src/llvm/lib/Transforms/ObjCARC/ProvenanceAnalysis.cpp

   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   // If the values are Selects with the same condition, we can do a more precise
  36   // check: just check for relations between the values on corresponding arms.
  37   if (const SelectInst *SB = dyn_cast<SelectInst>(B))
  38     if (A->getCondition() == SB->getCondition())
  39       return related(A->getTrueValue(), SB->getTrueValue()) ||
  40              related(A->getFalseValue(), SB->getFalseValue());
  41
  42   // Check both arms of the Select node individually.
  43   return related(A->getTrueValue(), B) ||
  44          related(A->getFalseValue(), B);
  45 }
  46
  47 bool ProvenanceAnalysis::relatedPHI(const PHINode *A,
  48                                     const Value *B) {
  49   // If the values are PHIs in the same block, we can do a more precise as well
  50   // as efficient check: just check for relations between the values on
  51   // corresponding edges.
  52   if (const PHINode *PNB = dyn_cast<PHINode>(B))
  53     if (PNB->getParent() == A->getParent()) {
  54       for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i)
  55         if (related(A->getIncomingValue(i),
  56                     PNB->getIncomingValueForBlock(A->getIncomingBlock(i))))
  57           return true;
  58       return false;
  59     }
  60
  61   // Check each unique source of the PHI node against B.
  62   SmallPtrSet<const Value *, 4> UniqueSrc;
  63   for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i) {
  64     const Value *PV1 = A->getIncomingValue(i);
  65     if (UniqueSrc.insert(PV1).second && related(PV1, B))
  66       return true;
  67   }
  68
  69   // All of the arms checked out.
  70   return false;
  71 }
  72
  73 /// Test if the value of P, or any value covered by its provenance, is ever
  74 /// stored within the function (not counting callees).
  75 static bool IsStoredObjCPointer(const Value *P) {
  76   SmallPtrSet<const Value *, 8> Visited;
  77   SmallVector<const Value *, 8> Worklist;
  78   Worklist.push_back(P);
  79   Visited.insert(P);
  80   do {
  81     P = Worklist.pop_back_val();
  82     for (const Use &U : P->uses()) {
  83       const User *Ur = U.getUser();
  84       if (isa<StoreInst>(Ur)) {
  85         if (U.getOperandNo() == 0)
  86           // The pointer is stored.
  87           return true;
  88         // The pointed is stored through.
  89         continue;
  90       }
  91       if (isa<CallInst>(Ur))
  92         // The pointer is passed as an argument, ignore this.
  93         continue;
  94       if (isa<PtrToIntInst>(P))
  95         // Assume the worst.
  96         return true;
  97       if (Visited.insert(Ur).second)
  98         Worklist.push_back(Ur);
  99     }
 100   } while (!Worklist.empty());
 101
 102   // Everything checked out.
 103   return false;
 104 }
 105
 106 bool ProvenanceAnalysis::relatedCheck(const Value *A,
 107                                       const Value *B) {
 108   // Skip past provenance pass-throughs.
 109   A = GetUnderlyingObjCPtr(A);
 110   B = GetUnderlyingObjCPtr(B);
 111
 112   // Quick check.
 113   if (A == B)
 114     return true;
 115
 116   // Ask regular AliasAnalysis, for a first approximation.
 117   switch (AA->alias(A, B)) {
 118   case AliasAnalysis::NoAlias:
 119     return false;
 120   case AliasAnalysis::MustAlias:
 121   case AliasAnalysis::PartialAlias:
 122     return true;
 123   case AliasAnalysis::MayAlias:
 124     break;
 125   }
 126
 127   bool AIsIdentified = IsObjCIdentifiedObject(A);
 128   bool BIsIdentified = IsObjCIdentifiedObject(B);
 129
 130   // An ObjC-Identified object can't alias a load if it is never locally stored.
 131   if (AIsIdentified) {
 132     // Check for an obvious escape.
 133     if (isa<LoadInst>(B))
 134       return IsStoredObjCPointer(A);
 135     if (BIsIdentified) {
 136       // Check for an obvious escape.
 137       if (isa<LoadInst>(A))
 138         return IsStoredObjCPointer(B);
 139       // Both pointers are identified and escapes aren't an evident problem.
 140       return false;
 141     }
 142   } else if (BIsIdentified) {
 143     // Check for an obvious escape.
 144     if (isa<LoadInst>(A))
 145       return IsStoredObjCPointer(B);
 146   }
 147
 148    // Special handling for PHI and Select.
 149   if (const PHINode *PN = dyn_cast<PHINode>(A))
 150     return relatedPHI(PN, B);
 151   if (const PHINode *PN = dyn_cast<PHINode>(B))
 152     return relatedPHI(PN, A);
 153   if (const SelectInst *S = dyn_cast<SelectInst>(A))
 154     return relatedSelect(S, B);
 155   if (const SelectInst *S = dyn_cast<SelectInst>(B))
 156     return relatedSelect(S, A);
 157
 158   // Conservative.
 159   return true;
 160 }
 161
 162 bool ProvenanceAnalysis::related(const Value *A,
 163                                  const Value *B) {
 164   // Begin by inserting a conservative value into the map. If the insertion
 165   // fails, we have the answer already. If it succeeds, leave it there until we
 166   // compute the real answer to guard against recursive queries.
 167   if (A > B) std::swap(A, B);
 168   std::pair<CachedResultsTy::iterator, bool> Pair =
 169     CachedResults.insert(std::make_pair(ValuePairTy(A, B), true));
 170   if (!Pair.second)
 171     return Pair.first->second;
 172
 173   bool Result = relatedCheck(A, B);
 174   CachedResults[ValuePairTy(A, B)] = Result;
 175   return Result;
 176 }