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