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 // This differs from traditional loop dependence analysis in that it tests
14 // for dependencies within a single iteration of a loop, rather than
15 // dependencies between different iterations.
17 // ScalarEvolution has a more complete understanding of pointer arithmetic
18 // than BasicAliasAnalysis' collection of ad-hoc analyses.
20 //===----------------------------------------------------------------------===//
22 #include "llvm/Analysis/AliasAnalysis.h"
23 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
24 #include "llvm/Analysis/Passes.h"
25 #include "llvm/Pass.h"
29 /// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
30 /// implementation that uses ScalarEvolution to answer queries.
31 class ScalarEvolutionAliasAnalysis : public FunctionPass,
32 public AliasAnalysis {
36 static char ID; // Class identification, replacement for typeinfo
37 ScalarEvolutionAliasAnalysis() : FunctionPass(&ID), SE(0) {}
39 /// getAdjustedAnalysisPointer - This method is used when a pass implements
40 /// an analysis interface through multiple inheritance. If needed, it
41 /// should override this to adjust the this pointer as needed for the
42 /// specified pass info.
43 virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
44 if (PI->isPassID(&AliasAnalysis::ID))
45 return (AliasAnalysis*)this;
50 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
51 virtual bool runOnFunction(Function &F);
52 virtual AliasResult alias(const Value *V1, unsigned V1Size,
53 const Value *V2, unsigned V2Size);
55 Value *GetBaseValue(const SCEV *S);
57 } // End of anonymous namespace
59 // Register this pass...
60 char ScalarEvolutionAliasAnalysis::ID = 0;
61 static RegisterPass<ScalarEvolutionAliasAnalysis>
62 X("scev-aa", "ScalarEvolution-based Alias Analysis", false, true);
64 // Declare that we implement the AliasAnalysis interface
65 static RegisterAnalysisGroup<AliasAnalysis> Y(X);
67 FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
68 return new ScalarEvolutionAliasAnalysis();
72 ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
73 AU.addRequiredTransitive<ScalarEvolution>();
75 AliasAnalysis::getAnalysisUsage(AU);
79 ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
80 InitializeAliasAnalysis(this);
81 SE = &getAnalysis<ScalarEvolution>();
85 /// GetBaseValue - Given an expression, try to find a
86 /// base value. Return null is none was found.
88 ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
89 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
90 // In an addrec, assume that the base will be in the start, rather
92 return GetBaseValue(AR->getStart());
93 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
94 // If there's a pointer operand, it'll be sorted at the end of the list.
95 const SCEV *Last = A->getOperand(A->getNumOperands()-1);
96 if (Last->getType()->isPointerTy())
97 return GetBaseValue(Last);
98 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
99 // This is a leaf node.
100 return U->getValue();
102 // No Identified object found.
106 AliasAnalysis::AliasResult
107 ScalarEvolutionAliasAnalysis::alias(const Value *A, unsigned ASize,
108 const Value *B, unsigned BSize) {
109 // If either of the memory references is empty, it doesn't matter what the
110 // pointer values are. This allows the code below to ignore this special
112 if (ASize == 0 || BSize == 0)
115 // This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
116 const SCEV *AS = SE->getSCEV(const_cast<Value *>(A));
117 const SCEV *BS = SE->getSCEV(const_cast<Value *>(B));
119 // If they evaluate to the same expression, it's a MustAlias.
120 if (AS == BS) return MustAlias;
122 // If something is known about the difference between the two addresses,
123 // see if it's enough to prove a NoAlias.
124 if (SE->getEffectiveSCEVType(AS->getType()) ==
125 SE->getEffectiveSCEVType(BS->getType())) {
126 unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
127 APInt ASizeInt(BitWidth, ASize);
128 APInt BSizeInt(BitWidth, BSize);
130 // Compute the difference between the two pointers.
131 const SCEV *BA = SE->getMinusSCEV(BS, AS);
133 // Test whether the difference is known to be great enough that memory of
134 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
135 // are non-zero, which is special-cased above.
136 if (ASizeInt.ule(SE->getUnsignedRange(BA).getUnsignedMin()) &&
137 (-BSizeInt).uge(SE->getUnsignedRange(BA).getUnsignedMax()))
140 // Folding the subtraction while preserving range information can be tricky
141 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
142 // and try again to see if things fold better that way.
144 // Compute the difference between the two pointers.
145 const SCEV *AB = SE->getMinusSCEV(AS, BS);
147 // Test whether the difference is known to be great enough that memory of
148 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
149 // are non-zero, which is special-cased above.
150 if (BSizeInt.ule(SE->getUnsignedRange(AB).getUnsignedMin()) &&
151 (-ASizeInt).uge(SE->getUnsignedRange(AB).getUnsignedMax()))
155 // If ScalarEvolution can find an underlying object, form a new query.
156 // The correctness of this depends on ScalarEvolution not recognizing
157 // inttoptr and ptrtoint operators.
158 Value *AO = GetBaseValue(AS);
159 Value *BO = GetBaseValue(BS);
160 if ((AO && AO != A) || (BO && BO != B))
161 if (alias(AO ? AO : A, AO ? ~0u : ASize,
162 BO ? BO : B, BO ? ~0u : BSize) == NoAlias)
165 // Forward the query to the next analysis.
166 return AliasAnalysis::alias(A, ASize, B, BSize);