ScopedNoAliasAA.cpp revision 4c5e43da7792f75567b693105cc53e3f1992ad98 
1//===- ScopedNoAliasAA.cpp - Scoped No-Alias 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 ScopedNoAlias alias-analysis pass, which implements
11// metadata-based scoped no-alias support.
12//
13// Alias-analysis scopes are defined by an id (which can be a string or some
14// other metadata node), a domain node, and an optional descriptive string.
15// A domain is defined by an id (which can be a string or some other metadata
16// node), and an optional descriptive string.
17//
18// !dom0 =   metadata !{ metadata !"domain of foo()" }
19// !scope1 = metadata !{ metadata !scope1, metadata !dom0, metadata !"scope 1" }
20// !scope2 = metadata !{ metadata !scope2, metadata !dom0, metadata !"scope 2" }
21//
22// Loads and stores can be tagged with an alias-analysis scope, and also, with
23// a noalias tag for a specific scope:
24//
25// ... = load %ptr1, !alias.scope !{ !scope1 }
26// ... = load %ptr2, !alias.scope !{ !scope1, !scope2 }, !noalias !{ !scope1 }
27//
28// When evaluating an aliasing query, if one of the instructions is associated
29// has a set of noalias scopes in some domain that is superset of the alias
30// scopes in that domain of some other instruction, then the two memory
31// accesses are assumed not to alias.
32//
33//===----------------------------------------------------------------------===//
34
35#include "llvm/ADT/SmallPtrSet.h"
36#include "llvm/Analysis/AliasAnalysis.h"
37#include "llvm/Analysis/Passes.h"
38#include "llvm/IR/Constants.h"
39#include "llvm/IR/LLVMContext.h"
40#include "llvm/IR/Metadata.h"
41#include "llvm/IR/Module.h"
42#include "llvm/Pass.h"
43#include "llvm/Support/CommandLine.h"
44using namespace llvm;
45
46// A handy option for disabling scoped no-alias functionality. The same effect
47// can also be achieved by stripping the associated metadata tags from IR, but
48// this option is sometimes more convenient.
49static cl::opt<bool>
50EnableScopedNoAlias("enable-scoped-noalias", cl::init(true));
51
52namespace {
53/// AliasScopeNode - This is a simple wrapper around an MDNode which provides
54/// a higher-level interface by hiding the details of how alias analysis
55/// information is encoded in its operands.
56class AliasScopeNode {
57  const MDNode *Node;
58
59public:
60  AliasScopeNode() : Node(0) {}
61  explicit AliasScopeNode(const MDNode *N) : Node(N) {}
62
63  /// getNode - Get the MDNode for this AliasScopeNode.
64  const MDNode *getNode() const { return Node; }
65
66  /// getDomain - Get the MDNode for this AliasScopeNode's domain.
67  const MDNode *getDomain() const {
68    if (Node->getNumOperands() < 2)
69      return nullptr;
70    return dyn_cast_or_null<MDNode>(Node->getOperand(1));
71  }
72};
73
74/// ScopedNoAliasAA - This is a simple alias analysis
75/// implementation that uses scoped-noalias metadata to answer queries.
76class ScopedNoAliasAA : public ImmutablePass, public AliasAnalysis {
77public:
78  static char ID; // Class identification, replacement for typeinfo
79  ScopedNoAliasAA() : ImmutablePass(ID) {
80    initializeScopedNoAliasAAPass(*PassRegistry::getPassRegistry());
81  }
82
83  bool doInitialization(Module &M) override;
84
85  /// getAdjustedAnalysisPointer - This method is used when a pass implements
86  /// an analysis interface through multiple inheritance.  If needed, it
87  /// should override this to adjust the this pointer as needed for the
88  /// specified pass info.
89  void *getAdjustedAnalysisPointer(const void *PI) override {
90    if (PI == &AliasAnalysis::ID)
91      return (AliasAnalysis*)this;
92    return this;
93  }
94
95protected:
96  bool mayAliasInScopes(const MDNode *Scopes, const MDNode *NoAlias) const;
97  void collectMDInDomain(const MDNode *List, const MDNode *Domain,
98                         SmallPtrSetImpl<const MDNode *> &Nodes) const;
99
100private:
101  void getAnalysisUsage(AnalysisUsage &AU) const override;
102  AliasResult alias(const Location &LocA, const Location &LocB) override;
103  bool pointsToConstantMemory(const Location &Loc, bool OrLocal) override;
104  ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override;
105  ModRefBehavior getModRefBehavior(const Function *F) override;
106  ModRefResult getModRefInfo(ImmutableCallSite CS,
107                             const Location &Loc) override;
108  ModRefResult getModRefInfo(ImmutableCallSite CS1,
109                             ImmutableCallSite CS2) override;
110};
111}  // End of anonymous namespace
112
113// Register this pass...
114char ScopedNoAliasAA::ID = 0;
115INITIALIZE_AG_PASS(ScopedNoAliasAA, AliasAnalysis, "scoped-noalias",
116                   "Scoped NoAlias Alias Analysis", false, true, false)
117
118ImmutablePass *llvm::createScopedNoAliasAAPass() {
119  return new ScopedNoAliasAA();
120}
121
122bool ScopedNoAliasAA::doInitialization(Module &M) {
123  InitializeAliasAnalysis(this, &M.getDataLayout());
124  return true;
125}
126
127void
128ScopedNoAliasAA::getAnalysisUsage(AnalysisUsage &AU) const {
129  AU.setPreservesAll();
130  AliasAnalysis::getAnalysisUsage(AU);
131}
132
133void
134ScopedNoAliasAA::collectMDInDomain(const MDNode *List, const MDNode *Domain,
135                   SmallPtrSetImpl<const MDNode *> &Nodes) const {
136  for (unsigned i = 0, ie = List->getNumOperands(); i != ie; ++i)
137    if (const MDNode *MD = dyn_cast<MDNode>(List->getOperand(i)))
138      if (AliasScopeNode(MD).getDomain() == Domain)
139        Nodes.insert(MD);
140}
141
142bool
143ScopedNoAliasAA::mayAliasInScopes(const MDNode *Scopes,
144                                  const MDNode *NoAlias) const {
145  if (!Scopes || !NoAlias)
146    return true;
147
148  // Collect the set of scope domains relevant to the noalias scopes.
149  SmallPtrSet<const MDNode *, 16> Domains;
150  for (unsigned i = 0, ie = NoAlias->getNumOperands(); i != ie; ++i)
151    if (const MDNode *NAMD = dyn_cast<MDNode>(NoAlias->getOperand(i)))
152      if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain())
153        Domains.insert(Domain);
154
155  // We alias unless, for some domain, the set of noalias scopes in that domain
156  // is a superset of the set of alias scopes in that domain.
157  for (const MDNode *Domain : Domains) {
158    SmallPtrSet<const MDNode *, 16> NANodes, ScopeNodes;
159    collectMDInDomain(NoAlias, Domain, NANodes);
160    collectMDInDomain(Scopes, Domain, ScopeNodes);
161    if (!ScopeNodes.size())
162      continue;
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
179AliasAnalysis::AliasResult
180ScopedNoAliasAA::alias(const Location &LocA, const Location &LocB) {
181  if (!EnableScopedNoAlias)
182    return AliasAnalysis::alias(LocA, LocB);
183
184  // Get the attached MDNodes.
185  const MDNode *AScopes = LocA.AATags.Scope,
186               *BScopes = LocB.AATags.Scope;
187
188  const MDNode *ANoAlias = LocA.AATags.NoAlias,
189               *BNoAlias = LocB.AATags.NoAlias;
190
191  if (!mayAliasInScopes(AScopes, BNoAlias))
192    return NoAlias;
193
194  if (!mayAliasInScopes(BScopes, ANoAlias))
195    return NoAlias;
196
197  // If they may alias, chain to the next AliasAnalysis.
198  return AliasAnalysis::alias(LocA, LocB);
199}
200
201bool ScopedNoAliasAA::pointsToConstantMemory(const Location &Loc,
202                                             bool OrLocal) {
203  return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
204}
205
206AliasAnalysis::ModRefBehavior
207ScopedNoAliasAA::getModRefBehavior(ImmutableCallSite CS) {
208  return AliasAnalysis::getModRefBehavior(CS);
209}
210
211AliasAnalysis::ModRefBehavior
212ScopedNoAliasAA::getModRefBehavior(const Function *F) {
213  return AliasAnalysis::getModRefBehavior(F);
214}
215
216AliasAnalysis::ModRefResult
217ScopedNoAliasAA::getModRefInfo(ImmutableCallSite CS, const Location &Loc) {
218  if (!EnableScopedNoAlias)
219    return AliasAnalysis::getModRefInfo(CS, Loc);
220
221  if (!mayAliasInScopes(Loc.AATags.Scope, CS.getInstruction()->getMetadata(
222                                              LLVMContext::MD_noalias)))
223    return NoModRef;
224
225  if (!mayAliasInScopes(
226          CS.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
227          Loc.AATags.NoAlias))
228    return NoModRef;
229
230  return AliasAnalysis::getModRefInfo(CS, Loc);
231}
232
233AliasAnalysis::ModRefResult
234ScopedNoAliasAA::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
235  if (!EnableScopedNoAlias)
236    return AliasAnalysis::getModRefInfo(CS1, CS2);
237
238  if (!mayAliasInScopes(
239          CS1.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
240          CS2.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
241    return NoModRef;
242
243  if (!mayAliasInScopes(
244          CS2.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
245          CS1.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
246    return NoModRef;
247
248  return AliasAnalysis::getModRefInfo(CS1, CS2);
249}
250
251