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//===- ScopedNoAliasAA.cpp - Scoped No-Alias Alias Analysis ---------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// 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), and an optional descriptive string.
// !dom0 = metadata !{ metadata !"domain of foo()" }
// !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 is associated
// 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.
#include "llvm/Analysis/ScopedNoAliasAA.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.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);
namespace {
/// This is a simple wrapper around an MDNode which provides a higher-level
/// interface by hiding the details of how alias analysis information is encoded
/// in its operands.
class AliasScopeNode {
const MDNode *Node = nullptr;
AliasScopeNode() = default;
explicit AliasScopeNode(const MDNode *N) : Node(N) {}
/// Get the MDNode for this AliasScopeNode.
const MDNode *getNode() const { return Node; }
/// Get the MDNode for this AliasScopeNode's domain.
const MDNode *getDomain() const {
if (Node->getNumOperands() < 2)
return nullptr;
return dyn_cast_or_null<MDNode>(Node->getOperand(1));
} // end anonymous namespace
AliasResult ScopedNoAliasAAResult::alias(const MemoryLocation &LocA,
const MemoryLocation &LocB) {
if (!EnableScopedNoAlias)
return AAResultBase::alias(LocA, LocB);
// 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 NoAlias;
if (!mayAliasInScopes(BScopes, ANoAlias))
return NoAlias;
// If they may alias, chain to the next AliasAnalysis.
return AAResultBase::alias(LocA, LocB);
ModRefInfo ScopedNoAliasAAResult::getModRefInfo(ImmutableCallSite CS,
const MemoryLocation &Loc) {
if (!EnableScopedNoAlias)
return AAResultBase::getModRefInfo(CS, Loc);
if (!mayAliasInScopes(Loc.AATags.Scope, CS.getInstruction()->getMetadata(
return ModRefInfo::NoModRef;
if (!mayAliasInScopes(
return ModRefInfo::NoModRef;
return AAResultBase::getModRefInfo(CS, Loc);
ModRefInfo ScopedNoAliasAAResult::getModRefInfo(ImmutableCallSite CS1,
ImmutableCallSite CS2) {
if (!EnableScopedNoAlias)
return AAResultBase::getModRefInfo(CS1, CS2);
if (!mayAliasInScopes(
return ModRefInfo::NoModRef;
if (!mayAliasInScopes(
return ModRefInfo::NoModRef;
return AAResultBase::getModRefInfo(CS1, CS2);
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)
bool ScopedNoAliasAAResult::mayAliasInScopes(const MDNode *Scopes,
const MDNode *NoAlias) const {
if (!Scopes || !NoAlias)
return true;
// Collect the set of scope domains relevant to the noalias scopes.
SmallPtrSet<const MDNode *, 16> Domains;
for (const MDOperand &MDOp : NoAlias->operands())
if (const MDNode *NAMD = dyn_cast<MDNode>(MDOp))
if (const MDNode *Domain = AliasScopeNode(NAMD).getDomain())
// 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())
SmallPtrSet<const MDNode *, 16> NANodes;
collectMDInDomain(NoAlias, Domain, NANodes);
// To not alias, all of the nodes in ScopeNodes must be in NANodes.
bool FoundAll = true;
for (const MDNode *SMD : ScopeNodes)
if (!NANodes.count(SMD)) {
FoundAll = false;
if (FoundAll)
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",
"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) {
return false;
void ScopedNoAliasAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {