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 }
134