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 a 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/Analysis/ScopedNoAliasAA.h"
36 #include "llvm/ADT/SmallPtrSet.h"
37 #include "llvm/IR/Constants.h"
38 #include "llvm/IR/LLVMContext.h"
39 #include "llvm/IR/Metadata.h"
40 #include "llvm/IR/Module.h"
41 #include "llvm/Pass.h"
42 #include "llvm/Support/CommandLine.h"
43 
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> EnableScopedNoAlias("enable-scoped-noalias",
50                                          cl::init(true));
51 
52 namespace {
53 /// This is a simple wrapper around an MDNode which provides a higher-level
54 /// interface by hiding the details of how alias analysis information is encoded
55 /// in its operands.
56 class AliasScopeNode {
57   const MDNode *Node;
58 
59 public:
60   AliasScopeNode() : Node(nullptr) {}
61   explicit AliasScopeNode(const MDNode *N) : Node(N) {}
62 
63   /// Get the MDNode for this AliasScopeNode.
64   const MDNode *getNode() const { return Node; }
65 
66   /// 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 } // end of anonymous namespace
74 
75 AliasResult ScopedNoAliasAAResult::alias(const MemoryLocation &LocA,
76                                          const MemoryLocation &LocB) {
77   if (!EnableScopedNoAlias)
78     return AAResultBase::alias(LocA, LocB);
79 
80   // Get the attached MDNodes.
81   const MDNode *AScopes = LocA.AATags.Scope, *BScopes = LocB.AATags.Scope;
82 
83   const MDNode *ANoAlias = LocA.AATags.NoAlias, *BNoAlias = LocB.AATags.NoAlias;
84 
85   if (!mayAliasInScopes(AScopes, BNoAlias))
86     return NoAlias;
87 
88   if (!mayAliasInScopes(BScopes, ANoAlias))
89     return NoAlias;
90 
91   // If they may alias, chain to the next AliasAnalysis.
92   return AAResultBase::alias(LocA, LocB);
93 }
94 
95 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(ImmutableCallSite CS,
96                                                 const MemoryLocation &Loc) {
97   if (!EnableScopedNoAlias)
98     return AAResultBase::getModRefInfo(CS, Loc);
99 
100   if (!mayAliasInScopes(Loc.AATags.Scope, CS.getInstruction()->getMetadata(
101                                               LLVMContext::MD_noalias)))
102     return MRI_NoModRef;
103 
104   if (!mayAliasInScopes(
105           CS.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
106           Loc.AATags.NoAlias))
107     return MRI_NoModRef;
108 
109   return AAResultBase::getModRefInfo(CS, Loc);
110 }
111 
112 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(ImmutableCallSite CS1,
113                                                 ImmutableCallSite CS2) {
114   if (!EnableScopedNoAlias)
115     return AAResultBase::getModRefInfo(CS1, CS2);
116 
117   if (!mayAliasInScopes(
118           CS1.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
119           CS2.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
120     return MRI_NoModRef;
121 
122   if (!mayAliasInScopes(
123           CS2.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
124           CS1.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
125     return MRI_NoModRef;
126 
127   return AAResultBase::getModRefInfo(CS1, CS2);
128 }
129 
130 static void collectMDInDomain(const MDNode *List, const MDNode *Domain,
131                               SmallPtrSetImpl<const MDNode *> &Nodes) {
132   for (const MDOperand &MDOp : List->operands())
133     if (const MDNode *MD = dyn_cast<MDNode>(MDOp))
134       if (AliasScopeNode(MD).getDomain() == Domain)
135         Nodes.insert(MD);
136 }
137 
138 bool ScopedNoAliasAAResult::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 (const MDOperand &MDOp : NoAlias->operands())
146     if (const MDNode *NAMD = dyn_cast<MDNode>(MDOp))
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> ScopeNodes;
154     collectMDInDomain(Scopes, Domain, ScopeNodes);
155     if (ScopeNodes.empty())
156       continue;
157 
158     SmallPtrSet<const MDNode *, 16> NANodes;
159     collectMDInDomain(NoAlias, Domain, NANodes);
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 AnalysisKey ScopedNoAliasAA::Key;
177 
178 ScopedNoAliasAAResult ScopedNoAliasAA::run(Function &F,
179                                            FunctionAnalysisManager &AM) {
180   return ScopedNoAliasAAResult();
181 }
182 
183 char ScopedNoAliasAAWrapperPass::ID = 0;
184 INITIALIZE_PASS(ScopedNoAliasAAWrapperPass, "scoped-noalias",
185                 "Scoped NoAlias Alias Analysis", false, true)
186 
187 ImmutablePass *llvm::createScopedNoAliasAAWrapperPass() {
188   return new ScopedNoAliasAAWrapperPass();
189 }
190 
191 ScopedNoAliasAAWrapperPass::ScopedNoAliasAAWrapperPass() : ImmutablePass(ID) {
192   initializeScopedNoAliasAAWrapperPassPass(*PassRegistry::getPassRegistry());
193 }
194 
195 bool ScopedNoAliasAAWrapperPass::doInitialization(Module &M) {
196   Result.reset(new ScopedNoAliasAAResult());
197   return false;
198 }
199 
200 bool ScopedNoAliasAAWrapperPass::doFinalization(Module &M) {
201   Result.reset();
202   return false;
203 }
204 
205 void ScopedNoAliasAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
206   AU.setPreservesAll();
207 }
208