ScalarEvolutionAliasAnalysis.cpp revision 2385e0e22ca482f2896dfa975a08db2f54926c09 
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"
23using namespace llvm;
24
25namespace {
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...
47char ScalarEvolutionAliasAnalysis::ID = 0;
48static RegisterPass<ScalarEvolutionAliasAnalysis>
49X("scev-aa", "ScalarEvolution-based Alias Analysis", false, true);
50
51// Declare that we implement the AliasAnalysis interface
52static RegisterAnalysisGroup<AliasAnalysis> Y(X);
53
54FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
55  return new ScalarEvolutionAliasAnalysis();
56}
57
58void
59ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
60  AU.addRequiredTransitive<ScalarEvolution>();
61  AU.setPreservesAll();
62  AliasAnalysis::getAnalysisUsage(AU);
63}
64
65bool
66ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
67  InitializeAliasAnalysis(this);
68  SE = &getAnalysis<ScalarEvolution>();
69  return false;
70}
71
72Value *
73ScalarEvolutionAliasAnalysis::GetUnderlyingIdentifiedObject(const SCEV *S) {
74  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
75    return GetUnderlyingIdentifiedObject(AR->getStart());
76  } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
77    // If there's a pointer operand, it'll be sorted at the end of the list.
78    const SCEV *Last = A->getOperand(A->getNumOperands()-1);
79    if (isa<PointerType>(Last->getType()))
80      return GetUnderlyingIdentifiedObject(Last);
81  } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
82    // Determine if we've found an Identified object.
83    Value *V = U->getValue();
84    if (isIdentifiedObject(V))
85      return V;
86  }
87  // No Identified object found.
88  return 0;
89}
90
91AliasAnalysis::AliasResult
92ScalarEvolutionAliasAnalysis::alias(const Value *A, unsigned ASize,
93                                    const Value *B, unsigned BSize) {
94  // This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
95  const SCEV *AS = SE->getSCEV(const_cast<Value *>(A));
96  const SCEV *BS = SE->getSCEV(const_cast<Value *>(B));
97
98  // If they evaluate to the same expression, it's a MustAlias.
99  if (AS == BS) return MustAlias;
100
101  // If something is known about the difference between the two addresses,
102  // see if it's enough to prove a NoAlias.
103  if (SE->getEffectiveSCEVType(AS->getType()) ==
104      SE->getEffectiveSCEVType(BS->getType())) {
105    unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
106    APInt AI(BitWidth, ASize);
107    const SCEV *BA = SE->getMinusSCEV(BS, AS);
108    if (AI.ule(SE->getUnsignedRange(BA).getUnsignedMin())) {
109      APInt BI(BitWidth, BSize);
110      const SCEV *AB = SE->getMinusSCEV(AS, BS);
111      if (BI.ule(SE->getUnsignedRange(AB).getUnsignedMin()))
112        return NoAlias;
113    }
114  }
115
116  // If ScalarEvolution can find an underlying object, form a new query.
117  // The correctness of this depends on ScalarEvolution not recognizing
118  // inttoptr and ptrtoint operators.
119  Value *AO = GetUnderlyingIdentifiedObject(AS);
120  Value *BO = GetUnderlyingIdentifiedObject(BS);
121  if ((AO && AO != A) || (BO && BO != B))
122    if (alias(AO ? AO : A, AO ? ~0u : ASize,
123              BO ? BO : B, BO ? ~0u : BSize) == NoAlias)
124      return NoAlias;
125
126  // Forward the query to the next analysis.
127  return AliasAnalysis::alias(A, ASize, B, BSize);
128}
129