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/Analysis/ScalarEvolutionExpressions.h"
24#include "llvm/InitializePasses.h"
25using namespace llvm;
26
27static bool canComputePointerDiff(ScalarEvolution &SE,
28 const SCEV *A, const SCEV *B) {
29 if (SE.getEffectiveSCEVType(Ty: A->getType()) !=
30 SE.getEffectiveSCEVType(Ty: B->getType()))
31 return false;
32
33 return SE.instructionCouldExistWithOperands(A, B);
34}
35
36AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,
37 const MemoryLocation &LocB, AAQueryInfo &AAQI,
38 const Instruction *) {
39 // If either of the memory references is empty, it doesn't matter what the
40 // pointer values are. This allows the code below to ignore this special
41 // case.
42 if (LocA.Size.isZero() || LocB.Size.isZero())
43 return AliasResult::NoAlias;
44
45 // This is SCEVAAResult. Get the SCEVs!
46 const SCEV *AS = SE.getSCEV(V: const_cast<Value *>(LocA.Ptr));
47 const SCEV *BS = SE.getSCEV(V: const_cast<Value *>(LocB.Ptr));
48
49 // If they evaluate to the same expression, it's a MustAlias.
50 if (AS == BS)
51 return AliasResult::MustAlias;
52
53 // If something is known about the difference between the two addresses,
54 // see if it's enough to prove a NoAlias.
55 if (canComputePointerDiff(SE, A: AS, B: BS)) {
56 unsigned BitWidth = SE.getTypeSizeInBits(Ty: AS->getType());
57 APInt ASizeInt(BitWidth, LocA.Size.hasValue()
58 ? static_cast<uint64_t>(LocA.Size.getValue())
59 : MemoryLocation::UnknownSize);
60 APInt BSizeInt(BitWidth, LocB.Size.hasValue()
61 ? static_cast<uint64_t>(LocB.Size.getValue())
62 : MemoryLocation::UnknownSize);
63
64 // Firstly, try to convert the two pointers into ptrtoint expressions to
65 // handle two pointers with different pointer bases.
66 // Either both pointers are used with ptrtoint or neither, so we can't end
67 // up with a ptr + int mix.
68 const SCEV *AInt =
69 SE.getPtrToIntExpr(Op: AS, Ty: SE.getEffectiveSCEVType(Ty: AS->getType()));
70 const SCEV *BInt =
71 SE.getPtrToIntExpr(Op: BS, Ty: SE.getEffectiveSCEVType(Ty: BS->getType()));
72 if (!isa<SCEVCouldNotCompute>(Val: AInt) && !isa<SCEVCouldNotCompute>(Val: BInt)) {
73 AS = AInt;
74 BS = BInt;
75 }
76
77 // Compute the difference between the two pointers.
78 const SCEV *BA = SE.getMinusSCEV(LHS: BS, RHS: AS);
79
80 // Test whether the difference is known to be great enough that memory of
81 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
82 // are non-zero, which is special-cased above.
83 if (!isa<SCEVCouldNotCompute>(Val: BA) &&
84 ASizeInt.ule(RHS: SE.getUnsignedRange(S: BA).getUnsignedMin()) &&
85 (-BSizeInt).uge(RHS: SE.getUnsignedRange(S: BA).getUnsignedMax()))
86 return AliasResult::NoAlias;
87
88 // Folding the subtraction while preserving range information can be tricky
89 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
90 // and try again to see if things fold better that way.
91
92 // Compute the difference between the two pointers.
93 const SCEV *AB = SE.getMinusSCEV(LHS: AS, RHS: BS);
94
95 // Test whether the difference is known to be great enough that memory of
96 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
97 // are non-zero, which is special-cased above.
98 if (!isa<SCEVCouldNotCompute>(Val: AB) &&
99 BSizeInt.ule(RHS: SE.getUnsignedRange(S: AB).getUnsignedMin()) &&
100 (-ASizeInt).uge(RHS: SE.getUnsignedRange(S: AB).getUnsignedMax()))
101 return AliasResult::NoAlias;
102 }
103
104 // If ScalarEvolution can find an underlying object, form a new query.
105 // The correctness of this depends on ScalarEvolution not recognizing
106 // inttoptr and ptrtoint operators.
107 Value *AO = GetBaseValue(S: AS);
108 Value *BO = GetBaseValue(S: BS);
109 if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
110 if (alias(LocA: MemoryLocation(AO ? AO : LocA.Ptr,
111 AO ? LocationSize::beforeOrAfterPointer()
112 : LocA.Size,
113 AO ? AAMDNodes() : LocA.AATags),
114 LocB: MemoryLocation(BO ? BO : LocB.Ptr,
115 BO ? LocationSize::beforeOrAfterPointer()
116 : LocB.Size,
117 BO ? AAMDNodes() : LocB.AATags),
118 AAQI, nullptr) == AliasResult::NoAlias)
119 return AliasResult::NoAlias;
120
121 return AliasResult::MayAlias;
122}
123
124/// Given an expression, try to find a base value.
125///
126/// Returns null if none was found.
127Value *SCEVAAResult::GetBaseValue(const SCEV *S) {
128 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(Val: S)) {
129 // In an addrec, assume that the base will be in the start, rather
130 // than the step.
131 return GetBaseValue(S: AR->getStart());
132 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(Val: S)) {
133 // If there's a pointer operand, it'll be sorted at the end of the list.
134 const SCEV *Last = A->getOperand(i: A->getNumOperands() - 1);
135 if (Last->getType()->isPointerTy())
136 return GetBaseValue(S: Last);
137 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(Val: S)) {
138 // This is a leaf node.
139 return U->getValue();
140 }
141 // No Identified object found.
142 return nullptr;
143}
144
145bool SCEVAAResult::invalidate(Function &Fn, const PreservedAnalyses &PA,
146 FunctionAnalysisManager::Invalidator &Inv) {
147 // We don't care if this analysis itself is preserved, it has no state. But
148 // we need to check that the analyses it depends on have been.
149 return Inv.invalidate<ScalarEvolutionAnalysis>(IR&: Fn, PA);
150}
151
152AnalysisKey SCEVAA::Key;
153
154SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) {
155 return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(IR&: F));
156}
157
158char SCEVAAWrapperPass::ID = 0;
159INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",
160 "ScalarEvolution-based Alias Analysis", false, true)
161INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
162INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",
163 "ScalarEvolution-based Alias Analysis", false, true)
164
165FunctionPass *llvm::createSCEVAAWrapperPass() {
166 return new SCEVAAWrapperPass();
167}
168
169SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {}
170
171bool SCEVAAWrapperPass::runOnFunction(Function &F) {
172 Result.reset(
173 p: new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
174 return false;
175}
176
177void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
178 AU.setPreservesAll();
179 AU.addRequired<ScalarEvolutionWrapperPass>();
180}
181