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/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
  38 using namespace llvm;
  39 using namespace llvm::objcarc;
  40
  41 bool 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
  56 bool 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).
  84 static 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
 115 bool 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
 163 bool 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 }