1 //===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
11 // simple alias analysis implemented in terms of ScalarEvolution queries.
13 // ScalarEvolution has a more complete understanding of pointer arithmetic
14 // than BasicAliasAnalysis' collection of ad-hoc analyses.
16 //===----------------------------------------------------------------------===//
18 #include "llvm/Analysis/AliasAnalysis.h"
19 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
20 #include "llvm/Analysis/Passes.h"
21 #include "llvm/Pass.h"
25 /// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
26 /// implementation that uses ScalarEvolution to answer queries.
27 class ScalarEvolutionAliasAnalysis : public FunctionPass,
28 public AliasAnalysis {
32 static char ID; // Class identification, replacement for typeinfo
33 ScalarEvolutionAliasAnalysis() : FunctionPass(&ID), SE(0) {}
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);
41 Value *GetUnderlyingIdentifiedObject(const SCEV *S);
43 } // End of anonymous namespace
45 // Register this pass...
46 char ScalarEvolutionAliasAnalysis::ID = 0;
47 static RegisterPass<ScalarEvolutionAliasAnalysis>
48 X("scev-aa", "ScalarEvolution-based Alias Analysis", false, true);
50 // Declare that we implement the AliasAnalysis interface
51 static RegisterAnalysisGroup<AliasAnalysis> Y(X);
53 FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
54 return new ScalarEvolutionAliasAnalysis();
58 ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
59 AU.addRequiredTransitive<ScalarEvolution>();
61 AliasAnalysis::getAnalysisUsage(AU);
65 ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
66 InitializeAliasAnalysis(this);
67 SE = &getAnalysis<ScalarEvolution>();
71 /// GetUnderlyingIdentifiedObject - Given an expression, try to find an
72 /// "identified object" (see AliasAnalysis::isIdentifiedObject) base
73 /// value. Return null is none was found.
75 ScalarEvolutionAliasAnalysis::GetUnderlyingIdentifiedObject(const SCEV *S) {
76 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
77 // In an addrec, assume that the base will be in the start, rather
79 return GetUnderlyingIdentifiedObject(AR->getStart());
80 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
81 // If there's a pointer operand, it'll be sorted at the end of the list.
82 const SCEV *Last = A->getOperand(A->getNumOperands()-1);
83 if (isa<PointerType>(Last->getType()))
84 return GetUnderlyingIdentifiedObject(Last);
85 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
86 // Determine if we've found an Identified object.
87 Value *V = U->getValue();
88 if (isIdentifiedObject(V))
91 // No Identified object found.
95 AliasAnalysis::AliasResult
96 ScalarEvolutionAliasAnalysis::alias(const Value *A, unsigned ASize,
97 const Value *B, unsigned BSize) {
98 // This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
99 const SCEV *AS = SE->getSCEV(const_cast<Value *>(A));
100 const SCEV *BS = SE->getSCEV(const_cast<Value *>(B));
102 // If they evaluate to the same expression, it's a MustAlias.
103 if (AS == BS) return MustAlias;
105 // If something is known about the difference between the two addresses,
106 // see if it's enough to prove a NoAlias.
107 if (SE->getEffectiveSCEVType(AS->getType()) ==
108 SE->getEffectiveSCEVType(BS->getType())) {
109 unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
110 APInt AI(BitWidth, ASize);
111 const SCEV *BA = SE->getMinusSCEV(BS, AS);
112 if (AI.ule(SE->getUnsignedRange(BA).getUnsignedMin())) {
113 APInt BI(BitWidth, BSize);
114 const SCEV *AB = SE->getMinusSCEV(AS, BS);
115 if (BI.ule(SE->getUnsignedRange(AB).getUnsignedMin()))
120 // If ScalarEvolution can find an underlying object, form a new query.
121 // The correctness of this depends on ScalarEvolution not recognizing
122 // inttoptr and ptrtoint operators.
123 Value *AO = GetUnderlyingIdentifiedObject(AS);
124 Value *BO = GetUnderlyingIdentifiedObject(BS);
125 if ((AO && AO != A) || (BO && BO != B))
126 if (alias(AO ? AO : A, AO ? ~0u : ASize,
127 BO ? BO : B, BO ? ~0u : BSize) == NoAlias)
130 // Forward the query to the next analysis.
131 return AliasAnalysis::alias(A, ASize, B, BSize);