1 //===- ScopedNoAliasAA.cpp - Scoped No-Alias Alias Analysis ---------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file defines the ScopedNoAlias alias-analysis pass, which implements
10 // metadata-based scoped no-alias support.
11 //
12 // Alias-analysis scopes are defined by an id (which can be a string or some
13 // other metadata node), a domain node, and an optional descriptive string.
14 // A domain is defined by an id (which can be a string or some other metadata
15 // node), and an optional descriptive string.
16 //
17 // !dom0 = metadata !{ metadata !"domain of foo()" }
18 // !scope1 = metadata !{ metadata !scope1, metadata !dom0, metadata !"scope 1" }
19 // !scope2 = metadata !{ metadata !scope2, metadata !dom0, metadata !"scope 2" }
20 //
21 // Loads and stores can be tagged with an alias-analysis scope, and also, with
22 // a noalias tag for a specific scope:
23 //
24 // ... = load %ptr1, !alias.scope !{ !scope1 }
25 // ... = load %ptr2, !alias.scope !{ !scope1, !scope2 }, !noalias !{ !scope1 }
26 //
27 // When evaluating an aliasing query, if one of the instructions is associated
28 // has a set of noalias scopes in some domain that is a superset of the alias
29 // scopes in that domain of some other instruction, then the two memory
30 // accesses are assumed not to alias.
31 //
32 //===----------------------------------------------------------------------===//
33
34 #include "llvm/Analysis/ScopedNoAliasAA.h"
35 #include "llvm/ADT/SmallPtrSet.h"
36 #include "llvm/Analysis/MemoryLocation.h"
37 #include "llvm/IR/Instruction.h"
38 #include "llvm/IR/LLVMContext.h"
39 #include "llvm/IR/Metadata.h"
40 #include "llvm/InitializePasses.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;
AliasScopeNode(const MDNode * N)63 explicit AliasScopeNode(const MDNode *N) : Node(N) {}
64
65 /// Get the MDNode for this AliasScopeNode.
getNode() const66 const MDNode *getNode() const { return Node; }
67
68 /// Get the MDNode for this AliasScopeNode's domain.
getDomain() const69 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
alias(const MemoryLocation & LocA,const MemoryLocation & LocB,AAQueryInfo & AAQI)78 AliasResult ScopedNoAliasAAResult::alias(const MemoryLocation &LocA,
79 const MemoryLocation &LocB,
80 AAQueryInfo &AAQI) {
81 if (!EnableScopedNoAlias)
82 return AAResultBase::alias(LocA, LocB, AAQI);
83
84 // Get the attached MDNodes.
85 const MDNode *AScopes = LocA.AATags.Scope, *BScopes = LocB.AATags.Scope;
86
87 const MDNode *ANoAlias = LocA.AATags.NoAlias, *BNoAlias = LocB.AATags.NoAlias;
88
89 if (!mayAliasInScopes(AScopes, BNoAlias))
90 return NoAlias;
91
92 if (!mayAliasInScopes(BScopes, ANoAlias))
93 return NoAlias;
94
95 // If they may alias, chain to the next AliasAnalysis.
96 return AAResultBase::alias(LocA, LocB, AAQI);
97 }
98
getModRefInfo(const CallBase * Call,const MemoryLocation & Loc,AAQueryInfo & AAQI)99 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call,
100 const MemoryLocation &Loc,
101 AAQueryInfo &AAQI) {
102 if (!EnableScopedNoAlias)
103 return AAResultBase::getModRefInfo(Call, Loc, AAQI);
104
105 if (!mayAliasInScopes(Loc.AATags.Scope,
106 Call->getMetadata(LLVMContext::MD_noalias)))
107 return ModRefInfo::NoModRef;
108
109 if (!mayAliasInScopes(Call->getMetadata(LLVMContext::MD_alias_scope),
110 Loc.AATags.NoAlias))
111 return ModRefInfo::NoModRef;
112
113 return AAResultBase::getModRefInfo(Call, Loc, AAQI);
114 }
115
getModRefInfo(const CallBase * Call1,const CallBase * Call2,AAQueryInfo & AAQI)116 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call1,
117 const CallBase *Call2,
118 AAQueryInfo &AAQI) {
119 if (!EnableScopedNoAlias)
120 return AAResultBase::getModRefInfo(Call1, Call2, AAQI);
121
122 if (!mayAliasInScopes(Call1->getMetadata(LLVMContext::MD_alias_scope),
123 Call2->getMetadata(LLVMContext::MD_noalias)))
124 return ModRefInfo::NoModRef;
125
126 if (!mayAliasInScopes(Call2->getMetadata(LLVMContext::MD_alias_scope),
127 Call1->getMetadata(LLVMContext::MD_noalias)))
128 return ModRefInfo::NoModRef;
129
130 return AAResultBase::getModRefInfo(Call1, Call2, AAQI);
131 }
132
collectMDInDomain(const MDNode * List,const MDNode * Domain,SmallPtrSetImpl<const MDNode * > & Nodes)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
mayAliasInScopes(const MDNode * Scopes,const MDNode * NoAlias) const141 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
run(Function & F,FunctionAnalysisManager & AM)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
createScopedNoAliasAAWrapperPass()191 ImmutablePass *llvm::createScopedNoAliasAAWrapperPass() {
192 return new ScopedNoAliasAAWrapperPass();
193 }
194
ScopedNoAliasAAWrapperPass()195 ScopedNoAliasAAWrapperPass::ScopedNoAliasAAWrapperPass() : ImmutablePass(ID) {
196 initializeScopedNoAliasAAWrapperPassPass(*PassRegistry::getPassRegistry());
197 }
198
doInitialization(Module & M)199 bool ScopedNoAliasAAWrapperPass::doInitialization(Module &M) {
200 Result.reset(new ScopedNoAliasAAResult());
201 return false;
202 }
203
doFinalization(Module & M)204 bool ScopedNoAliasAAWrapperPass::doFinalization(Module &M) {
205 Result.reset();
206 return false;
207 }
208
getAnalysisUsage(AnalysisUsage & AU) const209 void ScopedNoAliasAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
210 AU.setPreservesAll();
211 }
212