   A = GetUnderlyingObjCPtrCached(A, UnderlyingObjCPtrCache);
 160   B = GetUnderlyingObjCPtrCached(B, UnderlyingObjCPtrCache);
 161
 162   // Quick check.
 163   if (A == B)
 164     return true;
 165
 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   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);
 175   CachedResults[ValuePairTy(A, B)] = Result;
 176   return Result;
 177 }