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/Pass.h" 41 #include "llvm/Support/Casting.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), cl::Hidden); 51 52 namespace { 53 54 /// This is a simple wrapper around an MDNode which provides a higher-level 55 /// interface by hiding the details of how alias analysis information is encoded 56 /// in its operands. 57 class AliasScopeNode { 58 const MDNode *Node = nullptr; 59 60 public: 61 AliasScopeNode() = default; 62 explicit AliasScopeNode(const MDNode *N) : Node(N) {} 63 64 /// Get the MDNode for this AliasScopeNode. 65 const MDNode *getNode() const { return Node; } 66 67 /// Get the MDNode for this AliasScopeNode's domain. 68 const MDNode *getDomain() const { 69 if (Node->getNumOperands() < 2) 70 return nullptr; 71 return dyn_cast_or_null<MDNode>(Node->getOperand(1)); 72 } 73 }; 74 75 } // end anonymous namespace 76 77 AliasResult ScopedNoAliasAAResult::alias(const MemoryLocation &LocA, 78 const MemoryLocation &LocB, 79 AAQueryInfo &AAQI) { 80 if (!EnableScopedNoAlias) 81 return AAResultBase::alias(LocA, LocB, AAQI); 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, AAQI); 96 } 97 98 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call, 99 const MemoryLocation &Loc, 100 AAQueryInfo &AAQI) { 101 if (!EnableScopedNoAlias) 102 return AAResultBase::getModRefInfo(Call, Loc, AAQI); 103 104 if (!mayAliasInScopes(Loc.AATags.Scope, 105 Call->getMetadata(LLVMContext::MD_noalias))) 106 return ModRefInfo::NoModRef; 107 108 if (!mayAliasInScopes(Call->getMetadata(LLVMContext::MD_alias_scope), 109 Loc.AATags.NoAlias)) 110 return ModRefInfo::NoModRef; 111 112 return AAResultBase::getModRefInfo(Call, Loc, AAQI); 113 } 114 115 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call1, 116 const CallBase *Call2, 117 AAQueryInfo &AAQI) { 118 if (!EnableScopedNoAlias) 119 return AAResultBase::getModRefInfo(Call1, Call2, AAQI); 120 121 if (!mayAliasInScopes(Call1->getMetadata(LLVMContext::MD_alias_scope), 122 Call2->getMetadata(LLVMContext::MD_noalias))) 123 return ModRefInfo::NoModRef; 124 125 if (!mayAliasInScopes(Call2->getMetadata(LLVMContext::MD_alias_scope), 126 Call1->getMetadata(LLVMContext::MD_noalias))) 127 return ModRefInfo::NoModRef; 128 129 return AAResultBase::getModRefInfo(Call1, Call2, AAQI); 130 } 131 132 static void collectMDInDomain(const MDNode *List, const MDNode *Domain, 133 SmallPtrSetImpl<const MDNode *> &Nodes) { 134 for (const MDOperand &MDOp : List->operands()) 135 if (const MDNode *MD = dyn_cast<MDNode>(MDOp)) 136 if (AliasScopeNode(MD).getDomain() == Domain) 137 Nodes.insert(MD); 138 } 139 140 bool ScopedNoAliasAAResult::mayAliasInScopes(const MDNode *Scopes, 141 const MDNode *NoAlias) const { 142 if (!Scopes || !NoAlias) 143 return true; 144 145 // Collect the set of scope domains relevant to the noalias scopes. 146 SmallPtrSet<const MDNode *, 16> Domains; 147 for (const MDOperand &MDOp : NoAlias->operands()) 148 if (const MDNode *NAMD = dyn_cast<MDNode>(MDOp)) 149 if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain()) 150 Domains.insert(Domain); 151 152 // We alias unless, for some domain, the set of noalias scopes in that domain 153 // is a superset of the set of alias scopes in that domain. 154 for (const MDNode *Domain : Domains) { 155 SmallPtrSet<const MDNode *, 16> ScopeNodes; 156 collectMDInDomain(Scopes, Domain, ScopeNodes); 157 if (ScopeNodes.empty()) 158 continue; 159 160 SmallPtrSet<const MDNode *, 16> NANodes; 161 collectMDInDomain(NoAlias, Domain, NANodes); 162 163 // To not alias, all of the nodes in ScopeNodes must be in NANodes. 164 bool FoundAll = true; 165 for (const MDNode *SMD : ScopeNodes) 166 if (!NANodes.count(SMD)) { 167 FoundAll = false; 168 break; 169 } 170 171 if (FoundAll) 172 return false; 173 } 174 175 return true; 176 } 177 178 AnalysisKey ScopedNoAliasAA::Key; 179 180 ScopedNoAliasAAResult ScopedNoAliasAA::run(Function &F, 181 FunctionAnalysisManager &AM) { 182 return ScopedNoAliasAAResult(); 183 } 184 185 char ScopedNoAliasAAWrapperPass::ID = 0; 186 187 INITIALIZE_PASS(ScopedNoAliasAAWrapperPass, "scoped-noalias", 188 "Scoped NoAlias Alias Analysis", false, true) 189 190 ImmutablePass *llvm::createScopedNoAliasAAWrapperPass() { 191 return new ScopedNoAliasAAWrapperPass(); 192 } 193 194 ScopedNoAliasAAWrapperPass::ScopedNoAliasAAWrapperPass() : ImmutablePass(ID) { 195 initializeScopedNoAliasAAWrapperPassPass(*PassRegistry::getPassRegistry()); 196 } 197 198 bool ScopedNoAliasAAWrapperPass::doInitialization(Module &M) { 199 Result.reset(new ScopedNoAliasAAResult()); 200 return false; 201 } 202 203 bool ScopedNoAliasAAWrapperPass::doFinalization(Module &M) { 204 Result.reset(); 205 return false; 206 } 207 208 void ScopedNoAliasAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const { 209 AU.setPreservesAll(); 210 } 211