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//===- llvm/Analysis/AliasSetTracker.h - Build Alias Sets -------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file defines two classes: AliasSetTracker and AliasSet. These interfaces
// are used to classify a collection of pointer references into a maximal number
// of disjoint sets. Each AliasSet object constructed by the AliasSetTracker
// object refers to memory disjoint from the other sets.
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseMapInfo.h"
#include "llvm/ADT/ilist.h"
#include "llvm/ADT/ilist_node.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Support/Casting.h"
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <vector>
namespace llvm {
class AliasSetTracker;
class BasicBlock;
class LoadInst;
class Loop;
class MemorySSA;
class AnyMemSetInst;
class AnyMemTransferInst;
class raw_ostream;
class StoreInst;
class VAArgInst;
class Value;
class AliasSet : public ilist_node<AliasSet> {
friend class AliasSetTracker;
class PointerRec {
Value *Val; // The pointer this record corresponds to.
PointerRec **PrevInList = nullptr;
PointerRec *NextInList = nullptr;
AliasSet *AS = nullptr;
LocationSize Size = LocationSize::mapEmpty();
AAMDNodes AAInfo;
// Whether the size for this record has been set at all. This makes no
// guarantees about the size being known.
bool isSizeSet() const { return Size != LocationSize::mapEmpty(); }
PointerRec(Value *V)
: Val(V), AAInfo(DenseMapInfo<AAMDNodes>::getEmptyKey()) {}
Value *getValue() const { return Val; }
PointerRec *getNext() const { return NextInList; }
bool hasAliasSet() const { return AS != nullptr; }
PointerRec** setPrevInList(PointerRec **PIL) {
PrevInList = PIL;
return &NextInList;
bool updateSizeAndAAInfo(LocationSize NewSize, const AAMDNodes &NewAAInfo) {
bool SizeChanged = false;
if (NewSize != Size) {
LocationSize OldSize = Size;
Size = isSizeSet() ? Size.unionWith(NewSize) : NewSize;
SizeChanged = OldSize != Size;
if (AAInfo == DenseMapInfo<AAMDNodes>::getEmptyKey())
// We don't have a AAInfo yet. Set it to NewAAInfo.
AAInfo = NewAAInfo;
else {
AAMDNodes Intersection(AAInfo.intersect(NewAAInfo));
if (!Intersection) {
// NewAAInfo conflicts with AAInfo.
AAInfo = DenseMapInfo<AAMDNodes>::getTombstoneKey();
return SizeChanged;
AAInfo = Intersection;
return SizeChanged;
LocationSize getSize() const {
assert(isSizeSet() && "Getting an unset size!");
return Size;
/// Return the AAInfo, or null if there is no information or conflicting
/// information.
AAMDNodes getAAInfo() const {
// If we have missing or conflicting AAInfo, return null.
if (AAInfo == DenseMapInfo<AAMDNodes>::getEmptyKey() ||
AAInfo == DenseMapInfo<AAMDNodes>::getTombstoneKey())
return AAMDNodes();
return AAInfo;
AliasSet *getAliasSet(AliasSetTracker &AST) {
assert(AS && "No AliasSet yet!");
if (AS->Forward) {
AliasSet *OldAS = AS;
AS = OldAS->getForwardedTarget(AST);
return AS;
void setAliasSet(AliasSet *as) {
assert(!AS && "Already have an alias set!");
AS = as;
void eraseFromList() {
if (NextInList) NextInList->PrevInList = PrevInList;
*PrevInList = NextInList;
if (AS->PtrListEnd == &NextInList) {
AS->PtrListEnd = PrevInList;
assert(*AS->PtrListEnd == nullptr && "List not terminated right!");
delete this;
// Doubly linked list of nodes.
PointerRec *PtrList = nullptr;
PointerRec **PtrListEnd;
// Forwarding pointer.
AliasSet *Forward = nullptr;
/// All instructions without a specific address in this alias set.
/// In rare cases this vector can have a null'ed out WeakVH
/// instances (can happen if some other loop pass deletes an
/// instruction in this list).
std::vector<WeakVH> UnknownInsts;
/// Number of nodes pointing to this AliasSet plus the number of AliasSets
/// forwarding to it.
unsigned RefCount : 27;
// Signifies that this set should be considered to alias any pointer.
// Use when the tracker holding this set is saturated.
unsigned AliasAny : 1;
/// The kinds of access this alias set models.
/// We keep track of whether this alias set merely refers to the locations of
/// memory (and not any particular access), whether it modifies or references
/// the memory, or whether it does both. The lattice goes from "NoAccess" to
/// either RefAccess or ModAccess, then to ModRefAccess as necessary.
enum AccessLattice {
NoAccess = 0,
RefAccess = 1,
ModAccess = 2,
ModRefAccess = RefAccess | ModAccess
unsigned Access : 2;
/// The kind of alias relationship between pointers of the set.
/// These represent conservatively correct alias results between any members
/// of the set. We represent these independently of the values of alias
/// results in order to pack it into a single bit. Lattice goes from
/// MustAlias to MayAlias.
enum AliasLattice {
SetMustAlias = 0, SetMayAlias = 1
unsigned Alias : 1;
unsigned SetSize = 0;
void addRef() { ++RefCount; }
void dropRef(AliasSetTracker &AST) {
assert(RefCount >= 1 && "Invalid reference count detected!");
if (--RefCount == 0)
Instruction *getUnknownInst(unsigned i) const {
assert(i < UnknownInsts.size());
return cast_or_null<Instruction>(UnknownInsts[i]);
AliasSet(const AliasSet &) = delete;
AliasSet &operator=(const AliasSet &) = delete;
/// Accessors...
bool isRef() const { return Access & RefAccess; }
bool isMod() const { return Access & ModAccess; }
bool isMustAlias() const { return Alias == SetMustAlias; }
bool isMayAlias() const { return Alias == SetMayAlias; }
/// Return true if this alias set should be ignored as part of the
/// AliasSetTracker object.
bool isForwardingAliasSet() const { return Forward; }
/// Merge the specified alias set into this alias set.
void mergeSetIn(AliasSet &AS, AliasSetTracker &AST);
// Alias Set iteration - Allow access to all of the pointers which are part of
// this alias set.
class iterator;
iterator begin() const { return iterator(PtrList); }
iterator end() const { return iterator(); }
bool empty() const { return PtrList == nullptr; }
// Unfortunately, ilist::size() is linear, so we have to add code to keep
// track of the list's exact size.
unsigned size() { return SetSize; }
/// If this alias set is known to contain a single instruction and *only* a
/// single unique instruction, return it. Otherwise, return nullptr.
Instruction* getUniqueInstruction();
void print(raw_ostream &OS) const;
void dump() const;
/// Define an iterator for alias sets... this is just a forward iterator.
class iterator : public std::iterator<std::forward_iterator_tag,
PointerRec, ptrdiff_t> {
PointerRec *CurNode;
explicit iterator(PointerRec *CN = nullptr) : CurNode(CN) {}
bool operator==(const iterator& x) const {
return CurNode == x.CurNode;
bool operator!=(const iterator& x) const { return !operator==(x); }
value_type &operator*() const {
assert(CurNode && "Dereferencing AliasSet.end()!");
return *CurNode;
value_type *operator->() const { return &operator*(); }
Value *getPointer() const { return CurNode->getValue(); }
LocationSize getSize() const { return CurNode->getSize(); }
AAMDNodes getAAInfo() const { return CurNode->getAAInfo(); }
iterator& operator++() { // Preincrement
assert(CurNode && "Advancing past AliasSet.end()!");
CurNode = CurNode->getNext();
return *this;
iterator operator++(int) { // Postincrement
iterator tmp = *this; ++*this; return tmp;
// Can only be created by AliasSetTracker.
: PtrListEnd(&PtrList), RefCount(0), AliasAny(false), Access(NoAccess),
Alias(SetMustAlias) {}
PointerRec *getSomePointer() const {
return PtrList;
/// Return the real alias set this represents. If this has been merged with
/// another set and is forwarding, return the ultimate destination set. This
/// also implements the union-find collapsing as well.
AliasSet *getForwardedTarget(AliasSetTracker &AST) {
if (!Forward) return this;
AliasSet *Dest = Forward->getForwardedTarget(AST);
if (Dest != Forward) {
Forward = Dest;
return Dest;
void removeFromTracker(AliasSetTracker &AST);
void addPointer(AliasSetTracker &AST, PointerRec &Entry, LocationSize Size,
const AAMDNodes &AAInfo, bool KnownMustAlias = false,
bool SkipSizeUpdate = false);
void addUnknownInst(Instruction *I, AliasAnalysis &AA);
void removeUnknownInst(AliasSetTracker &AST, Instruction *I) {
bool WasEmpty = UnknownInsts.empty();
for (size_t i = 0, e = UnknownInsts.size(); i != e; ++i)
if (UnknownInsts[i] == I) {
UnknownInsts[i] = UnknownInsts.back();
--i; --e; // Revisit the moved entry.
if (!WasEmpty && UnknownInsts.empty())
/// If the specified pointer "may" (or must) alias one of the members in the
/// set return the appropriate AliasResult. Otherwise return NoAlias.
AliasResult aliasesPointer(const Value *Ptr, LocationSize Size,
const AAMDNodes &AAInfo, AliasAnalysis &AA) const;
bool aliasesUnknownInst(const Instruction *Inst, AliasAnalysis &AA) const;
inline raw_ostream& operator<<(raw_ostream &OS, const AliasSet &AS) {
return OS;
class AliasSetTracker {
/// A CallbackVH to arrange for AliasSetTracker to be notified whenever a
/// Value is deleted.
class ASTCallbackVH final : public CallbackVH {
AliasSetTracker *AST;
void deleted() override;
void allUsesReplacedWith(Value *) override;
ASTCallbackVH(Value *V, AliasSetTracker *AST = nullptr);
ASTCallbackVH &operator=(Value *V);
/// Traits to tell DenseMap that tell us how to compare and hash the value
/// handle.
struct ASTCallbackVHDenseMapInfo : public DenseMapInfo<Value *> {};
AliasAnalysis &AA;
MemorySSA *MSSA;
Loop *L;
ilist<AliasSet> AliasSets;
using PointerMapType = DenseMap<ASTCallbackVH, AliasSet::PointerRec *,
// Map from pointers to their node
PointerMapType PointerMap;
/// Create an empty collection of AliasSets, and use the specified alias
/// analysis object to disambiguate load and store addresses.
explicit AliasSetTracker(AliasAnalysis &aa) : AA(aa) {}
explicit AliasSetTracker(AliasAnalysis &aa, MemorySSA *mssa, Loop *l)
: AA(aa), MSSA(mssa), L(l) {}
~AliasSetTracker() { clear(); }
/// These methods are used to add different types of instructions to the alias
/// sets. Adding a new instruction can result in one of three actions
/// happening:
/// 1. If the instruction doesn't alias any other sets, create a new set.
/// 2. If the instruction aliases exactly one set, add it to the set
/// 3. If the instruction aliases multiple sets, merge the sets, and add
/// the instruction to the result.
/// These methods return true if inserting the instruction resulted in the
/// addition of a new alias set (i.e., the pointer did not alias anything).
void add(Value *Ptr, LocationSize Size, const AAMDNodes &AAInfo); // Add a loc
void add(LoadInst *LI);
void add(StoreInst *SI);
void add(VAArgInst *VAAI);
void add(AnyMemSetInst *MSI);
void add(AnyMemTransferInst *MTI);
void add(Instruction *I); // Dispatch to one of the other add methods...
void add(BasicBlock &BB); // Add all instructions in basic block
void add(const AliasSetTracker &AST); // Add alias relations from another AST
void addUnknown(Instruction *I);
void addAllInstructionsInLoopUsingMSSA();
void clear();
/// Return the alias sets that are active.
const ilist<AliasSet> &getAliasSets() const { return AliasSets; }
/// Return the alias set which contains the specified memory location. If
/// the memory location aliases two or more existing alias sets, will have
/// the effect of merging those alias sets before the single resulting alias
/// set is returned.
AliasSet &getAliasSetFor(const MemoryLocation &MemLoc);
/// Return the underlying alias analysis object used by this tracker.
AliasAnalysis &getAliasAnalysis() const { return AA; }
/// This method is used to remove a pointer value from the AliasSetTracker
/// entirely. It should be used when an instruction is deleted from the
/// program to update the AST. If you don't use this, you would have dangling
/// pointers to deleted instructions.
void deleteValue(Value *PtrVal);
/// This method should be used whenever a preexisting value in the program is
/// copied or cloned, introducing a new value. Note that it is ok for clients
/// that use this method to introduce the same value multiple times: if the
/// tracker already knows about a value, it will ignore the request.
void copyValue(Value *From, Value *To);
using iterator = ilist<AliasSet>::iterator;
using const_iterator = ilist<AliasSet>::const_iterator;
const_iterator begin() const { return AliasSets.begin(); }
const_iterator end() const { return AliasSets.end(); }
iterator begin() { return AliasSets.begin(); }
iterator end() { return AliasSets.end(); }
void print(raw_ostream &OS) const;
void dump() const;
friend class AliasSet;
// The total number of pointers contained in all "may" alias sets.
unsigned TotalMayAliasSetSize = 0;
// A non-null value signifies this AST is saturated. A saturated AST lumps
// all pointers into a single "May" set.
AliasSet *AliasAnyAS = nullptr;
void removeAliasSet(AliasSet *AS);
/// Just like operator[] on the map, except that it creates an entry for the
/// pointer if it doesn't already exist.
AliasSet::PointerRec &getEntryFor(Value *V) {
AliasSet::PointerRec *&Entry = PointerMap[ASTCallbackVH(V, this)];
if (!Entry)
Entry = new AliasSet::PointerRec(V);
return *Entry;
AliasSet &addPointer(MemoryLocation Loc, AliasSet::AccessLattice E);
AliasSet *mergeAliasSetsForPointer(const Value *Ptr, LocationSize Size,
const AAMDNodes &AAInfo,
bool &MustAliasAll);
/// Merge all alias sets into a single set that is considered to alias any
/// pointer.
AliasSet &mergeAllAliasSets();
AliasSet *findAliasSetForUnknownInst(Instruction *Inst);
inline raw_ostream& operator<<(raw_ostream &OS, const AliasSetTracker &AST) {
return OS;
} // end namespace llvm