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
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.
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)
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
75 ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
76   AU.addRequiredTransitive<ScalarEvolution>();
77   AU.setPreservesAll();
78   AliasAnalysis::getAnalysisUsage(AU);
79 }
80 
81 bool
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 *
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
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