LLVM API Documentation

ProvenanceAnalysis.cpp
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00001 //===- ProvenanceAnalysis.cpp - ObjC ARC Optimization ---------------------===//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file is distributed under the University of Illinois Open Source
00006 // License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 /// \file
00010 ///
00011 /// This file defines a special form of Alias Analysis called ``Provenance
00012 /// Analysis''. The word ``provenance'' refers to the history of the ownership
00013 /// of an object. Thus ``Provenance Analysis'' is an analysis which attempts to
00014 /// use various techniques to determine if locally
00015 ///
00016 /// WARNING: This file knows about certain library functions. It recognizes them
00017 /// by name, and hardwires knowledge of their semantics.
00018 ///
00019 /// WARNING: This file knows about how certain Objective-C library functions are
00020 /// used. Naive LLVM IR transformations which would otherwise be
00021 /// behavior-preserving may break these assumptions.
00022 ///
00023 //===----------------------------------------------------------------------===//
00024 
00025 #include "ObjCARC.h"
00026 #include "ProvenanceAnalysis.h"
00027 #include "llvm/ADT/STLExtras.h"
00028 #include "llvm/ADT/SmallPtrSet.h"
00029 
00030 using namespace llvm;
00031 using namespace llvm::objcarc;
00032 
00033 bool ProvenanceAnalysis::relatedSelect(const SelectInst *A,
00034                                        const Value *B) {
00035   // If the values are Selects with the same condition, we can do a more precise
00036   // check: just check for relations between the values on corresponding arms.
00037   if (const SelectInst *SB = dyn_cast<SelectInst>(B))
00038     if (A->getCondition() == SB->getCondition())
00039       return related(A->getTrueValue(), SB->getTrueValue()) ||
00040              related(A->getFalseValue(), SB->getFalseValue());
00041 
00042   // Check both arms of the Select node individually.
00043   return related(A->getTrueValue(), B) ||
00044          related(A->getFalseValue(), B);
00045 }
00046 
00047 bool ProvenanceAnalysis::relatedPHI(const PHINode *A,
00048                                     const Value *B) {
00049   // If the values are PHIs in the same block, we can do a more precise as well
00050   // as efficient check: just check for relations between the values on
00051   // corresponding edges.
00052   if (const PHINode *PNB = dyn_cast<PHINode>(B))
00053     if (PNB->getParent() == A->getParent()) {
00054       for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i)
00055         if (related(A->getIncomingValue(i),
00056                     PNB->getIncomingValueForBlock(A->getIncomingBlock(i))))
00057           return true;
00058       return false;
00059     }
00060 
00061   // Check each unique source of the PHI node against B.
00062   SmallPtrSet<const Value *, 4> UniqueSrc;
00063   for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i) {
00064     const Value *PV1 = A->getIncomingValue(i);
00065     if (UniqueSrc.insert(PV1).second && related(PV1, B))
00066       return true;
00067   }
00068 
00069   // All of the arms checked out.
00070   return false;
00071 }
00072 
00073 /// Test if the value of P, or any value covered by its provenance, is ever
00074 /// stored within the function (not counting callees).
00075 static bool IsStoredObjCPointer(const Value *P) {
00076   SmallPtrSet<const Value *, 8> Visited;
00077   SmallVector<const Value *, 8> Worklist;
00078   Worklist.push_back(P);
00079   Visited.insert(P);
00080   do {
00081     P = Worklist.pop_back_val();
00082     for (const Use &U : P->uses()) {
00083       const User *Ur = U.getUser();
00084       if (isa<StoreInst>(Ur)) {
00085         if (U.getOperandNo() == 0)
00086           // The pointer is stored.
00087           return true;
00088         // The pointed is stored through.
00089         continue;
00090       }
00091       if (isa<CallInst>(Ur))
00092         // The pointer is passed as an argument, ignore this.
00093         continue;
00094       if (isa<PtrToIntInst>(P))
00095         // Assume the worst.
00096         return true;
00097       if (Visited.insert(Ur).second)
00098         Worklist.push_back(Ur);
00099     }
00100   } while (!Worklist.empty());
00101 
00102   // Everything checked out.
00103   return false;
00104 }
00105 
00106 bool ProvenanceAnalysis::relatedCheck(const Value *A,
00107                                       const Value *B) {
00108   // Skip past provenance pass-throughs.
00109   A = GetUnderlyingObjCPtr(A);
00110   B = GetUnderlyingObjCPtr(B);
00111 
00112   // Quick check.
00113   if (A == B)
00114     return true;
00115 
00116   // Ask regular AliasAnalysis, for a first approximation.
00117   switch (AA->alias(A, B)) {
00118   case AliasAnalysis::NoAlias:
00119     return false;
00120   case AliasAnalysis::MustAlias:
00121   case AliasAnalysis::PartialAlias:
00122     return true;
00123   case AliasAnalysis::MayAlias:
00124     break;
00125   }
00126 
00127   bool AIsIdentified = IsObjCIdentifiedObject(A);
00128   bool BIsIdentified = IsObjCIdentifiedObject(B);
00129 
00130   // An ObjC-Identified object can't alias a load if it is never locally stored.
00131   if (AIsIdentified) {
00132     // Check for an obvious escape.
00133     if (isa<LoadInst>(B))
00134       return IsStoredObjCPointer(A);
00135     if (BIsIdentified) {
00136       // Check for an obvious escape.
00137       if (isa<LoadInst>(A))
00138         return IsStoredObjCPointer(B);
00139       // Both pointers are identified and escapes aren't an evident problem.
00140       return false;
00141     }
00142   } else if (BIsIdentified) {
00143     // Check for an obvious escape.
00144     if (isa<LoadInst>(A))
00145       return IsStoredObjCPointer(B);
00146   }
00147 
00148    // Special handling for PHI and Select.
00149   if (const PHINode *PN = dyn_cast<PHINode>(A))
00150     return relatedPHI(PN, B);
00151   if (const PHINode *PN = dyn_cast<PHINode>(B))
00152     return relatedPHI(PN, A);
00153   if (const SelectInst *S = dyn_cast<SelectInst>(A))
00154     return relatedSelect(S, B);
00155   if (const SelectInst *S = dyn_cast<SelectInst>(B))
00156     return relatedSelect(S, A);
00157 
00158   // Conservative.
00159   return true;
00160 }
00161 
00162 bool ProvenanceAnalysis::related(const Value *A,
00163                                  const Value *B) {
00164   // Begin by inserting a conservative value into the map. If the insertion
00165   // fails, we have the answer already. If it succeeds, leave it there until we
00166   // compute the real answer to guard against recursive queries.
00167   if (A > B) std::swap(A, B);
00168   std::pair<CachedResultsTy::iterator, bool> Pair =
00169     CachedResults.insert(std::make_pair(ValuePairTy(A, B), true));
00170   if (!Pair.second)
00171     return Pair.first->second;
00172 
00173   bool Result = relatedCheck(A, B);
00174   CachedResults[ValuePairTy(A, B)] = Result;
00175   return Result;
00176 }