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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/ScalarEvolutionAliasAnalysis.h"
23 using namespace llvm;
24 
alias(const MemoryLocation & LocA,const MemoryLocation & LocB)25 AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,
26                                 const MemoryLocation &LocB) {
27   // If either of the memory references is empty, it doesn't matter what the
28   // pointer values are. This allows the code below to ignore this special
29   // case.
30   if (LocA.Size == 0 || LocB.Size == 0)
31     return NoAlias;
32 
33   // This is SCEVAAResult. Get the SCEVs!
34   const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr));
35   const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr));
36 
37   // If they evaluate to the same expression, it's a MustAlias.
38   if (AS == BS)
39     return MustAlias;
40 
41   // If something is known about the difference between the two addresses,
42   // see if it's enough to prove a NoAlias.
43   if (SE.getEffectiveSCEVType(AS->getType()) ==
44       SE.getEffectiveSCEVType(BS->getType())) {
45     unsigned BitWidth = SE.getTypeSizeInBits(AS->getType());
46     APInt ASizeInt(BitWidth, LocA.Size);
47     APInt BSizeInt(BitWidth, LocB.Size);
48 
49     // Compute the difference between the two pointers.
50     const SCEV *BA = SE.getMinusSCEV(BS, AS);
51 
52     // Test whether the difference is known to be great enough that memory of
53     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
54     // are non-zero, which is special-cased above.
55     if (ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&
56         (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))
57       return NoAlias;
58 
59     // Folding the subtraction while preserving range information can be tricky
60     // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
61     // and try again to see if things fold better that way.
62 
63     // Compute the difference between the two pointers.
64     const SCEV *AB = SE.getMinusSCEV(AS, BS);
65 
66     // Test whether the difference is known to be great enough that memory of
67     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
68     // are non-zero, which is special-cased above.
69     if (BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&
70         (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))
71       return NoAlias;
72   }
73 
74   // If ScalarEvolution can find an underlying object, form a new query.
75   // The correctness of this depends on ScalarEvolution not recognizing
76   // inttoptr and ptrtoint operators.
77   Value *AO = GetBaseValue(AS);
78   Value *BO = GetBaseValue(BS);
79   if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
80     if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
81                              AO ? +MemoryLocation::UnknownSize : LocA.Size,
82                              AO ? AAMDNodes() : LocA.AATags),
83               MemoryLocation(BO ? BO : LocB.Ptr,
84                              BO ? +MemoryLocation::UnknownSize : LocB.Size,
85                              BO ? AAMDNodes() : LocB.AATags)) == NoAlias)
86       return NoAlias;
87 
88   // Forward the query to the next analysis.
89   return AAResultBase::alias(LocA, LocB);
90 }
91 
92 /// Given an expression, try to find a base value.
93 ///
94 /// Returns null if none was found.
GetBaseValue(const SCEV * S)95 Value *SCEVAAResult::GetBaseValue(const SCEV *S) {
96   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
97     // In an addrec, assume that the base will be in the start, rather
98     // than the step.
99     return GetBaseValue(AR->getStart());
100   } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
101     // If there's a pointer operand, it'll be sorted at the end of the list.
102     const SCEV *Last = A->getOperand(A->getNumOperands() - 1);
103     if (Last->getType()->isPointerTy())
104       return GetBaseValue(Last);
105   } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
106     // This is a leaf node.
107     return U->getValue();
108   }
109   // No Identified object found.
110   return nullptr;
111 }
112 
113 char SCEVAA::PassID;
114 
run(Function & F,AnalysisManager<Function> & AM)115 SCEVAAResult SCEVAA::run(Function &F, AnalysisManager<Function> &AM) {
116   return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F));
117 }
118 
119 char SCEVAAWrapperPass::ID = 0;
120 INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",
121                       "ScalarEvolution-based Alias Analysis", false, true)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)122 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
123 INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",
124                     "ScalarEvolution-based Alias Analysis", false, true)
125 
126 FunctionPass *llvm::createSCEVAAWrapperPass() {
127   return new SCEVAAWrapperPass();
128 }
129 
SCEVAAWrapperPass()130 SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {
131   initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry());
132 }
133 
runOnFunction(Function & F)134 bool SCEVAAWrapperPass::runOnFunction(Function &F) {
135   Result.reset(
136       new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
137   return false;
138 }
139 
getAnalysisUsage(AnalysisUsage & AU) const140 void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
141   AU.setPreservesAll();
142   AU.addRequired<ScalarEvolutionWrapperPass>();
143 }
144