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/Pass.h"
26 using namespace llvm;
27
28 namespace {
29 /// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
30 /// implementation that uses ScalarEvolution to answer queries.
31 class ScalarEvolutionAliasAnalysis : public FunctionPass,
32 public AliasAnalysis {
33 ScalarEvolution *SE;
34
35 public:
36 static char ID; // Class identification, replacement for typeinfo
ScalarEvolutionAliasAnalysis()37 ScalarEvolutionAliasAnalysis() : FunctionPass(ID), SE(0) {
38 initializeScalarEvolutionAliasAnalysisPass(
39 *PassRegistry::getPassRegistry());
40 }
41
42 /// getAdjustedAnalysisPointer - This method is used when a pass implements
43 /// an analysis interface through multiple inheritance. If needed, it
44 /// should override this to adjust the this pointer as needed for the
45 /// specified pass info.
getAdjustedAnalysisPointer(AnalysisID PI)46 virtual void *getAdjustedAnalysisPointer(AnalysisID PI) {
47 if (PI == &AliasAnalysis::ID)
48 return (AliasAnalysis*)this;
49 return this;
50 }
51
52 private:
53 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
54 virtual bool runOnFunction(Function &F);
55 virtual AliasResult alias(const Location &LocA, const Location &LocB);
56
57 Value *GetBaseValue(const SCEV *S);
58 };
59 } // End of anonymous namespace
60
61 // Register this pass...
62 char ScalarEvolutionAliasAnalysis::ID = 0;
63 INITIALIZE_AG_PASS_BEGIN(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
64 "ScalarEvolution-based Alias Analysis", false, true, false)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)65 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
66 INITIALIZE_AG_PASS_END(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
67 "ScalarEvolution-based Alias Analysis", false, true, false)
68
69 FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
70 return new ScalarEvolutionAliasAnalysis();
71 }
72
73 void
getAnalysisUsage(AnalysisUsage & AU) const74 ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
75 AU.addRequiredTransitive<ScalarEvolution>();
76 AU.setPreservesAll();
77 AliasAnalysis::getAnalysisUsage(AU);
78 }
79
80 bool
runOnFunction(Function & F)81 ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
82 InitializeAliasAnalysis(this);
83 SE = &getAnalysis<ScalarEvolution>();
84 return false;
85 }
86
87 /// GetBaseValue - Given an expression, try to find a
88 /// base value. Return null is none was found.
89 Value *
GetBaseValue(const SCEV * S)90 ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
91 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
92 // In an addrec, assume that the base will be in the start, rather
93 // than the step.
94 return GetBaseValue(AR->getStart());
95 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
96 // If there's a pointer operand, it'll be sorted at the end of the list.
97 const SCEV *Last = A->getOperand(A->getNumOperands()-1);
98 if (Last->getType()->isPointerTy())
99 return GetBaseValue(Last);
100 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
101 // This is a leaf node.
102 return U->getValue();
103 }
104 // No Identified object found.
105 return 0;
106 }
107
108 AliasAnalysis::AliasResult
alias(const Location & LocA,const Location & LocB)109 ScalarEvolutionAliasAnalysis::alias(const Location &LocA,
110 const Location &LocB) {
111 // If either of the memory references is empty, it doesn't matter what the
112 // pointer values are. This allows the code below to ignore this special
113 // case.
114 if (LocA.Size == 0 || LocB.Size == 0)
115 return NoAlias;
116
117 // This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
118 const SCEV *AS = SE->getSCEV(const_cast<Value *>(LocA.Ptr));
119 const SCEV *BS = SE->getSCEV(const_cast<Value *>(LocB.Ptr));
120
121 // If they evaluate to the same expression, it's a MustAlias.
122 if (AS == BS) return MustAlias;
123
124 // If something is known about the difference between the two addresses,
125 // see if it's enough to prove a NoAlias.
126 if (SE->getEffectiveSCEVType(AS->getType()) ==
127 SE->getEffectiveSCEVType(BS->getType())) {
128 unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
129 APInt ASizeInt(BitWidth, LocA.Size);
130 APInt BSizeInt(BitWidth, LocB.Size);
131
132 // Compute the difference between the two pointers.
133 const SCEV *BA = SE->getMinusSCEV(BS, AS);
134
135 // Test whether the difference is known to be great enough that memory of
136 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
137 // are non-zero, which is special-cased above.
138 if (ASizeInt.ule(SE->getUnsignedRange(BA).getUnsignedMin()) &&
139 (-BSizeInt).uge(SE->getUnsignedRange(BA).getUnsignedMax()))
140 return NoAlias;
141
142 // Folding the subtraction while preserving range information can be tricky
143 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
144 // and try again to see if things fold better that way.
145
146 // Compute the difference between the two pointers.
147 const SCEV *AB = SE->getMinusSCEV(AS, BS);
148
149 // Test whether the difference is known to be great enough that memory of
150 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
151 // are non-zero, which is special-cased above.
152 if (BSizeInt.ule(SE->getUnsignedRange(AB).getUnsignedMin()) &&
153 (-ASizeInt).uge(SE->getUnsignedRange(AB).getUnsignedMax()))
154 return NoAlias;
155 }
156
157 // If ScalarEvolution can find an underlying object, form a new query.
158 // The correctness of this depends on ScalarEvolution not recognizing
159 // inttoptr and ptrtoint operators.
160 Value *AO = GetBaseValue(AS);
161 Value *BO = GetBaseValue(BS);
162 if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
163 if (alias(Location(AO ? AO : LocA.Ptr,
164 AO ? +UnknownSize : LocA.Size,
165 AO ? 0 : LocA.TBAATag),
166 Location(BO ? BO : LocB.Ptr,
167 BO ? +UnknownSize : LocB.Size,
168 BO ? 0 : LocB.TBAATag)) == NoAlias)
169 return NoAlias;
170
171 // Forward the query to the next analysis.
172 return AliasAnalysis::alias(LocA, LocB);
173 }
174