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