lib/Analysis/ScopedNoAliasAA.cpp

   1 //===- ScopedNoAliasAA.cpp - Scoped No-Alias Alias Analysis ---------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file defines the ScopedNoAlias alias-analysis pass, which implements
  11 // metadata-based scoped no-alias support.
  12 //
  13 // Alias-analysis scopes are defined by an id (which can be a string or some
  14 // other metadata node), a domain node, and an optional descriptive string.
  15 // A domain is defined by an id (which can be a string or some other metadata
  16 // node), and an optional descriptive string.
  17 //
  18 // !dom0 =   metadata !{ metadata !"domain of foo()" }
  19 // !scope1 = metadata !{ metadata !scope1, metadata !dom0, metadata !"scope 1" }
  20 // !scope2 = metadata !{ metadata !scope2, metadata !dom0, metadata !"scope 2" }
  21 //
  22 // Loads and stores can be tagged with an alias-analysis scope, and also, with
  23 // a noalias tag for a specific scope:
  24 //
  25 // ... = load %ptr1, !alias.scope !{ !scope1 }
  26 // ... = load %ptr2, !alias.scope !{ !scope1, !scope2 }, !noalias !{ !scope1 }
  27 //
  28 // When evaluating an aliasing query, if one of the instructions is associated
  29 // has a set of noalias scopes in some domain that is a superset of the alias
  30 // scopes in that domain of some other instruction, then the two memory
  31 // accesses are assumed not to alias.
  32 //
  33 //===----------------------------------------------------------------------===//
  34
  35 #include "llvm/Analysis/ScopedNoAliasAA.h"
  36 #include "llvm/ADT/SmallPtrSet.h"
  37 #include "llvm/Analysis/MemoryLocation.h"
  38 #include "llvm/IR/Instruction.h"
  39 #include "llvm/IR/LLVMContext.h"
  40 #include "llvm/IR/Metadata.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   if (!EnableScopedNoAlias)
  81     return AAResultBase::alias(LocA, LocB);
  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);
  96 }
  97
  98 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(ImmutableCallSite CS,
  99                                                 const MemoryLocation &Loc) {
 100   if (!EnableScopedNoAlias)
 101     return AAResultBase::getModRefInfo(CS, Loc);
 102
 103   if (!mayAliasInScopes(Loc.AATags.Scope, CS.getInstruction()->getMetadata(
 104                                               LLVMContext::MD_noalias)))
 105     return ModRefInfo::NoModRef;
 106
 107   if (!mayAliasInScopes(
 108           CS.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
 109           Loc.AATags.NoAlias))
 110     return ModRefInfo::NoModRef;
 111
 112   return AAResultBase::getModRefInfo(CS, Loc);
 113 }
 114
 115 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(ImmutableCallSite CS1,
 116                                                 ImmutableCallSite CS2) {
 117   if (!EnableScopedNoAlias)
 118     return AAResultBase::getModRefInfo(CS1, CS2);
 119
 120   if (!mayAliasInScopes(
 121           CS1.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
 122           CS2.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
 123     return ModRefInfo::NoModRef;
 124
 125   if (!mayAliasInScopes(
 126           CS2.getInstruction()->getMetadata(LLVMContext::MD_alias_scope),
 127           CS1.getInstruction()->getMetadata(LLVMContext::MD_noalias)))
 128     return ModRefInfo::NoModRef;
 129
 130   return AAResultBase::getModRefInfo(CS1, CS2);
 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 }