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; 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 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 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 } 115 116 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 } 132 133 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