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 INITIALIZE_AG_PASS(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
62 "ScalarEvolution-based Alias Analysis", false, true, false);
64 FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
65 return new ScalarEvolutionAliasAnalysis();
69 ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
70 AU.addRequiredTransitive<ScalarEvolution>();
72 AliasAnalysis::getAnalysisUsage(AU);
76 ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
77 InitializeAliasAnalysis(this);
78 SE = &getAnalysis<ScalarEvolution>();
82 /// GetBaseValue - Given an expression, try to find a
83 /// base value. Return null is none was found.
85 ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
86 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
87 // In an addrec, assume that the base will be in the start, rather
89 return GetBaseValue(AR->getStart());
90 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
91 // If there's a pointer operand, it'll be sorted at the end of the list.
92 const SCEV *Last = A->getOperand(A->getNumOperands()-1);
93 if (Last->getType()->isPointerTy())
94 return GetBaseValue(Last);
95 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
96 // This is a leaf node.
99 // No Identified object found.
103 AliasAnalysis::AliasResult
104 ScalarEvolutionAliasAnalysis::alias(const Value *A, unsigned ASize,
105 const Value *B, unsigned BSize) {
106 // If either of the memory references is empty, it doesn't matter what the
107 // pointer values are. This allows the code below to ignore this special
109 if (ASize == 0 || BSize == 0)
112 // This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
113 const SCEV *AS = SE->getSCEV(const_cast<Value *>(A));
114 const SCEV *BS = SE->getSCEV(const_cast<Value *>(B));
116 // If they evaluate to the same expression, it's a MustAlias.
117 if (AS == BS) return MustAlias;
119 // If something is known about the difference between the two addresses,
120 // see if it's enough to prove a NoAlias.
121 if (SE->getEffectiveSCEVType(AS->getType()) ==
122 SE->getEffectiveSCEVType(BS->getType())) {
123 unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
124 APInt ASizeInt(BitWidth, ASize);
125 APInt BSizeInt(BitWidth, BSize);
127 // Compute the difference between the two pointers.
128 const SCEV *BA = SE->getMinusSCEV(BS, AS);
130 // Test whether the difference is known to be great enough that memory of
131 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
132 // are non-zero, which is special-cased above.
133 if (ASizeInt.ule(SE->getUnsignedRange(BA).getUnsignedMin()) &&
134 (-BSizeInt).uge(SE->getUnsignedRange(BA).getUnsignedMax()))
137 // Folding the subtraction while preserving range information can be tricky
138 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
139 // and try again to see if things fold better that way.
141 // Compute the difference between the two pointers.
142 const SCEV *AB = SE->getMinusSCEV(AS, BS);
144 // Test whether the difference is known to be great enough that memory of
145 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
146 // are non-zero, which is special-cased above.
147 if (BSizeInt.ule(SE->getUnsignedRange(AB).getUnsignedMin()) &&
148 (-ASizeInt).uge(SE->getUnsignedRange(AB).getUnsignedMax()))
152 // If ScalarEvolution can find an underlying object, form a new query.
153 // The correctness of this depends on ScalarEvolution not recognizing
154 // inttoptr and ptrtoint operators.
155 Value *AO = GetBaseValue(AS);
156 Value *BO = GetBaseValue(BS);
157 if ((AO && AO != A) || (BO && BO != B))
158 if (alias(AO ? AO : A, AO ? UnknownSize : ASize,
159 BO ? BO : B, BO ? UnknownSize : BSize) == NoAlias)
162 // Forward the query to the next analysis.
163 return AliasAnalysis::alias(A, ASize, B, BSize);