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   virtual void initializePass() {
84     InitializeAliasAnalysis(this);
85   }
86 
87   /// getAdjustedAnalysisPointer - This method is used when a pass implements
88   /// an analysis interface through multiple inheritance.  If needed, it
89   /// should override this to adjust the this pointer as needed for the
90   /// specified pass info.
91   virtual void *getAdjustedAnalysisPointer(const void *PI) {
92     if (PI == &AliasAnalysis::ID)
93       return (AliasAnalysis*)this;
94     return this;
95   }
96 
97 protected:
98   bool mayAliasInScopes(const MDNode *Scopes, const MDNode *NoAlias) const;
99   void collectMDInDomain(const MDNode *List, const MDNode *Domain,
100                          SmallPtrSetImpl<const MDNode *> &Nodes) const;
101 
102 private:
103   virtual void getAnalysisUsage(AnalysisUsage &AU) const;
104   virtual AliasResult alias(const Location &LocA, const Location &LocB);
105   virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal);
106   virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
107   virtual ModRefBehavior getModRefBehavior(const Function *F);
108   virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
109                                      const Location &Loc);
110   virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
111                                      ImmutableCallSite CS2);
112 };
113 }  // End of anonymous namespace
114 
115 // Register this pass...
116 char ScopedNoAliasAA::ID = 0;
117 INITIALIZE_AG_PASS(ScopedNoAliasAA, AliasAnalysis, "scoped-noalias",
118                    "Scoped NoAlias Alias Analysis", false, true, false)
119 
120 ImmutablePass *llvm::createScopedNoAliasAAPass() {
121   return new ScopedNoAliasAA();
122 }
123 
124 void
125 ScopedNoAliasAA::getAnalysisUsage(AnalysisUsage &AU) const {
126   AU.setPreservesAll();
127   AliasAnalysis::getAnalysisUsage(AU);
128 }
129 
130 void
131 ScopedNoAliasAA::collectMDInDomain(const MDNode *List, const MDNode *Domain,
132                    SmallPtrSetImpl<const MDNode *> &Nodes) const {
133   for (unsigned i = 0, ie = List->getNumOperands(); i != ie; ++i)
134     if (const MDNode *MD = dyn_cast<MDNode>(List->getOperand(i)))
135       if (AliasScopeNode(MD).getDomain() == Domain)
136         Nodes.insert(MD);
137 }
138 
139 bool
140 ScopedNoAliasAA::mayAliasInScopes(const MDNode *Scopes,
141                                   const MDNode *NoAlias) const {
142   if (!Scopes || !NoAlias)
143     return true;
144 
145   // Collect the set of scope domains relevant to the noalias scopes.
146   SmallPtrSet<const MDNode *, 16> Domains;
147   for (unsigned i = 0, ie = NoAlias->getNumOperands(); i != ie; ++i)
148     if (const MDNode *NAMD = dyn_cast<MDNode>(NoAlias->getOperand(i)))
149       if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain())
150         Domains.insert(Domain);
151 
152   // We alias unless, for some domain, the set of noalias scopes in that domain
153   // is a superset of the set of alias scopes in that domain.
154   for (const MDNode *Domain : Domains) {
155     SmallPtrSet<const MDNode *, 16> NANodes, ScopeNodes;
156     collectMDInDomain(NoAlias, Domain, NANodes);
157     collectMDInDomain(Scopes, Domain, ScopeNodes);
158     if (!ScopeNodes.size())
159       continue;
160 
161     // To not alias, all of the nodes in ScopeNodes must be in NANodes.
162     bool FoundAll = true;
163     for (const MDNode *SMD : ScopeNodes)
164       if (!NANodes.count(SMD)) {
165         FoundAll = false;
166         break;
167       }
168 
169     if (FoundAll)
170       return false;
171   }
172 
173   return true;
174 }
175 
176 AliasAnalysis::AliasResult
177 ScopedNoAliasAA::alias(const Location &LocA, const Location &LocB) {
178   if (!EnableScopedNoAlias)
179     return AliasAnalysis::alias(LocA, LocB);
180 
181   // Get the attached MDNodes.
182   const MDNode *AScopes = LocA.AATags.Scope,
183                *BScopes = LocB.AATags.Scope;
184 
185   const MDNode *ANoAlias = LocA.AATags.NoAlias,
186                *BNoAlias = LocB.AATags.NoAlias;
187 
188   if (!mayAliasInScopes(AScopes, BNoAlias))
189     return NoAlias;
190 
191   if (!mayAliasInScopes(BScopes, ANoAlias))
192     return NoAlias;
193 
194   // If they may alias, chain to the next AliasAnalysis.
195   return AliasAnalysis::alias(LocA, LocB);
196 }
197 
198 bool ScopedNoAliasAA::pointsToConstantMemory(const Location &Loc,
199                                              bool OrLocal) {
200   return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
201 }
202 
203 AliasAnalysis::ModRefBehavior
204 ScopedNoAliasAA::getModRefBehavior(ImmutableCallSite CS) {
205   return AliasAnalysis::getModRefBehavior(CS);
206 }
207 
208 AliasAnalysis::ModRefBehavior
209 ScopedNoAliasAA::getModRefBehavior(const Function *F) {
210   return AliasAnalysis::getModRefBehavior(F);
211 }
212 
213 AliasAnalysis::ModRefResult
214 ScopedNoAliasAA::getModRefInfo(ImmutableCallSite CS, const Location &Loc) {
215   if (!EnableScopedNoAlias)
216     return AliasAnalysis::getModRefInfo(CS, Loc);
217 
218   if (!mayAliasInScopes(Loc.AATags.Scope,
219         CS.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
220     return NoModRef;
221 
222   if (!mayAliasInScopes(
223         CS.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
224         Loc.AATags.NoAlias))
225     return NoModRef;
226 
227   return AliasAnalysis::getModRefInfo(CS, Loc);
228 }
229 
230 AliasAnalysis::ModRefResult
231 ScopedNoAliasAA::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
232   if (!EnableScopedNoAlias)
233     return AliasAnalysis::getModRefInfo(CS1, CS2);
234 
235   if (!mayAliasInScopes(
236         CS1.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
237         CS2.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
238     return NoModRef;
239 
240   if (!mayAliasInScopes(
241         CS2.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
242         CS1.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
243     return NoModRef;
244 
245   return AliasAnalysis::getModRefInfo(CS1, CS2);
246 }
247 
248