ScopedNoAliasAA.cpp

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//===- ScopedNoAliasAA.cpp - Scoped No-Alias Alias Analysis ---------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file defines the ScopedNoAlias alias-analysis pass, which implements
// metadata-based scoped no-alias support.
//
// Alias-analysis scopes are defined by an id (which can be a string or some
// other metadata node), a domain node, and an optional descriptive string.
// A domain is defined by an id (which can be a string or some other metadata
// node), a flag saying whether its scopes are disjoint, and an optional
// descriptive string.
//
// !dom0 =   metadata !{ metadata !"domain of foo()", i1 false }
// !scope1 = metadata !{ metadata !scope1, metadata !dom0, metadata !"scope 1" }
// !scope2 = metadata !{ metadata !scope2, metadata !dom0, metadata !"scope 2" }
//
// Loads and stores can be tagged with an alias-analysis scope, and also, with
// a noalias tag for a specific scope:
//
// ... = load %ptr1, !alias.scope !{ !scope1 }
// ... = load %ptr2, !alias.scope !{ !scope1, !scope2 }, !noalias !{ !scope1 }
//
// When evaluating an aliasing query, if one of the instructions
// has a set of noalias scopes in some domain that is a superset of the alias
// scopes in that domain of some other instruction, then the two memory
// accesses are assumed not to alias.
//
// If a domain is declared as having disjoint scopes, two memory accesses are
// assumed not to alias if the they both have entries for that domain in their
// `alias.scope` list and their `alias.scope` lists have no scopes in common for
// that domain. Equivalently, an instruction with a set of scopes from a
// disjoint-scope domain in its `alias.scope` list implicitly has all other
// scopes in that domain in its `noalias` set. For example,
//
// !dom1 =   metadata !{ metadata !dom1, i1 true, metadata !"disjoint domain" }
// !scope3 = metadata !{ metadata !scope3, metadata !dom1 }
// !scope4 = metadata !{ metadata !scope4, metadata !dom1 }
//
// ... = load %ptr3, !alias.scope !{ !scope3 } ; doesn't alias the load below
// ... = load %ptr4, !alias.scope !{ !scope4 }
//
//===----------------------------------------------------------------------===//
 
#include "llvm/Analysis/ScopedNoAliasAA.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
 
using namespace llvm;
 
// A handy option for disabling scoped no-alias functionality. The same effect
// can also be achieved by stripping the associated metadata tags from IR, but
// this option is sometimes more convenient.
static cl::opt<bool> EnableScopedNoAlias("enable-scoped-noalias",
                                         cl::init(true), cl::Hidden);
 
AliasResult ScopedNoAliasAAResult::alias(const MemoryLocation &LocA,
                                         const MemoryLocation &LocB) {
  if (!EnableScopedNoAlias)
    return AliasResult::MayAlias;
 
  // Get the attached MDNodes.
  const MDNode *AScopes = LocA.AATags.Scope, *BScopes = LocB.AATags.Scope;
 
  const MDNode *ANoAlias = LocA.AATags.NoAlias, *BNoAlias = LocB.AATags.NoAlias;
 
  if (!mayAliasInScopes(AScopes, BNoAlias))
    return AliasResult::NoAlias;
 
  if (!mayAliasInScopes(BScopes, ANoAlias))
    return AliasResult::NoAlias;
 
  if (!mayAliasInDisjointDomains(AScopes, BScopes))
    return AliasResult::NoAlias;
 
  return AliasResult::MayAlias;
}
 
AliasResult ScopedNoAliasAAResult::alias(const MemoryLocation &LocA,
                                         const MemoryLocation &LocB,
                                         AAQueryInfo &, const Instruction *) {
  return alias(LocA, LocB);
}
 
ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call,
                                                const MemoryLocation &Loc,
                                                AAQueryInfo &AAQI) {
  if (!EnableScopedNoAlias)
    return ModRefInfo::ModRef;
 
  if (!mayAliasInScopes(Loc.AATags.Scope,
                        Call->getMetadata(LLVMContext::MD_noalias)))
    return ModRefInfo::NoModRef;
 
  if (!mayAliasInScopes(Call->getMetadata(LLVMContext::MD_alias_scope),
                        Loc.AATags.NoAlias))
    return ModRefInfo::NoModRef;
 
  if (!mayAliasInDisjointDomains(
          Loc.AATags.Scope, Call->getMetadata(LLVMContext::MD_alias_scope)))
    return ModRefInfo::NoModRef;
 
  return ModRefInfo::ModRef;
}
 
ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const FenceInst *F,
                                                const MemoryLocation &Loc,
                                                AAQueryInfo &AAQI) {
  if (!EnableScopedNoAlias)
    return ModRefInfo::ModRef;
 
  if (!mayAliasInScopes(Loc.AATags.Scope,
                        F->getMetadata(LLVMContext::MD_noalias)))
    return ModRefInfo::NoModRef;
 
  if (!mayAliasInScopes(F->getMetadata(LLVMContext::MD_alias_scope),
                        Loc.AATags.NoAlias))
    return ModRefInfo::NoModRef;
 
  if (!mayAliasInDisjointDomains(Loc.AATags.Scope,
                                 F->getMetadata(LLVMContext::MD_alias_scope)))
    return ModRefInfo::NoModRef;
 
  return ModRefInfo::ModRef;
}
 
ModRefInfo ScopedNoAliasAAResult::getModRefInfo(const CallBase *Call1,
                                                const CallBase *Call2,
                                                AAQueryInfo &AAQI) {
  if (!EnableScopedNoAlias)
    return ModRefInfo::ModRef;
 
  if (!mayAliasInScopes(Call1->getMetadata(LLVMContext::MD_alias_scope),
                        Call2->getMetadata(LLVMContext::MD_noalias)))
    return ModRefInfo::NoModRef;
 
  if (!mayAliasInScopes(Call2->getMetadata(LLVMContext::MD_alias_scope),
                        Call1->getMetadata(LLVMContext::MD_noalias)))
    return ModRefInfo::NoModRef;
 
  if (!mayAliasInDisjointDomains(
          Call1->getMetadata(LLVMContext::MD_alias_scope),
          Call2->getMetadata(LLVMContext::MD_alias_scope)))
    return ModRefInfo::NoModRef;
 
  return ModRefInfo::ModRef;
}
 
static void collectMDInDomain(const MDNode *List, const MDNode *Domain,
                              SmallPtrSetImpl<const MDNode *> &Nodes) {
  for (const MDOperand &MDOp : List->operands())
    if (const MDNode *MD = dyn_cast<MDNode>(MDOp))
      if (AliasScopeNode(MD).getDomain() == Domain)
        Nodes.insert(MD);
}
 
/// Collect the set of scoped domains relevant to the noalias scopes.
void ScopedNoAliasAAResult::collectScopedDomains(
    const MDNode *NoAlias, SmallPtrSetImpl<const MDNode *> &Domains,
    bool DisjointOnly) {
  if (!NoAlias)
    return;
  assert(Domains.empty() && "Domains should be empty");
  for (const MDOperand &MDOp : NoAlias->operands())
    if (const MDNode *NAMD = dyn_cast<MDNode>(MDOp))
      if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain())
        if (!DisjointOnly || AliasScopeDomainNode(Domain).hasDisjointScopes())
          Domains.insert(Domain);
}
 
bool ScopedNoAliasAAResult::mayAliasInScopes(const MDNode *Scopes,
                                             const MDNode *NoAlias) {
  if (!Scopes || !NoAlias)
    return true;
 
  // Collect the set of scope domains relevant to the noalias scopes.
  SmallPtrSet<const MDNode *, 16> Domains;
  collectScopedDomains(NoAlias, Domains);
 
  // We alias unless, for some domain, the set of noalias scopes in that domain
  // is a superset of the set of alias scopes in that domain.
  for (const MDNode *Domain : Domains) {
    SmallPtrSet<const MDNode *, 16> ScopeNodes;
    collectMDInDomain(Scopes, Domain, ScopeNodes);
    if (ScopeNodes.empty())
      continue;
 
    SmallPtrSet<const MDNode *, 16> NANodes;
    collectMDInDomain(NoAlias, Domain, NANodes);
 
    // To not alias, all of the nodes in ScopeNodes must be in NANodes.
    if (llvm::set_is_subset(ScopeNodes, NANodes))
      return false;
  }
 
  return true;
}
 
bool ScopedNoAliasAAResult::mayAliasInDisjointDomains(const MDNode *Scopes1,
                                                      const MDNode *Scopes2) {
  if (!Scopes1 || !Scopes2)
    return true;
 
  // Collect the domains with disjoint scopes that the first access is in.
  SmallPtrSet<const MDNode *, 16> Domains;
  collectScopedDomains(Scopes1, Domains, /*DisjointOnly=*/true);
 
  // The accesses don't alias if, for some domain, both accesses use at least
  // one of its scopes and have no scopes in that domain in common.
  for (const MDNode *Domain : Domains) {
    SmallPtrSet<const MDNode *, 16> Nodes2;
    collectMDInDomain(Scopes2, Domain, Nodes2);
    if (Nodes2.empty())
      continue;
 
    SmallPtrSet<const MDNode *, 16> Nodes1;
    collectMDInDomain(Scopes1, Domain, Nodes1);
    if (!llvm::set_intersects(Nodes1, Nodes2))
      return false;
  }
 
  return true;
}
 
AnalysisKey ScopedNoAliasAA::Key;
 
ScopedNoAliasAAResult ScopedNoAliasAA::run(Function &F,
                                           FunctionAnalysisManager &AM) {
  return ScopedNoAliasAAResult();
}
 
char ScopedNoAliasAAWrapperPass::ID = 0;
 
INITIALIZE_PASS(ScopedNoAliasAAWrapperPass, "scoped-noalias-aa",
                "Scoped NoAlias Alias Analysis", false, true)
 
ImmutablePass *llvm::createScopedNoAliasAAWrapperPass() {
  return new ScopedNoAliasAAWrapperPass();
}
 
ScopedNoAliasAAWrapperPass::ScopedNoAliasAAWrapperPass() : ImmutablePass(ID) {}
 
bool ScopedNoAliasAAWrapperPass::doInitialization(Module &M) {
  Result.reset(new ScopedNoAliasAAResult());
  return false;
}
 
bool ScopedNoAliasAAWrapperPass::doFinalization(Module &M) {
  Result.reset();
  return false;
}
 
void ScopedNoAliasAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesAll();
}