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/Analysis/MemoryLocation.h"
38 #include "llvm/IR/Instruction.h"
39 #include "llvm/IR/LLVMContext.h"
40 #include "llvm/IR/Metadata.h"
41 #include "llvm/Pass.h"
42 #include "llvm/Support/Casting.h"
43 #include "llvm/Support/CommandLine.h"
44 
45 using namespace llvm;
46 
47 // A handy option for disabling scoped no-alias functionality. The same effect
48 // can also be achieved by stripping the associated metadata tags from IR, but
49 // this option is sometimes more convenient.
50 static cl::opt<bool> EnableScopedNoAlias("enable-scoped-noalias",
51                                          cl::init(true), cl::Hidden);
52 
53 namespace {
54 
55 /// This is a simple wrapper around an MDNode which provides a higher-level
56 /// interface by hiding the details of how alias analysis information is encoded
57 /// in its operands.
58 class AliasScopeNode {
59   const MDNode *Node = nullptr;
60 
61 public:
62   AliasScopeNode() = default;
63   explicit AliasScopeNode(const MDNode *N) : Node(N) {}
64 
65   /// Get the MDNode for this AliasScopeNode.
66   const MDNode *getNode() const { return Node; }
67 
68   /// Get the MDNode for this AliasScopeNode's domain.
69   const MDNode *getDomain() const {
70     if (Node->getNumOperands() < 2)
71       return nullptr;
72     return dyn_cast_or_null<MDNode>(Node->getOperand(1));
73   }
74 };
75 
76 } // end anonymous namespace
77 
78 AliasResult ScopedNoAliasAAResult::alias(const MemoryLocation &LocA,
79                                          const MemoryLocation &LocB) {
80   if (!EnableScopedNoAlias)
81     return AAResultBase::alias(LocA, LocB);
82 
83   // Get the attached MDNodes.
84   const MDNode *AScopes = LocA.AATags.Scope, *BScopes = LocB.AATags.Scope;
85 
86   const MDNode *ANoAlias = LocA.AATags.NoAlias, *BNoAlias = LocB.AATags.NoAlias;
87 
88   if (!mayAliasInScopes(AScopes, BNoAlias))
89     return NoAlias;
90 
91   if (!mayAliasInScopes(BScopes, ANoAlias))
92     return NoAlias;
93 
94   // If they may alias, chain to the next AliasAnalysis.
95   return AAResultBase::alias(LocA, LocB);
96 }
97 
98 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(ImmutableCallSite CS,
99                                                 const MemoryLocation &Loc) {
100   if (!EnableScopedNoAlias)
101     return AAResultBase::getModRefInfo(CS, Loc);
102 
103   if (!mayAliasInScopes(Loc.AATags.Scope, CS.getInstruction()->getMetadata(
104                                               LLVMContext::MD_noalias)))
105     return ModRefInfo::NoModRef;
106 
107   if (!mayAliasInScopes(
108           CS.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
109           Loc.AATags.NoAlias))
110     return ModRefInfo::NoModRef;
111 
112   return AAResultBase::getModRefInfo(CS, Loc);
113 }
114 
115 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(ImmutableCallSite CS1,
116                                                 ImmutableCallSite CS2) {
117   if (!EnableScopedNoAlias)
118     return AAResultBase::getModRefInfo(CS1, CS2);
119 
120   if (!mayAliasInScopes(
121           CS1.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
122           CS2.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
123     return ModRefInfo::NoModRef;
124 
125   if (!mayAliasInScopes(
126           CS2.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
127           CS1.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
128     return ModRefInfo::NoModRef;
129 
130   return AAResultBase::getModRefInfo(CS1, CS2);
131 }
132 
133 static void collectMDInDomain(const MDNode *List, const MDNode *Domain,
134                               SmallPtrSetImpl<const MDNode *> &Nodes) {
135   for (const MDOperand &MDOp : List->operands())
136     if (const MDNode *MD = dyn_cast<MDNode>(MDOp))
137       if (AliasScopeNode(MD).getDomain() == Domain)
138         Nodes.insert(MD);
139 }
140 
141 bool ScopedNoAliasAAResult::mayAliasInScopes(const MDNode *Scopes,
142                                              const MDNode *NoAlias) const {
143   if (!Scopes || !NoAlias)
144     return true;
145 
146   // Collect the set of scope domains relevant to the noalias scopes.
147   SmallPtrSet<const MDNode *, 16> Domains;
148   for (const MDOperand &MDOp : NoAlias->operands())
149     if (const MDNode *NAMD = dyn_cast<MDNode>(MDOp))
150       if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain())
151         Domains.insert(Domain);
152 
153   // We alias unless, for some domain, the set of noalias scopes in that domain
154   // is a superset of the set of alias scopes in that domain.
155   for (const MDNode *Domain : Domains) {
156     SmallPtrSet<const MDNode *, 16> ScopeNodes;
157     collectMDInDomain(Scopes, Domain, ScopeNodes);
158     if (ScopeNodes.empty())
159       continue;
160 
161     SmallPtrSet<const MDNode *, 16> NANodes;
162     collectMDInDomain(NoAlias, Domain, NANodes);
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 
179 AnalysisKey ScopedNoAliasAA::Key;
180 
181 ScopedNoAliasAAResult ScopedNoAliasAA::run(Function &F,
182                                            FunctionAnalysisManager &AM) {
183   return ScopedNoAliasAAResult();
184 }
185 
186 char ScopedNoAliasAAWrapperPass::ID = 0;
187 
188 INITIALIZE_PASS(ScopedNoAliasAAWrapperPass, "scoped-noalias",
189                 "Scoped NoAlias Alias Analysis", false, true)
190 
191 ImmutablePass *llvm::createScopedNoAliasAAWrapperPass() {
192   return new ScopedNoAliasAAWrapperPass();
193 }
194 
195 ScopedNoAliasAAWrapperPass::ScopedNoAliasAAWrapperPass() : ImmutablePass(ID) {
196   initializeScopedNoAliasAAWrapperPassPass(*PassRegistry::getPassRegistry());
197 }
198 
199 bool ScopedNoAliasAAWrapperPass::doInitialization(Module &M) {
200   Result.reset(new ScopedNoAliasAAResult());
201   return false;
202 }
203 
204 bool ScopedNoAliasAAWrapperPass::doFinalization(Module &M) {
205   Result.reset();
206   return false;
207 }
208 
209 void ScopedNoAliasAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
210   AU.setPreservesAll();
211 }
212