Home | History | Annotate | Line # | Download | only in Analysis 
      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 }
    182