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/Passes.h"
37 #include "llvm/Analysis/AliasAnalysis.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:
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.
76 class ScopedNoAliasAA : public ImmutablePass, public AliasAnalysis {
77 public:
78   static char ID; // Class identification, replacement for typeinfo
79   ScopedNoAliasAA() : ImmutablePass(ID) {
80     initializeScopedNoAliasAAPass(*PassRegistry::getPassRegistry());
81   }
82 
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.
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 
118 ImmutablePass *llvm::createScopedNoAliasAAPass() {
119   return new ScopedNoAliasAA();
120 }
121 
122 void
123 ScopedNoAliasAA::getAnalysisUsage(AnalysisUsage &AU) const {
124   AU.setPreservesAll();
125   AliasAnalysis::getAnalysisUsage(AU);
126 }
127 
128 void
129 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
138 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
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 
196 bool ScopedNoAliasAA::pointsToConstantMemory(const Location &Loc,
197                                              bool OrLocal) {
198   return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
199 }
200 
201 AliasAnalysis::ModRefBehavior
202 ScopedNoAliasAA::getModRefBehavior(ImmutableCallSite CS) {
203   return AliasAnalysis::getModRefBehavior(CS);
204 }
205 
206 AliasAnalysis::ModRefBehavior
207 ScopedNoAliasAA::getModRefBehavior(const Function *F) {
208   return AliasAnalysis::getModRefBehavior(F);
209 }
210 
211 AliasAnalysis::ModRefResult
212 ScopedNoAliasAA::getModRefInfo(ImmutableCallSite CS, const Location &Loc) {
213   if (!EnableScopedNoAlias)
214     return AliasAnalysis::getModRefInfo(CS, Loc);
215 
216   if (!mayAliasInScopes(Loc.AATags.Scope,
217         CS.getInstruction()->getMetadata(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
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