1 //===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
10 // simple alias analysis implemented in terms of ScalarEvolution queries.
11 //
12 // This differs from traditional loop dependence analysis in that it tests
13 // for dependencies within a single iteration of a loop, rather than
14 // dependencies between different iterations.
15 //
16 // ScalarEvolution has a more complete understanding of pointer arithmetic
17 // than BasicAliasAnalysis' collection of ad-hoc analyses.
18 //
19 //===----------------------------------------------------------------------===//
20
21 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
22 #include "llvm/Analysis/ScalarEvolution.h"
23 #include "llvm/InitializePasses.h"
24 using namespace llvm;
25
alias(const MemoryLocation & LocA,const MemoryLocation & LocB,AAQueryInfo & AAQI)26 AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,
27 const MemoryLocation &LocB, AAQueryInfo &AAQI) {
28 // If either of the memory references is empty, it doesn't matter what the
29 // pointer values are. This allows the code below to ignore this special
30 // case.
31 if (LocA.Size.isZero() || LocB.Size.isZero())
32 return AliasResult::NoAlias;
33
34 // This is SCEVAAResult. Get the SCEVs!
35 const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr));
36 const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr));
37
38 // If they evaluate to the same expression, it's a MustAlias.
39 if (AS == BS)
40 return AliasResult::MustAlias;
41
42 // If something is known about the difference between the two addresses,
43 // see if it's enough to prove a NoAlias.
44 if (SE.getEffectiveSCEVType(AS->getType()) ==
45 SE.getEffectiveSCEVType(BS->getType())) {
46 unsigned BitWidth = SE.getTypeSizeInBits(AS->getType());
47 APInt ASizeInt(BitWidth, LocA.Size.hasValue()
48 ? LocA.Size.getValue()
49 : MemoryLocation::UnknownSize);
50 APInt BSizeInt(BitWidth, LocB.Size.hasValue()
51 ? LocB.Size.getValue()
52 : MemoryLocation::UnknownSize);
53
54 // Compute the difference between the two pointers.
55 const SCEV *BA = SE.getMinusSCEV(BS, AS);
56
57 // Test whether the difference is known to be great enough that memory of
58 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
59 // are non-zero, which is special-cased above.
60 if (ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&
61 (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))
62 return AliasResult::NoAlias;
63
64 // Folding the subtraction while preserving range information can be tricky
65 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
66 // and try again to see if things fold better that way.
67
68 // Compute the difference between the two pointers.
69 const SCEV *AB = SE.getMinusSCEV(AS, BS);
70
71 // Test whether the difference is known to be great enough that memory of
72 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
73 // are non-zero, which is special-cased above.
74 if (BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&
75 (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))
76 return AliasResult::NoAlias;
77 }
78
79 // If ScalarEvolution can find an underlying object, form a new query.
80 // The correctness of this depends on ScalarEvolution not recognizing
81 // inttoptr and ptrtoint operators.
82 Value *AO = GetBaseValue(AS);
83 Value *BO = GetBaseValue(BS);
84 if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
85 if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
86 AO ? LocationSize::beforeOrAfterPointer()
87 : LocA.Size,
88 AO ? AAMDNodes() : LocA.AATags),
89 MemoryLocation(BO ? BO : LocB.Ptr,
90 BO ? LocationSize::beforeOrAfterPointer()
91 : LocB.Size,
92 BO ? AAMDNodes() : LocB.AATags),
93 AAQI) == AliasResult::NoAlias)
94 return AliasResult::NoAlias;
95
96 // Forward the query to the next analysis.
97 return AAResultBase::alias(LocA, LocB, AAQI);
98 }
99
100 /// Given an expression, try to find a base value.
101 ///
102 /// Returns null if none was found.
GetBaseValue(const SCEV * S)103 Value *SCEVAAResult::GetBaseValue(const SCEV *S) {
104 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
105 // In an addrec, assume that the base will be in the start, rather
106 // than the step.
107 return GetBaseValue(AR->getStart());
108 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
109 // If there's a pointer operand, it'll be sorted at the end of the list.
110 const SCEV *Last = A->getOperand(A->getNumOperands() - 1);
111 if (Last->getType()->isPointerTy())
112 return GetBaseValue(Last);
113 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
114 // This is a leaf node.
115 return U->getValue();
116 }
117 // No Identified object found.
118 return nullptr;
119 }
120
invalidate(Function & Fn,const PreservedAnalyses & PA,FunctionAnalysisManager::Invalidator & Inv)121 bool SCEVAAResult::invalidate(Function &Fn, const PreservedAnalyses &PA,
122 FunctionAnalysisManager::Invalidator &Inv) {
123 // We don't care if this analysis itself is preserved, it has no state. But
124 // we need to check that the analyses it depends on have been.
125 return Inv.invalidate<ScalarEvolutionAnalysis>(Fn, PA);
126 }
127
128 AnalysisKey SCEVAA::Key;
129
run(Function & F,FunctionAnalysisManager & AM)130 SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) {
131 return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F));
132 }
133
134 char SCEVAAWrapperPass::ID = 0;
135 INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",
136 "ScalarEvolution-based Alias Analysis", false, true)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)137 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
138 INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",
139 "ScalarEvolution-based Alias Analysis", false, true)
140
141 FunctionPass *llvm::createSCEVAAWrapperPass() {
142 return new SCEVAAWrapperPass();
143 }
144
SCEVAAWrapperPass()145 SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {
146 initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry());
147 }
148
runOnFunction(Function & F)149 bool SCEVAAWrapperPass::runOnFunction(Function &F) {
150 Result.reset(
151 new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
152 return false;
153 }
154
getAnalysisUsage(AnalysisUsage & AU) const155 void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
156 AU.setPreservesAll();
157 AU.addRequired<ScalarEvolutionWrapperPass>();
158 }
159