lib/Analysis/ScopedNoAliasAA.cpp

   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 }