lib/Analysis/ScalarEvolutionAliasAnalysis.cpp

   1 //===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
   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 //
  10 // This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
  11 // simple alias analysis implemented in terms of ScalarEvolution queries.
  12 //
  13 // ScalarEvolution has a more complete understanding of pointer arithmetic
  14 // than BasicAliasAnalysis' collection of ad-hoc analyses.
  15 //
  16 //===----------------------------------------------------------------------===//
  17
  18 #include "llvm/Analysis/AliasAnalysis.h"
  19 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
  20 #include "llvm/Analysis/Passes.h"
  21 #include "llvm/Pass.h"
  22 #include "llvm/Support/Compiler.h"
  23 using namespace llvm;
  24
  25 namespace {
  26   /// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
  27   /// implementation that uses ScalarEvolution to answer queries.
  28   class VISIBILITY_HIDDEN ScalarEvolutionAliasAnalysis : public FunctionPass,
  29                                                          public AliasAnalysis {
  30     ScalarEvolution *SE;
  31
  32   public:
  33     static char ID; // Class identification, replacement for typeinfo
  34     ScalarEvolutionAliasAnalysis() : FunctionPass(&ID), SE(0) {}
  35
  36   private:
  37     virtual void getAnalysisUsage(AnalysisUsage &AU) const;
  38     virtual bool runOnFunction(Function &F);
  39     virtual AliasResult alias(const Value *V1, unsigned V1Size,
  40                               const Value *V2, unsigned V2Size);
  41
  42     Value *GetUnderlyingIdentifiedObject(const SCEV *S);
  43   };
  44 }  // End of anonymous namespace
  45
  46 // Register this pass...
  47 char ScalarEvolutionAliasAnalysis::ID = 0;
  48 static RegisterPass<ScalarEvolutionAliasAnalysis>
  49 X("scev-aa", "ScalarEvolution-based Alias Analysis", false, true);
  50
  51 // Declare that we implement the AliasAnalysis interface
  52 static RegisterAnalysisGroup<AliasAnalysis> Y(X);
  53
  54 FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
  55   return new ScalarEvolutionAliasAnalysis();
  56 }
  57
  58 void
  59 ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
  60   AU.addRequiredTransitive<ScalarEvolution>();
  61   AU.setPreservesAll();
  62   AliasAnalysis::getAnalysisUsage(AU);
  63 }
  64
  65 bool
  66 ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
  67   InitializeAliasAnalysis(this);
  68   SE = &getAnalysis<ScalarEvolution>();
  69   return false;
  70 }
  71
  72 /// GetUnderlyingIdentifiedObject - Given an expression, try to find an
  73 /// "identified object" (see AliasAnalysis::isIdentifiedObject) base
  74 /// value. Return null is none was found.
  75 Value *
  76 ScalarEvolutionAliasAnalysis::GetUnderlyingIdentifiedObject(const SCEV *S) {
  77   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
  78     // In an addrec, assume that the base will be in the start, rather
  79     // than the step.
  80     return GetUnderlyingIdentifiedObject(AR->getStart());
  81   } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
  82     // If there's a pointer operand, it'll be sorted at the end of the list.
  83     const SCEV *Last = A->getOperand(A->getNumOperands()-1);
  84     if (isa<PointerType>(Last->getType()))
  85       return GetUnderlyingIdentifiedObject(Last);
  86   } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
  87     // Determine if we've found an Identified object.
  88     Value *V = U->getValue();
  89     if (isIdentifiedObject(V))
  90       return V;
  91   }
  92   // No Identified object found.
  93   return 0;
  94 }
  95
  96 AliasAnalysis::AliasResult
  97 ScalarEvolutionAliasAnalysis::alias(const Value *A, unsigned ASize,
  98                                     const Value *B, unsigned BSize) {
  99   // This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
 100   const SCEV *AS = SE->getSCEV(const_cast<Value *>(A));
 101   const SCEV *BS = SE->getSCEV(const_cast<Value *>(B));
 102
 103   // If they evaluate to the same expression, it's a MustAlias.
 104   if (AS == BS) return MustAlias;
 105
 106   // If something is known about the difference between the two addresses,
 107   // see if it's enough to prove a NoAlias.
 108   if (SE->getEffectiveSCEVType(AS->getType()) ==
 109       SE->getEffectiveSCEVType(BS->getType())) {
 110     unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
 111     APInt AI(BitWidth, ASize);
 112     const SCEV *BA = SE->getMinusSCEV(BS, AS);
 113     if (AI.ule(SE->getUnsignedRange(BA).getUnsignedMin())) {
 114       APInt BI(BitWidth, BSize);
 115       const SCEV *AB = SE->getMinusSCEV(AS, BS);
 116       if (BI.ule(SE->getUnsignedRange(AB).getUnsignedMin()))
 117         return NoAlias;
 118     }
 119   }
 120
 121   // If ScalarEvolution can find an underlying object, form a new query.
 122   // The correctness of this depends on ScalarEvolution not recognizing
 123   // inttoptr and ptrtoint operators.
 124   Value *AO = GetUnderlyingIdentifiedObject(AS);
 125   Value *BO = GetUnderlyingIdentifiedObject(BS);
 126   if ((AO && AO != A) || (BO && BO != B))
 127     if (alias(AO ? AO : A, AO ? ~0u : ASize,
 128               BO ? BO : B, BO ? ~0u : BSize) == NoAlias)
 129       return NoAlias;
 130
 131   // Forward the query to the next analysis.
 132   return AliasAnalysis::alias(A, ASize, B, BSize);
 133 }