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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/AliasAnalysis.h"
23 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
24 #include "llvm/Analysis/Passes.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