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   if (!EnableScopedNoAlias)
  80     return AAResultBase::alias(LocA, LocB);
  81
  82   // Get the attached MDNodes.
  83   const MDNode *AScopes = LocA.AATags.Scope, *BScopes = LocB.AATags.Scope;
  84
  85   const MDNode *ANoAlias = LocA.AATags.NoAlias, *BNoAlias = LocB.AATags.NoAlias;
  86
  87   if (!mayAliasInScopes(AScopes, BNoAlias))
  88     return NoAlias;
  89
  90   if (!mayAliasInScopes(BScopes, ANoAlias))
  91     return NoAlias;
  92
  93   // If they may alias, chain to the next AliasAnalysis.
  94   return AAResultBase::alias(LocA, LocB);
  95 }
  96
  97 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call,
  98                                                 const MemoryLocation &Loc) {
  99   if (!EnableScopedNoAlias)
 100     return AAResultBase::getModRefInfo(Call, Loc);
 101
 102   if (!mayAliasInScopes(Loc.AATags.Scope,
 103                         Call->getMetadata(LLVMContext::MD_noalias)))
 104     return ModRefInfo::NoModRef;
 105
 106   if (!mayAliasInScopes(Call->getMetadata(LLVMContext::MD_alias_scope),
 107                         Loc.AATags.NoAlias))
 108     return ModRefInfo::NoModRef;
 109
 110   return AAResultBase::getModRefInfo(Call, Loc);
 111 }
 112
 113 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call1,
 114                                                 const CallBase *Call2) {
 115   if (!EnableScopedNoAlias)
 116     return AAResultBase::getModRefInfo(Call1, Call2);
 117
 118   if (!mayAliasInScopes(Call1->getMetadata(LLVMContext::MD_alias_scope),
 119                         Call2->getMetadata(LLVMContext::MD_noalias)))
 120     return ModRefInfo::NoModRef;
 121
 122   if (!mayAliasInScopes(Call2->getMetadata(LLVMContext::MD_alias_scope),
 123                         Call1->getMetadata(LLVMContext::MD_noalias)))
 124     return ModRefInfo::NoModRef;
 125
 126   return AAResultBase::getModRefInfo(Call1, Call2);
 127 }
 128
 129 static void collectMDInDomain(const MDNode *List, const MDNode *Domain,
 130                               SmallPtrSetImpl<const MDNode *> &Nodes) {
 131   for (const MDOperand &MDOp : List->operands())
 132     if (const MDNode *MD = dyn_cast<MDNode>(MDOp))
 133       if (AliasScopeNode(MD).getDomain() == Domain)
 134         Nodes.insert(MD);
 135 }
 136
 137 bool ScopedNoAliasAAResult::mayAliasInScopes(const MDNode *Scopes,
 138                                              const MDNode *NoAlias) const {
 139   if (!Scopes || !NoAlias)
 140     return true;
 141
 142   // Collect the set of scope domains relevant to the noalias scopes.
 143   SmallPtrSet<const MDNode *, 16> Domains;
 144   for (const MDOperand &MDOp : NoAlias->operands())
 145     if (const MDNode *NAMD = dyn_cast<MDNode>(MDOp))
 146       if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain())
 147         Domains.insert(Domain);
 148
 149   // We alias unless, for some domain, the set of noalias scopes in that domain
 150   // is a superset of the set of alias scopes in that domain.
 151   for (const MDNode *Domain : Domains) {
 152     SmallPtrSet<const MDNode *, 16> ScopeNodes;
 153     collectMDInDomain(Scopes, Domain, ScopeNodes);
 154     if (ScopeNodes.empty())
 155       continue;
 156
 157     SmallPtrSet<const MDNode *, 16> NANodes;
 158     collectMDInDomain(NoAlias, Domain, NANodes);
 159
 160     // To not alias, all of the nodes in ScopeNodes must be in NANodes.
 161     bool FoundAll = true;
 162     for (const MDNode *SMD : ScopeNodes)
 163       if (!NANodes.count(SMD)) {
 164         FoundAll = false;
 165         break;
 166       }
 167
 168     if (FoundAll)
 169       return false;
 170   }
 171
 172   return true;
 173 }
 174
 175 AnalysisKey ScopedNoAliasAA::Key;
 176
 177 ScopedNoAliasAAResult ScopedNoAliasAA::run(Function &F,
 178                                            FunctionAnalysisManager &AM) {
 179   return ScopedNoAliasAAResult();
 180 }
 181
 182 char ScopedNoAliasAAWrapperPass::ID = 0;
 183
 184 INITIALIZE_PASS(ScopedNoAliasAAWrapperPass, "scoped-noalias",
 185                 "Scoped NoAlias Alias Analysis", false, true)
 186
 187 ImmutablePass *llvm::createScopedNoAliasAAWrapperPass() {
 188   return new ScopedNoAliasAAWrapperPass();
 189 }
 190
 191 ScopedNoAliasAAWrapperPass::ScopedNoAliasAAWrapperPass() : ImmutablePass(ID) {
 192   initializeScopedNoAliasAAWrapperPassPass(*PassRegistry::getPassRegistry());
 193 }
 194
 195 bool ScopedNoAliasAAWrapperPass::doInitialization(Module &M) {
 196   Result.reset(new ScopedNoAliasAAResult());
 197   return false;
 198 }
 199
 200 bool ScopedNoAliasAAWrapperPass::doFinalization(Module &M) {
 201   Result.reset();
 202   return false;
 203 }
 204
 205 void ScopedNoAliasAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
 206   AU.setPreservesAll();
 207 }