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/InstrTypes.h"
38 #include "llvm/IR/Instruction.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 
54 AliasResult ScopedNoAliasAAResult::alias(const MemoryLocation &LocA,
55                                          const MemoryLocation &LocB,
56                                          AAQueryInfo &AAQI) {
57   if (!EnableScopedNoAlias)
58     return AAResultBase::alias(LocA, LocB, AAQI);
59 
60   // Get the attached MDNodes.
61   const MDNode *AScopes = LocA.AATags.Scope, *BScopes = LocB.AATags.Scope;
62 
63   const MDNode *ANoAlias = LocA.AATags.NoAlias, *BNoAlias = LocB.AATags.NoAlias;
64 
65   if (!mayAliasInScopes(AScopes, BNoAlias))
66     return NoAlias;
67 
68   if (!mayAliasInScopes(BScopes, ANoAlias))
69     return NoAlias;
70 
71   // If they may alias, chain to the next AliasAnalysis.
72   return AAResultBase::alias(LocA, LocB, AAQI);
73 }
74 
75 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call,
76                                                 const MemoryLocation &Loc,
77                                                 AAQueryInfo &AAQI) {
78   if (!EnableScopedNoAlias)
79     return AAResultBase::getModRefInfo(Call, Loc, AAQI);
80 
81   if (!mayAliasInScopes(Loc.AATags.Scope,
82                         Call->getMetadata(LLVMContext::MD_noalias)))
83     return ModRefInfo::NoModRef;
84 
85   if (!mayAliasInScopes(Call->getMetadata(LLVMContext::MD_alias_scope),
86                         Loc.AATags.NoAlias))
87     return ModRefInfo::NoModRef;
88 
89   return AAResultBase::getModRefInfo(Call, Loc, AAQI);
90 }
91 
92 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call1,
93                                                 const CallBase *Call2,
94                                                 AAQueryInfo &AAQI) {
95   if (!EnableScopedNoAlias)
96     return AAResultBase::getModRefInfo(Call1, Call2, AAQI);
97 
98   if (!mayAliasInScopes(Call1->getMetadata(LLVMContext::MD_alias_scope),
99                         Call2->getMetadata(LLVMContext::MD_noalias)))
100     return ModRefInfo::NoModRef;
101 
102   if (!mayAliasInScopes(Call2->getMetadata(LLVMContext::MD_alias_scope),
103                         Call1->getMetadata(LLVMContext::MD_noalias)))
104     return ModRefInfo::NoModRef;
105 
106   return AAResultBase::getModRefInfo(Call1, Call2, AAQI);
107 }
108 
109 static void collectMDInDomain(const MDNode *List, const MDNode *Domain,
110                               SmallPtrSetImpl<const MDNode *> &Nodes) {
111   for (const MDOperand &MDOp : List->operands())
112     if (const MDNode *MD = dyn_cast<MDNode>(MDOp))
113       if (AliasScopeNode(MD).getDomain() == Domain)
114         Nodes.insert(MD);
115 }
116 
117 bool ScopedNoAliasAAResult::mayAliasInScopes(const MDNode *Scopes,
118                                              const MDNode *NoAlias) const {
119   if (!Scopes || !NoAlias)
120     return true;
121 
122   // Collect the set of scope domains relevant to the noalias scopes.
123   SmallPtrSet<const MDNode *, 16> Domains;
124   for (const MDOperand &MDOp : NoAlias->operands())
125     if (const MDNode *NAMD = dyn_cast<MDNode>(MDOp))
126       if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain())
127         Domains.insert(Domain);
128 
129   // We alias unless, for some domain, the set of noalias scopes in that domain
130   // is a superset of the set of alias scopes in that domain.
131   for (const MDNode *Domain : Domains) {
132     SmallPtrSet<const MDNode *, 16> ScopeNodes;
133     collectMDInDomain(Scopes, Domain, ScopeNodes);
134     if (ScopeNodes.empty())
135       continue;
136 
137     SmallPtrSet<const MDNode *, 16> NANodes;
138     collectMDInDomain(NoAlias, Domain, NANodes);
139 
140     // To not alias, all of the nodes in ScopeNodes must be in NANodes.
141     bool FoundAll = true;
142     for (const MDNode *SMD : ScopeNodes)
143       if (!NANodes.count(SMD)) {
144         FoundAll = false;
145         break;
146       }
147 
148     if (FoundAll)
149       return false;
150   }
151 
152   return true;
153 }
154 
155 AnalysisKey ScopedNoAliasAA::Key;
156 
157 ScopedNoAliasAAResult ScopedNoAliasAA::run(Function &F,
158                                            FunctionAnalysisManager &AM) {
159   return ScopedNoAliasAAResult();
160 }
161 
162 char ScopedNoAliasAAWrapperPass::ID = 0;
163 
164 INITIALIZE_PASS(ScopedNoAliasAAWrapperPass, "scoped-noalias-aa",
165                 "Scoped NoAlias Alias Analysis", false, true)
166 
167 ImmutablePass *llvm::createScopedNoAliasAAWrapperPass() {
168   return new ScopedNoAliasAAWrapperPass();
169 }
170 
171 ScopedNoAliasAAWrapperPass::ScopedNoAliasAAWrapperPass() : ImmutablePass(ID) {
172   initializeScopedNoAliasAAWrapperPassPass(*PassRegistry::getPassRegistry());
173 }
174 
175 bool ScopedNoAliasAAWrapperPass::doInitialization(Module &M) {
176   Result.reset(new ScopedNoAliasAAResult());
177   return false;
178 }
179 
180 bool ScopedNoAliasAAWrapperPass::doFinalization(Module &M) {
181   Result.reset();
182   return false;
183 }
184 
185 void ScopedNoAliasAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
186   AU.setPreservesAll();
187 }
188