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/InitializePasses.h"
23using namespace llvm;
24
25AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,
26                                const MemoryLocation &LocB, AAQueryInfo &AAQI) {
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.isZero() || LocB.Size.isZero())
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.hasValue()
47                                 ? LocA.Size.getValue()
48                                 : MemoryLocation::UnknownSize);
49    APInt BSizeInt(BitWidth, LocB.Size.hasValue()
50                                 ? LocB.Size.getValue()
51                                 : MemoryLocation::UnknownSize);
52
53    // Compute the difference between the two pointers.
54    const SCEV *BA = SE.getMinusSCEV(BS, AS);
55
56    // Test whether the difference is known to be great enough that memory of
57    // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
58    // are non-zero, which is special-cased above.
59    if (ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&
60        (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))
61      return NoAlias;
62
63    // Folding the subtraction while preserving range information can be tricky
64    // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
65    // and try again to see if things fold better that way.
66
67    // Compute the difference between the two pointers.
68    const SCEV *AB = SE.getMinusSCEV(AS, BS);
69
70    // Test whether the difference is known to be great enough that memory of
71    // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
72    // are non-zero, which is special-cased above.
73    if (BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&
74        (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))
75      return NoAlias;
76  }
77
78  // If ScalarEvolution can find an underlying object, form a new query.
79  // The correctness of this depends on ScalarEvolution not recognizing
80  // inttoptr and ptrtoint operators.
81  Value *AO = GetBaseValue(AS);
82  Value *BO = GetBaseValue(BS);
83  if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
84    if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
85                             AO ? LocationSize::unknown() : LocA.Size,
86                             AO ? AAMDNodes() : LocA.AATags),
87              MemoryLocation(BO ? BO : LocB.Ptr,
88                             BO ? LocationSize::unknown() : LocB.Size,
89                             BO ? AAMDNodes() : LocB.AATags),
90              AAQI) == NoAlias)
91      return NoAlias;
92
93  // Forward the query to the next analysis.
94  return AAResultBase::alias(LocA, LocB, AAQI);
95}
96
97/// Given an expression, try to find a base value.
98///
99/// Returns null if none was found.
100Value *SCEVAAResult::GetBaseValue(const SCEV *S) {
101  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
102    // In an addrec, assume that the base will be in the start, rather
103    // than the step.
104    return GetBaseValue(AR->getStart());
105  } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
106    // If there's a pointer operand, it'll be sorted at the end of the list.
107    const SCEV *Last = A->getOperand(A->getNumOperands() - 1);
108    if (Last->getType()->isPointerTy())
109      return GetBaseValue(Last);
110  } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
111    // This is a leaf node.
112    return U->getValue();
113  }
114  // No Identified object found.
115  return nullptr;
116}
117
118AnalysisKey SCEVAA::Key;
119
120SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) {
121  return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F));
122}
123
124char SCEVAAWrapperPass::ID = 0;
125INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",
126                      "ScalarEvolution-based Alias Analysis", false, true)
127INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
128INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",
129                    "ScalarEvolution-based Alias Analysis", false, true)
130
131FunctionPass *llvm::createSCEVAAWrapperPass() {
132  return new SCEVAAWrapperPass();
133}
134
135SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {
136  initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry());
137}
138
139bool SCEVAAWrapperPass::runOnFunction(Function &F) {
140  Result.reset(
141      new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
142  return false;
143}
144
145void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
146  AU.setPreservesAll();
147  AU.addRequired<ScalarEvolutionWrapperPass>();
148}
149