llvm/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/SetOperations.h"
  36 #include "llvm/ADT/SmallPtrSet.h"
  37 #include "llvm/Analysis/MemoryLocation.h"
  38 #include "llvm/IR/InstrTypes.h"
  39 #include "llvm/IR/Instruction.h"
  40 #include "llvm/IR/LLVMContext.h"
  41 #include "llvm/IR/Metadata.h"
  42 #include "llvm/InitializePasses.h"
  43 #include "llvm/Pass.h"
  44 #include "llvm/Support/Casting.h"
  45 #include "llvm/Support/CommandLine.h"
  46
  47 using namespace llvm;
  48
  49 // A handy option for disabling scoped no-alias functionality. The same effect
  50 // can also be achieved by stripping the associated metadata tags from IR, but
  51 // this option is sometimes more convenient.
  52 static cl::opt<bool> EnableScopedNoAlias("enable-scoped-noalias",
  53                                          cl::init(true), cl::Hidden);
  54
  55 AliasResult ScopedNoAliasAAResult::alias(const MemoryLocation &LocA,
  56                                          const MemoryLocation &LocB,
  57                                          AAQueryInfo &AAQI) {
  58   if (!EnableScopedNoAlias)
  59     return AAResultBase::alias(LocA, LocB, AAQI);
  60
  61   // Get the attached MDNodes.
  62   const MDNode *AScopes = LocA.AATags.Scope, *BScopes = LocB.AATags.Scope;
  63
  64   const MDNode *ANoAlias = LocA.AATags.NoAlias, *BNoAlias = LocB.AATags.NoAlias;
  65
  66   if (!mayAliasInScopes(AScopes, BNoAlias))
  67     return AliasResult::NoAlias;
  68
  69   if (!mayAliasInScopes(BScopes, ANoAlias))
  70     return AliasResult::NoAlias;
  71
  72   // If they may alias, chain to the next AliasAnalysis.
  73   return AAResultBase::alias(LocA, LocB, AAQI);
  74 }
  75
  76 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call,
  77                                                 const MemoryLocation &Loc,
  78                                                 AAQueryInfo &AAQI) {
  79   if (!EnableScopedNoAlias)
  80     return AAResultBase::getModRefInfo(Call, Loc, AAQI);
  81
  82   if (!mayAliasInScopes(Loc.AATags.Scope,
  83                         Call->getMetadata(LLVMContext::MD_noalias)))
  84     return ModRefInfo::NoModRef;
  85
  86   if (!mayAliasInScopes(Call->getMetadata(LLVMContext::MD_alias_scope),
  87                         Loc.AATags.NoAlias))
  88     return ModRefInfo::NoModRef;
  89
  90   return AAResultBase::getModRefInfo(Call, Loc, AAQI);
  91 }
  92
  93 ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call1,
  94                                                 const CallBase *Call2,
  95                                                 AAQueryInfo &AAQI) {
  96   if (!EnableScopedNoAlias)
  97     return AAResultBase::getModRefInfo(Call1, Call2, AAQI);
  98
  99   if (!mayAliasInScopes(Call1->getMetadata(LLVMContext::MD_alias_scope),
 100                         Call2->getMetadata(LLVMContext::MD_noalias)))
 101     return ModRefInfo::NoModRef;
 102
 103   if (!mayAliasInScopes(Call2->getMetadata(LLVMContext::MD_alias_scope),
 104                         Call1->getMetadata(LLVMContext::MD_noalias)))
 105     return ModRefInfo::NoModRef;
 106
 107   return AAResultBase::getModRefInfo(Call1, Call2, AAQI);
 108 }
 109
 110 static void collectMDInDomain(const MDNode *List, const MDNode *Domain,
 111                               SmallPtrSetImpl<const MDNode *> &Nodes) {
 112   for (const MDOperand &MDOp : List->operands())
 113     if (const MDNode *MD = dyn_cast<MDNode>(MDOp))
 114       if (AliasScopeNode(MD).getDomain() == Domain)
 115         Nodes.insert(MD);
 116 }
 117
 118 bool ScopedNoAliasAAResult::mayAliasInScopes(const MDNode *Scopes,
 119                                              const MDNode *NoAlias) const {
 120   if (!Scopes || !NoAlias)
 121     return true;
 122
 123   // Collect the set of scope domains relevant to the noalias scopes.
 124   SmallPtrSet<const MDNode *, 16> Domains;
 125   for (const MDOperand &MDOp : NoAlias->operands())
 126     if (const MDNode *NAMD = dyn_cast<MDNode>(MDOp))
 127       if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain())
 128         Domains.insert(Domain);
 129
 130   // We alias unless, for some domain, the set of noalias scopes in that domain
 131   // is a superset of the set of alias scopes in that domain.
 132   for (const MDNode *Domain : Domains) {
 133     SmallPtrSet<const MDNode *, 16> ScopeNodes;
 134     collectMDInDomain(Scopes, Domain, ScopeNodes);
 135     if (ScopeNodes.empty())
 136       continue;
 137
 138     SmallPtrSet<const MDNode *, 16> NANodes;
 139     collectMDInDomain(NoAlias, Domain, NANodes);
 140
 141     // To not alias, all of the nodes in ScopeNodes must be in NANodes.
 142     if (llvm::set_is_subset(ScopeNodes, NANodes))
 143       return false;
 144   }
 145
 146   return true;
 147 }
 148
 149 AnalysisKey ScopedNoAliasAA::Key;
 150
 151 ScopedNoAliasAAResult ScopedNoAliasAA::run(Function &F,
 152                                            FunctionAnalysisManager &AM) {
 153   return ScopedNoAliasAAResult();
 154 }
 155
 156 char ScopedNoAliasAAWrapperPass::ID = 0;
 157
 158 INITIALIZE_PASS(ScopedNoAliasAAWrapperPass, "scoped-noalias-aa",
 159                 "Scoped NoAlias Alias Analysis", false, true)
 160
 161 ImmutablePass *llvm::createScopedNoAliasAAWrapperPass() {
 162   return new ScopedNoAliasAAWrapperPass();
 163 }
 164
 165 ScopedNoAliasAAWrapperPass::ScopedNoAliasAAWrapperPass() : ImmutablePass(ID) {
 166   initializeScopedNoAliasAAWrapperPassPass(*PassRegistry::getPassRegistry());
 167 }
 168
 169 bool ScopedNoAliasAAWrapperPass::doInitialization(Module &M) {
 170   Result.reset(new ScopedNoAliasAAResult());
 171   return false;
 172 }
 173
 174 bool ScopedNoAliasAAWrapperPass::doFinalization(Module &M) {
 175   Result.reset();
 176   return false;
 177 }
 178
 179 void ScopedNoAliasAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
 180   AU.setPreservesAll();
 181 }