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"
44 using 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.
49 static cl::opt<bool>
50 EnableScopedNoAlias("enable-scoped-noalias", cl::init(true));
51
52 namespace {
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.
56 class AliasScopeNode {
57 const MDNode *Node;
58
59 public:
AliasScopeNode()60 AliasScopeNode() : Node(0) {}
AliasScopeNode(const MDNode * N)61 explicit AliasScopeNode(const MDNode *N) : Node(N) {}
62
63 /// getNode - Get the MDNode for this AliasScopeNode.
getNode() const64 const MDNode *getNode() const { return Node; }
65
66 /// getDomain - Get the MDNode for this AliasScopeNode's domain.
getDomain() const67 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.
76 class ScopedNoAliasAA : public ImmutablePass, public AliasAnalysis {
77 public:
78 static char ID; // Class identification, replacement for typeinfo
ScopedNoAliasAA()79 ScopedNoAliasAA() : ImmutablePass(ID) {
80 initializeScopedNoAliasAAPass(*PassRegistry::getPassRegistry());
81 }
82
initializePass()83 void initializePass() override { InitializeAliasAnalysis(this); }
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.
getAdjustedAnalysisPointer(const void * PI)89 void *getAdjustedAnalysisPointer(const void *PI) override {
90 if (PI == &AliasAnalysis::ID)
91 return (AliasAnalysis*)this;
92 return this;
93 }
94
95 protected:
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
100 private:
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...
114 char ScopedNoAliasAA::ID = 0;
115 INITIALIZE_AG_PASS(ScopedNoAliasAA, AliasAnalysis, "scoped-noalias",
116 "Scoped NoAlias Alias Analysis", false, true, false)
117
createScopedNoAliasAAPass()118 ImmutablePass *llvm::createScopedNoAliasAAPass() {
119 return new ScopedNoAliasAA();
120 }
121
122 void
getAnalysisUsage(AnalysisUsage & AU) const123 ScopedNoAliasAA::getAnalysisUsage(AnalysisUsage &AU) const {
124 AU.setPreservesAll();
125 AliasAnalysis::getAnalysisUsage(AU);
126 }
127
128 void
collectMDInDomain(const MDNode * List,const MDNode * Domain,SmallPtrSetImpl<const MDNode * > & Nodes) const129 ScopedNoAliasAA::collectMDInDomain(const MDNode *List, const MDNode *Domain,
130 SmallPtrSetImpl<const MDNode *> &Nodes) const {
131 for (unsigned i = 0, ie = List->getNumOperands(); i != ie; ++i)
132 if (const MDNode *MD = dyn_cast<MDNode>(List->getOperand(i)))
133 if (AliasScopeNode(MD).getDomain() == Domain)
134 Nodes.insert(MD);
135 }
136
137 bool
mayAliasInScopes(const MDNode * Scopes,const MDNode * NoAlias) const138 ScopedNoAliasAA::mayAliasInScopes(const MDNode *Scopes,
139 const MDNode *NoAlias) const {
140 if (!Scopes || !NoAlias)
141 return true;
142
143 // Collect the set of scope domains relevant to the noalias scopes.
144 SmallPtrSet<const MDNode *, 16> Domains;
145 for (unsigned i = 0, ie = NoAlias->getNumOperands(); i != ie; ++i)
146 if (const MDNode *NAMD = dyn_cast<MDNode>(NoAlias->getOperand(i)))
147 if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain())
148 Domains.insert(Domain);
149
150 // We alias unless, for some domain, the set of noalias scopes in that domain
151 // is a superset of the set of alias scopes in that domain.
152 for (const MDNode *Domain : Domains) {
153 SmallPtrSet<const MDNode *, 16> NANodes, ScopeNodes;
154 collectMDInDomain(NoAlias, Domain, NANodes);
155 collectMDInDomain(Scopes, Domain, ScopeNodes);
156 if (!ScopeNodes.size())
157 continue;
158
159 // To not alias, all of the nodes in ScopeNodes must be in NANodes.
160 bool FoundAll = true;
161 for (const MDNode *SMD : ScopeNodes)
162 if (!NANodes.count(SMD)) {
163 FoundAll = false;
164 break;
165 }
166
167 if (FoundAll)
168 return false;
169 }
170
171 return true;
172 }
173
174 AliasAnalysis::AliasResult
alias(const Location & LocA,const Location & LocB)175 ScopedNoAliasAA::alias(const Location &LocA, const Location &LocB) {
176 if (!EnableScopedNoAlias)
177 return AliasAnalysis::alias(LocA, LocB);
178
179 // Get the attached MDNodes.
180 const MDNode *AScopes = LocA.AATags.Scope,
181 *BScopes = LocB.AATags.Scope;
182
183 const MDNode *ANoAlias = LocA.AATags.NoAlias,
184 *BNoAlias = LocB.AATags.NoAlias;
185
186 if (!mayAliasInScopes(AScopes, BNoAlias))
187 return NoAlias;
188
189 if (!mayAliasInScopes(BScopes, ANoAlias))
190 return NoAlias;
191
192 // If they may alias, chain to the next AliasAnalysis.
193 return AliasAnalysis::alias(LocA, LocB);
194 }
195
pointsToConstantMemory(const Location & Loc,bool OrLocal)196 bool ScopedNoAliasAA::pointsToConstantMemory(const Location &Loc,
197 bool OrLocal) {
198 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
199 }
200
201 AliasAnalysis::ModRefBehavior
getModRefBehavior(ImmutableCallSite CS)202 ScopedNoAliasAA::getModRefBehavior(ImmutableCallSite CS) {
203 return AliasAnalysis::getModRefBehavior(CS);
204 }
205
206 AliasAnalysis::ModRefBehavior
getModRefBehavior(const Function * F)207 ScopedNoAliasAA::getModRefBehavior(const Function *F) {
208 return AliasAnalysis::getModRefBehavior(F);
209 }
210
211 AliasAnalysis::ModRefResult
getModRefInfo(ImmutableCallSite CS,const Location & Loc)212 ScopedNoAliasAA::getModRefInfo(ImmutableCallSite CS, const Location &Loc) {
213 if (!EnableScopedNoAlias)
214 return AliasAnalysis::getModRefInfo(CS, Loc);
215
216 if (!mayAliasInScopes(Loc.AATags.Scope, CS.getInstruction()->getMetadata(
217 LLVMContext::MD_noalias)))
218 return NoModRef;
219
220 if (!mayAliasInScopes(
221 CS.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
222 Loc.AATags.NoAlias))
223 return NoModRef;
224
225 return AliasAnalysis::getModRefInfo(CS, Loc);
226 }
227
228 AliasAnalysis::ModRefResult
getModRefInfo(ImmutableCallSite CS1,ImmutableCallSite CS2)229 ScopedNoAliasAA::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
230 if (!EnableScopedNoAlias)
231 return AliasAnalysis::getModRefInfo(CS1, CS2);
232
233 if (!mayAliasInScopes(
234 CS1.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
235 CS2.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
236 return NoModRef;
237
238 if (!mayAliasInScopes(
239 CS2.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
240 CS1.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
241 return NoModRef;
242
243 return AliasAnalysis::getModRefInfo(CS1, CS2);
244 }
245
246