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/SetOperations.h"
36 #include "llvm/ADT/SmallPtrSet.h"
37 #include "llvm/Analysis/MemoryLocation.h"
38 #include "llvm/IR/InstrTypes.h"
39 #include "llvm/IR/LLVMContext.h"
40 #include "llvm/IR/Metadata.h"
41 #include "llvm/InitializePasses.h"
42 #include "llvm/Pass.h"
43 #include "llvm/Support/Casting.h"
44 #include "llvm/Support/CommandLine.h"
45
46 using namespace llvm;
47
48 // A handy option for disabling scoped no-alias functionality. The same effect
49 // can also be achieved by stripping the associated metadata tags from IR, but
50 // this option is sometimes more convenient.
51 static cl::opt<bool> EnableScopedNoAlias("enable-scoped-noalias",
52 cl::init(true), cl::Hidden);
53
alias(const MemoryLocation & LocA,const MemoryLocation & LocB,AAQueryInfo & AAQI,const Instruction *)54 AliasResult ScopedNoAliasAAResult::alias(const MemoryLocation &LocA,
55 const MemoryLocation &LocB,
56 AAQueryInfo &AAQI,
57 const Instruction *) {
58 if (!EnableScopedNoAlias)
59 return AAResultBase::alias(LocA, LocB, AAQI, nullptr);
60
61 // Get the attached MDNodes.
62 const MDNode *AScopes = LocA.AATags.Scope, *BScopes = LocB.AATags.Scope;
63
64 const MDNode *ANoAlias = LocA.AATags.NoAlias, *BNoAlias = LocB.AATags.NoAlias;
65
66 if (!mayAliasInScopes(AScopes, BNoAlias))
67 return AliasResult::NoAlias;
68
69 if (!mayAliasInScopes(BScopes, ANoAlias))
70 return AliasResult::NoAlias;
71
72 // If they may alias, chain to the next AliasAnalysis.
73 return AAResultBase::alias(LocA, LocB, AAQI, nullptr);
74 }
75
getModRefInfo(const CallBase * Call,const MemoryLocation & Loc,AAQueryInfo & AAQI)76 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call,
77 const MemoryLocation &Loc,
78 AAQueryInfo &AAQI) {
79 if (!EnableScopedNoAlias)
80 return AAResultBase::getModRefInfo(Call, Loc, AAQI);
81
82 if (!mayAliasInScopes(Loc.AATags.Scope,
83 Call->getMetadata(LLVMContext::MD_noalias)))
84 return ModRefInfo::NoModRef;
85
86 if (!mayAliasInScopes(Call->getMetadata(LLVMContext::MD_alias_scope),
87 Loc.AATags.NoAlias))
88 return ModRefInfo::NoModRef;
89
90 return AAResultBase::getModRefInfo(Call, Loc, AAQI);
91 }
92
getModRefInfo(const CallBase * Call1,const CallBase * Call2,AAQueryInfo & AAQI)93 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call1,
94 const CallBase *Call2,
95 AAQueryInfo &AAQI) {
96 if (!EnableScopedNoAlias)
97 return AAResultBase::getModRefInfo(Call1, Call2, AAQI);
98
99 if (!mayAliasInScopes(Call1->getMetadata(LLVMContext::MD_alias_scope),
100 Call2->getMetadata(LLVMContext::MD_noalias)))
101 return ModRefInfo::NoModRef;
102
103 if (!mayAliasInScopes(Call2->getMetadata(LLVMContext::MD_alias_scope),
104 Call1->getMetadata(LLVMContext::MD_noalias)))
105 return ModRefInfo::NoModRef;
106
107 return AAResultBase::getModRefInfo(Call1, Call2, AAQI);
108 }
109
collectMDInDomain(const MDNode * List,const MDNode * Domain,SmallPtrSetImpl<const MDNode * > & Nodes)110 static void collectMDInDomain(const MDNode *List, const MDNode *Domain,
111 SmallPtrSetImpl<const MDNode *> &Nodes) {
112 for (const MDOperand &MDOp : List->operands())
113 if (const MDNode *MD = dyn_cast<MDNode>(MDOp))
114 if (AliasScopeNode(MD).getDomain() == Domain)
115 Nodes.insert(MD);
116 }
117
mayAliasInScopes(const MDNode * Scopes,const MDNode * NoAlias) const118 bool ScopedNoAliasAAResult::mayAliasInScopes(const MDNode *Scopes,
119 const MDNode *NoAlias) const {
120 if (!Scopes || !NoAlias)
121 return true;
122
123 // Collect the set of scope domains relevant to the noalias scopes.
124 SmallPtrSet<const MDNode *, 16> Domains;
125 for (const MDOperand &MDOp : NoAlias->operands())
126 if (const MDNode *NAMD = dyn_cast<MDNode>(MDOp))
127 if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain())
128 Domains.insert(Domain);
129
130 // We alias unless, for some domain, the set of noalias scopes in that domain
131 // is a superset of the set of alias scopes in that domain.
132 for (const MDNode *Domain : Domains) {
133 SmallPtrSet<const MDNode *, 16> ScopeNodes;
134 collectMDInDomain(Scopes, Domain, ScopeNodes);
135 if (ScopeNodes.empty())
136 continue;
137
138 SmallPtrSet<const MDNode *, 16> NANodes;
139 collectMDInDomain(NoAlias, Domain, NANodes);
140
141 // To not alias, all of the nodes in ScopeNodes must be in NANodes.
142 if (llvm::set_is_subset(ScopeNodes, NANodes))
143 return false;
144 }
145
146 return true;
147 }
148
149 AnalysisKey ScopedNoAliasAA::Key;
150
run(Function & F,FunctionAnalysisManager & AM)151 ScopedNoAliasAAResult ScopedNoAliasAA::run(Function &F,
152 FunctionAnalysisManager &AM) {
153 return ScopedNoAliasAAResult();
154 }
155
156 char ScopedNoAliasAAWrapperPass::ID = 0;
157
158 INITIALIZE_PASS(ScopedNoAliasAAWrapperPass, "scoped-noalias-aa",
159 "Scoped NoAlias Alias Analysis", false, true)
160
createScopedNoAliasAAWrapperPass()161 ImmutablePass *llvm::createScopedNoAliasAAWrapperPass() {
162 return new ScopedNoAliasAAWrapperPass();
163 }
164
ScopedNoAliasAAWrapperPass()165 ScopedNoAliasAAWrapperPass::ScopedNoAliasAAWrapperPass() : ImmutablePass(ID) {
166 initializeScopedNoAliasAAWrapperPassPass(*PassRegistry::getPassRegistry());
167 }
168
doInitialization(Module & M)169 bool ScopedNoAliasAAWrapperPass::doInitialization(Module &M) {
170 Result.reset(new ScopedNoAliasAAResult());
171 return false;
172 }
173
doFinalization(Module & M)174 bool ScopedNoAliasAAWrapperPass::doFinalization(Module &M) {
175 Result.reset();
176 return false;
177 }
178
getAnalysisUsage(AnalysisUsage & AU) const179 void ScopedNoAliasAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
180 AU.setPreservesAll();
181 }
182