ScalarEvolutionAliasAnalysis.cpp revision 198892 
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"
22using namespace llvm;
23
24namespace {
25  /// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
26  /// implementation that uses ScalarEvolution to answer queries.
27  class ScalarEvolutionAliasAnalysis : public FunctionPass,
28                                       public AliasAnalysis {
29    ScalarEvolution *SE;
30
31  public:
32    static char ID; // Class identification, replacement for typeinfo
33    ScalarEvolutionAliasAnalysis() : FunctionPass(&ID), SE(0) {}
34
35  private:
36    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
37    virtual bool runOnFunction(Function &F);
38    virtual AliasResult alias(const Value *V1, unsigned V1Size,
39                              const Value *V2, unsigned V2Size);
40
41    Value *GetBaseValue(const SCEV *S);
42  };
43}  // End of anonymous namespace
44
45// Register this pass...
46char ScalarEvolutionAliasAnalysis::ID = 0;
47static RegisterPass<ScalarEvolutionAliasAnalysis>
48X("scev-aa", "ScalarEvolution-based Alias Analysis", false, true);
49
50// Declare that we implement the AliasAnalysis interface
51static RegisterAnalysisGroup<AliasAnalysis> Y(X);
52
53FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
54  return new ScalarEvolutionAliasAnalysis();
55}
56
57void
58ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
59  AU.addRequiredTransitive<ScalarEvolution>();
60  AU.setPreservesAll();
61  AliasAnalysis::getAnalysisUsage(AU);
62}
63
64bool
65ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
66  InitializeAliasAnalysis(this);
67  SE = &getAnalysis<ScalarEvolution>();
68  return false;
69}
70
71/// GetBaseValue - Given an expression, try to find a
72/// base value. Return null is none was found.
73Value *
74ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
75  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
76    // In an addrec, assume that the base will be in the start, rather
77    // than the step.
78    return GetBaseValue(AR->getStart());
79  } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
80    // If there's a pointer operand, it'll be sorted at the end of the list.
81    const SCEV *Last = A->getOperand(A->getNumOperands()-1);
82    if (isa<PointerType>(Last->getType()))
83      return GetBaseValue(Last);
84  } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
85    // This is a leaf node.
86    return U->getValue();
87  }
88  // No Identified object found.
89  return 0;
90}
91
92AliasAnalysis::AliasResult
93ScalarEvolutionAliasAnalysis::alias(const Value *A, unsigned ASize,
94                                    const Value *B, unsigned BSize) {
95  // This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
96  const SCEV *AS = SE->getSCEV(const_cast<Value *>(A));
97  const SCEV *BS = SE->getSCEV(const_cast<Value *>(B));
98
99  // If they evaluate to the same expression, it's a MustAlias.
100  if (AS == BS) return MustAlias;
101
102  // If something is known about the difference between the two addresses,
103  // see if it's enough to prove a NoAlias.
104  if (SE->getEffectiveSCEVType(AS->getType()) ==
105      SE->getEffectiveSCEVType(BS->getType())) {
106    unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
107    APInt AI(BitWidth, ASize);
108    const SCEV *BA = SE->getMinusSCEV(BS, AS);
109    if (AI.ule(SE->getUnsignedRange(BA).getUnsignedMin())) {
110      APInt BI(BitWidth, BSize);
111      const SCEV *AB = SE->getMinusSCEV(AS, BS);
112      if (BI.ule(SE->getUnsignedRange(AB).getUnsignedMin()))
113        return NoAlias;
114    }
115  }
116
117  // If ScalarEvolution can find an underlying object, form a new query.
118  // The correctness of this depends on ScalarEvolution not recognizing
119  // inttoptr and ptrtoint operators.
120  Value *AO = GetBaseValue(AS);
121  Value *BO = GetBaseValue(BS);
122  if ((AO && AO != A) || (BO && BO != B))
123    if (alias(AO ? AO : A, AO ? ~0u : ASize,
124              BO ? BO : B, BO ? ~0u : BSize) == NoAlias)
125      return NoAlias;
126
127  // Forward the query to the next analysis.
128  return AliasAnalysis::alias(A, ASize, B, BSize);
129}
130