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 // 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.
16 //
17 // ScalarEvolution has a more complete understanding of pointer arithmetic
18 // than BasicAliasAnalysis' collection of ad-hoc analyses.
19 //
20 //===----------------------------------------------------------------------===//
21
22 #include "llvm/Analysis/Passes.h"
23 #include "llvm/Analysis/AliasAnalysis.h"
24 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
25 #include "llvm/IR/Module.h"
26 #include "llvm/Pass.h"
27 using namespace llvm;
28
29 namespace {
30 /// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
31 /// implementation that uses ScalarEvolution to answer queries.
32 class ScalarEvolutionAliasAnalysis : public FunctionPass,
33 public AliasAnalysis {
34 ScalarEvolution *SE;
35
36 public:
37 static char ID; // Class identification, replacement for typeinfo
ScalarEvolutionAliasAnalysis()38 ScalarEvolutionAliasAnalysis() : FunctionPass(ID), SE(nullptr) {
39 initializeScalarEvolutionAliasAnalysisPass(
40 *PassRegistry::getPassRegistry());
41 }
42
43 /// getAdjustedAnalysisPointer - This method is used when a pass implements
44 /// an analysis interface through multiple inheritance. If needed, it
45 /// should override this to adjust the this pointer as needed for the
46 /// specified pass info.
getAdjustedAnalysisPointer(AnalysisID PI)47 void *getAdjustedAnalysisPointer(AnalysisID PI) override {
48 if (PI == &AliasAnalysis::ID)
49 return (AliasAnalysis*)this;
50 return this;
51 }
52
53 private:
54 void getAnalysisUsage(AnalysisUsage &AU) const override;
55 bool runOnFunction(Function &F) override;
56 AliasResult alias(const Location &LocA, const Location &LocB) override;
57
58 Value *GetBaseValue(const SCEV *S);
59 };
60 } // End of anonymous namespace
61
62 // Register this pass...
63 char ScalarEvolutionAliasAnalysis::ID = 0;
64 INITIALIZE_AG_PASS_BEGIN(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
65 "ScalarEvolution-based Alias Analysis", false, true, false)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)66 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
67 INITIALIZE_AG_PASS_END(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
68 "ScalarEvolution-based Alias Analysis", false, true, false)
69
70 FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
71 return new ScalarEvolutionAliasAnalysis();
72 }
73
74 void
getAnalysisUsage(AnalysisUsage & AU) const75 ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
76 AU.addRequiredTransitive<ScalarEvolution>();
77 AU.setPreservesAll();
78 AliasAnalysis::getAnalysisUsage(AU);
79 }
80
81 bool
runOnFunction(Function & F)82 ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
83 InitializeAliasAnalysis(this, &F.getParent()->getDataLayout());
84 SE = &getAnalysis<ScalarEvolution>();
85 return false;
86 }
87
88 /// GetBaseValue - Given an expression, try to find a
89 /// base value. Return null is none was found.
90 Value *
GetBaseValue(const SCEV * S)91 ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
92 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
93 // In an addrec, assume that the base will be in the start, rather
94 // than the step.
95 return GetBaseValue(AR->getStart());
96 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
97 // If there's a pointer operand, it'll be sorted at the end of the list.
98 const SCEV *Last = A->getOperand(A->getNumOperands()-1);
99 if (Last->getType()->isPointerTy())
100 return GetBaseValue(Last);
101 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
102 // This is a leaf node.
103 return U->getValue();
104 }
105 // No Identified object found.
106 return nullptr;
107 }
108
109 AliasAnalysis::AliasResult
alias(const Location & LocA,const Location & LocB)110 ScalarEvolutionAliasAnalysis::alias(const Location &LocA,
111 const Location &LocB) {
112 // If either of the memory references is empty, it doesn't matter what the
113 // pointer values are. This allows the code below to ignore this special
114 // case.
115 if (LocA.Size == 0 || LocB.Size == 0)
116 return NoAlias;
117
118 // This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
119 const SCEV *AS = SE->getSCEV(const_cast<Value *>(LocA.Ptr));
120 const SCEV *BS = SE->getSCEV(const_cast<Value *>(LocB.Ptr));
121
122 // If they evaluate to the same expression, it's a MustAlias.
123 if (AS == BS) return MustAlias;
124
125 // If something is known about the difference between the two addresses,
126 // see if it's enough to prove a NoAlias.
127 if (SE->getEffectiveSCEVType(AS->getType()) ==
128 SE->getEffectiveSCEVType(BS->getType())) {
129 unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
130 APInt ASizeInt(BitWidth, LocA.Size);
131 APInt BSizeInt(BitWidth, LocB.Size);
132
133 // Compute the difference between the two pointers.
134 const SCEV *BA = SE->getMinusSCEV(BS, AS);
135
136 // Test whether the difference is known to be great enough that memory of
137 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
138 // are non-zero, which is special-cased above.
139 if (ASizeInt.ule(SE->getUnsignedRange(BA).getUnsignedMin()) &&
140 (-BSizeInt).uge(SE->getUnsignedRange(BA).getUnsignedMax()))
141 return NoAlias;
142
143 // Folding the subtraction while preserving range information can be tricky
144 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
145 // and try again to see if things fold better that way.
146
147 // Compute the difference between the two pointers.
148 const SCEV *AB = SE->getMinusSCEV(AS, BS);
149
150 // Test whether the difference is known to be great enough that memory of
151 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
152 // are non-zero, which is special-cased above.
153 if (BSizeInt.ule(SE->getUnsignedRange(AB).getUnsignedMin()) &&
154 (-ASizeInt).uge(SE->getUnsignedRange(AB).getUnsignedMax()))
155 return NoAlias;
156 }
157
158 // If ScalarEvolution can find an underlying object, form a new query.
159 // The correctness of this depends on ScalarEvolution not recognizing
160 // inttoptr and ptrtoint operators.
161 Value *AO = GetBaseValue(AS);
162 Value *BO = GetBaseValue(BS);
163 if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
164 if (alias(Location(AO ? AO : LocA.Ptr,
165 AO ? +UnknownSize : LocA.Size,
166 AO ? AAMDNodes() : LocA.AATags),
167 Location(BO ? BO : LocB.Ptr,
168 BO ? +UnknownSize : LocB.Size,
169 BO ? AAMDNodes() : LocB.AATags)) == NoAlias)
170 return NoAlias;
171
172 // Forward the query to the next analysis.
173 return AliasAnalysis::alias(LocA, LocB);
174 }
175