ScopedNoAliasAA.cpp revision 360784 
1//===- ScopedNoAliasAA.cpp - Scoped No-Alias 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 ScopedNoAlias alias-analysis pass, which implements
10// metadata-based scoped no-alias support.
11//
12// Alias-analysis scopes are defined by an id (which can be a string or some
13// other metadata node), a domain node, and an optional descriptive string.
14// A domain is defined by an id (which can be a string or some other metadata
15// node), and an optional descriptive string.
16//
17// !dom0 =   metadata !{ metadata !"domain of foo()" }
18// !scope1 = metadata !{ metadata !scope1, metadata !dom0, metadata !"scope 1" }
19// !scope2 = metadata !{ metadata !scope2, metadata !dom0, metadata !"scope 2" }
20//
21// Loads and stores can be tagged with an alias-analysis scope, and also, with
22// a noalias tag for a specific scope:
23//
24// ... = load %ptr1, !alias.scope !{ !scope1 }
25// ... = load %ptr2, !alias.scope !{ !scope1, !scope2 }, !noalias !{ !scope1 }
26//
27// When evaluating an aliasing query, if one of the instructions is associated
28// has a set of noalias scopes in some domain that is a superset of the alias
29// scopes in that domain of some other instruction, then the two memory
30// accesses are assumed not to alias.
31//
32//===----------------------------------------------------------------------===//
33
34#include "llvm/Analysis/ScopedNoAliasAA.h"
35#include "llvm/ADT/SmallPtrSet.h"
36#include "llvm/Analysis/MemoryLocation.h"
37#include "llvm/IR/Instruction.h"
38#include "llvm/IR/LLVMContext.h"
39#include "llvm/IR/Metadata.h"
40#include "llvm/InitializePasses.h"
41#include "llvm/Pass.h"
42#include "llvm/Support/Casting.h"
43#include "llvm/Support/CommandLine.h"
44
45using namespace llvm;
46
47// A handy option for disabling scoped no-alias functionality. The same effect
48// can also be achieved by stripping the associated metadata tags from IR, but
49// this option is sometimes more convenient.
50static cl::opt<bool> EnableScopedNoAlias("enable-scoped-noalias",
51                                         cl::init(true), cl::Hidden);
52
53namespace {
54
55/// This is a simple wrapper around an MDNode which provides a higher-level
56/// interface by hiding the details of how alias analysis information is encoded
57/// in its operands.
58class AliasScopeNode {
59  const MDNode *Node = nullptr;
60
61public:
62  AliasScopeNode() = default;
63  explicit AliasScopeNode(const MDNode *N) : Node(N) {}
64
65  /// Get the MDNode for this AliasScopeNode.
66  const MDNode *getNode() const { return Node; }
67
68  /// Get the MDNode for this AliasScopeNode's domain.
69  const MDNode *getDomain() const {
70    if (Node->getNumOperands() < 2)
71      return nullptr;
72    return dyn_cast_or_null<MDNode>(Node->getOperand(1));
73  }
74};
75
76} // end anonymous namespace
77
78AliasResult ScopedNoAliasAAResult::alias(const MemoryLocation &LocA,
79                                         const MemoryLocation &LocB,
80                                         AAQueryInfo &AAQI) {
81  if (!EnableScopedNoAlias)
82    return AAResultBase::alias(LocA, LocB, AAQI);
83
84  // Get the attached MDNodes.
85  const MDNode *AScopes = LocA.AATags.Scope, *BScopes = LocB.AATags.Scope;
86
87  const MDNode *ANoAlias = LocA.AATags.NoAlias, *BNoAlias = LocB.AATags.NoAlias;
88
89  if (!mayAliasInScopes(AScopes, BNoAlias))
90    return NoAlias;
91
92  if (!mayAliasInScopes(BScopes, ANoAlias))
93    return NoAlias;
94
95  // If they may alias, chain to the next AliasAnalysis.
96  return AAResultBase::alias(LocA, LocB, AAQI);
97}
98
99ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call,
100                                                const MemoryLocation &Loc,
101                                                AAQueryInfo &AAQI) {
102  if (!EnableScopedNoAlias)
103    return AAResultBase::getModRefInfo(Call, Loc, AAQI);
104
105  if (!mayAliasInScopes(Loc.AATags.Scope,
106                        Call->getMetadata(LLVMContext::MD_noalias)))
107    return ModRefInfo::NoModRef;
108
109  if (!mayAliasInScopes(Call->getMetadata(LLVMContext::MD_alias_scope),
110                        Loc.AATags.NoAlias))
111    return ModRefInfo::NoModRef;
112
113  return AAResultBase::getModRefInfo(Call, Loc, AAQI);
114}
115
116ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call1,
117                                                const CallBase *Call2,
118                                                AAQueryInfo &AAQI) {
119  if (!EnableScopedNoAlias)
120    return AAResultBase::getModRefInfo(Call1, Call2, AAQI);
121
122  if (!mayAliasInScopes(Call1->getMetadata(LLVMContext::MD_alias_scope),
123                        Call2->getMetadata(LLVMContext::MD_noalias)))
124    return ModRefInfo::NoModRef;
125
126  if (!mayAliasInScopes(Call2->getMetadata(LLVMContext::MD_alias_scope),
127                        Call1->getMetadata(LLVMContext::MD_noalias)))
128    return ModRefInfo::NoModRef;
129
130  return AAResultBase::getModRefInfo(Call1, Call2, AAQI);
131}
132
133static void collectMDInDomain(const MDNode *List, const MDNode *Domain,
134                              SmallPtrSetImpl<const MDNode *> &Nodes) {
135  for (const MDOperand &MDOp : List->operands())
136    if (const MDNode *MD = dyn_cast<MDNode>(MDOp))
137      if (AliasScopeNode(MD).getDomain() == Domain)
138        Nodes.insert(MD);
139}
140
141bool ScopedNoAliasAAResult::mayAliasInScopes(const MDNode *Scopes,
142                                             const MDNode *NoAlias) const {
143  if (!Scopes || !NoAlias)
144    return true;
145
146  // Collect the set of scope domains relevant to the noalias scopes.
147  SmallPtrSet<const MDNode *, 16> Domains;
148  for (const MDOperand &MDOp : NoAlias->operands())
149    if (const MDNode *NAMD = dyn_cast<MDNode>(MDOp))
150      if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain())
151        Domains.insert(Domain);
152
153  // We alias unless, for some domain, the set of noalias scopes in that domain
154  // is a superset of the set of alias scopes in that domain.
155  for (const MDNode *Domain : Domains) {
156    SmallPtrSet<const MDNode *, 16> ScopeNodes;
157    collectMDInDomain(Scopes, Domain, ScopeNodes);
158    if (ScopeNodes.empty())
159      continue;
160
161    SmallPtrSet<const MDNode *, 16> NANodes;
162    collectMDInDomain(NoAlias, Domain, NANodes);
163
164    // To not alias, all of the nodes in ScopeNodes must be in NANodes.
165    bool FoundAll = true;
166    for (const MDNode *SMD : ScopeNodes)
167      if (!NANodes.count(SMD)) {
168        FoundAll = false;
169        break;
170      }
171
172    if (FoundAll)
173      return false;
174  }
175
176  return true;
177}
178
179AnalysisKey ScopedNoAliasAA::Key;
180
181ScopedNoAliasAAResult ScopedNoAliasAA::run(Function &F,
182                                           FunctionAnalysisManager &AM) {
183  return ScopedNoAliasAAResult();
184}
185
186char ScopedNoAliasAAWrapperPass::ID = 0;
187
188INITIALIZE_PASS(ScopedNoAliasAAWrapperPass, "scoped-noalias",
189                "Scoped NoAlias Alias Analysis", false, true)
190
191ImmutablePass *llvm::createScopedNoAliasAAWrapperPass() {
192  return new ScopedNoAliasAAWrapperPass();
193}
194
195ScopedNoAliasAAWrapperPass::ScopedNoAliasAAWrapperPass() : ImmutablePass(ID) {
196  initializeScopedNoAliasAAWrapperPassPass(*PassRegistry::getPassRegistry());
197}
198
199bool ScopedNoAliasAAWrapperPass::doInitialization(Module &M) {
200  Result.reset(new ScopedNoAliasAAResult());
201  return false;
202}
203
204bool ScopedNoAliasAAWrapperPass::doFinalization(Module &M) {
205  Result.reset();
206  return false;
207}
208
209void ScopedNoAliasAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
210  AU.setPreservesAll();
211}
212