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//===- MemorySSA.h - Build Memory SSA ---------------------------*- 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
/// \file
/// This file exposes an interface to building/using memory SSA to
/// walk memory instructions using a use/def graph.
/// Memory SSA class builds an SSA form that links together memory access
/// instructions such as loads, stores, atomics, and calls. Additionally, it
/// does a trivial form of "heap versioning" Every time the memory state changes
/// in the program, we generate a new heap version. It generates
/// MemoryDef/Uses/Phis that are overlayed on top of the existing instructions.
/// As a trivial example,
/// define i32 @main() #0 {
/// entry:
/// %call = call noalias i8* @_Znwm(i64 4) #2
/// %0 = bitcast i8* %call to i32*
/// %call1 = call noalias i8* @_Znwm(i64 4) #2
/// %1 = bitcast i8* %call1 to i32*
/// store i32 5, i32* %0, align 4
/// store i32 7, i32* %1, align 4
/// %2 = load i32* %0, align 4
/// %3 = load i32* %1, align 4
/// %add = add nsw i32 %2, %3
/// ret i32 %add
/// }
/// Will become
/// define i32 @main() #0 {
/// entry:
/// ; 1 = MemoryDef(0)
/// %call = call noalias i8* @_Znwm(i64 4) #3
/// %2 = bitcast i8* %call to i32*
/// ; 2 = MemoryDef(1)
/// %call1 = call noalias i8* @_Znwm(i64 4) #3
/// %4 = bitcast i8* %call1 to i32*
/// ; 3 = MemoryDef(2)
/// store i32 5, i32* %2, align 4
/// ; 4 = MemoryDef(3)
/// store i32 7, i32* %4, align 4
/// ; MemoryUse(3)
/// %7 = load i32* %2, align 4
/// ; MemoryUse(4)
/// %8 = load i32* %4, align 4
/// %add = add nsw i32 %7, %8
/// ret i32 %add
/// }
/// Given this form, all the stores that could ever effect the load at %8 can be
/// gotten by using the MemoryUse associated with it, and walking from use to
/// def until you hit the top of the function.
/// Each def also has a list of users associated with it, so you can walk from
/// both def to users, and users to defs. Note that we disambiguate MemoryUses,
/// but not the RHS of MemoryDefs. You can see this above at %7, which would
/// otherwise be a MemoryUse(4). Being disambiguated means that for a given
/// store, all the MemoryUses on its use lists are may-aliases of that store
/// (but the MemoryDefs on its use list may not be).
/// MemoryDefs are not disambiguated because it would require multiple reaching
/// definitions, which would require multiple phis, and multiple memoryaccesses
/// per instruction.
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/GraphTraits.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/ilist.h"
#include "llvm/ADT/ilist_node.h"
#include "llvm/ADT/iterator.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/ADT/simple_ilist.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/PHITransAddr.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/DerivedUser.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <iterator>
#include <memory>
#include <utility>
namespace llvm {
class Function;
class Instruction;
class MemoryAccess;
class MemorySSAWalker;
class LLVMContext;
class raw_ostream;
namespace MSSAHelpers {
struct AllAccessTag {};
struct DefsOnlyTag {};
} // end namespace MSSAHelpers
enum : unsigned {
// Used to signify what the default invalid ID is for MemoryAccess's
// getID()
template <class T> class memoryaccess_def_iterator_base;
using memoryaccess_def_iterator = memoryaccess_def_iterator_base<MemoryAccess>;
using const_memoryaccess_def_iterator =
memoryaccess_def_iterator_base<const MemoryAccess>;
// The base for all memory accesses. All memory accesses in a block are
// linked together using an intrusive list.
class MemoryAccess
: public DerivedUser,
public ilist_node<MemoryAccess, ilist_tag<MSSAHelpers::AllAccessTag>>,
public ilist_node<MemoryAccess, ilist_tag<MSSAHelpers::DefsOnlyTag>> {
using AllAccessType =
ilist_node<MemoryAccess, ilist_tag<MSSAHelpers::AllAccessTag>>;
using DefsOnlyType =
ilist_node<MemoryAccess, ilist_tag<MSSAHelpers::DefsOnlyTag>>;
MemoryAccess(const MemoryAccess &) = delete;
MemoryAccess &operator=(const MemoryAccess &) = delete;
void *operator new(size_t) = delete;
// Methods for support type inquiry through isa, cast, and
// dyn_cast
static bool classof(const Value *V) {
unsigned ID = V->getValueID();
return ID == MemoryUseVal || ID == MemoryPhiVal || ID == MemoryDefVal;
BasicBlock *getBlock() const { return Block; }
void print(raw_ostream &OS) const;
void dump() const;
/// The user iterators for a memory access
using iterator = user_iterator;
using const_iterator = const_user_iterator;
/// This iterator walks over all of the defs in a given
/// MemoryAccess. For MemoryPhi nodes, this walks arguments. For
/// MemoryUse/MemoryDef, this walks the defining access.
memoryaccess_def_iterator defs_begin();
const_memoryaccess_def_iterator defs_begin() const;
memoryaccess_def_iterator defs_end();
const_memoryaccess_def_iterator defs_end() const;
/// Get the iterators for the all access list and the defs only list
/// We default to the all access list.
AllAccessType::self_iterator getIterator() {
return this->AllAccessType::getIterator();
AllAccessType::const_self_iterator getIterator() const {
return this->AllAccessType::getIterator();
AllAccessType::reverse_self_iterator getReverseIterator() {
return this->AllAccessType::getReverseIterator();
AllAccessType::const_reverse_self_iterator getReverseIterator() const {
return this->AllAccessType::getReverseIterator();
DefsOnlyType::self_iterator getDefsIterator() {
return this->DefsOnlyType::getIterator();
DefsOnlyType::const_self_iterator getDefsIterator() const {
return this->DefsOnlyType::getIterator();
DefsOnlyType::reverse_self_iterator getReverseDefsIterator() {
return this->DefsOnlyType::getReverseIterator();
DefsOnlyType::const_reverse_self_iterator getReverseDefsIterator() const {
return this->DefsOnlyType::getReverseIterator();
friend class MemoryDef;
friend class MemoryPhi;
friend class MemorySSA;
friend class MemoryUse;
friend class MemoryUseOrDef;
/// Used by MemorySSA to change the block of a MemoryAccess when it is
/// moved.
void setBlock(BasicBlock *BB) { Block = BB; }
/// Used for debugging and tracking things about MemoryAccesses.
/// Guaranteed unique among MemoryAccesses, no guarantees otherwise.
inline unsigned getID() const;
MemoryAccess(LLVMContext &C, unsigned Vty, DeleteValueTy DeleteValue,
BasicBlock *BB, unsigned NumOperands)
: DerivedUser(Type::getVoidTy(C), Vty, nullptr, NumOperands, DeleteValue),
Block(BB) {}
// Use deleteValue() to delete a generic MemoryAccess.
~MemoryAccess() = default;
BasicBlock *Block;
template <>
struct ilist_alloc_traits<MemoryAccess> {
static void deleteNode(MemoryAccess *MA) { MA->deleteValue(); }
inline raw_ostream &operator<<(raw_ostream &OS, const MemoryAccess &MA) {
return OS;
/// Class that has the common methods + fields of memory uses/defs. It's
/// a little awkward to have, but there are many cases where we want either a
/// use or def, and there are many cases where uses are needed (defs aren't
/// acceptable), and vice-versa.
/// This class should never be instantiated directly; make a MemoryUse or
/// MemoryDef instead.
class MemoryUseOrDef : public MemoryAccess {
void *operator new(size_t) = delete;
/// Get the instruction that this MemoryUse represents.
Instruction *getMemoryInst() const { return MemoryInstruction; }
/// Get the access that produces the memory state used by this Use.
MemoryAccess *getDefiningAccess() const { return getOperand(0); }
static bool classof(const Value *MA) {
return MA->getValueID() == MemoryUseVal || MA->getValueID() == MemoryDefVal;
// Sadly, these have to be public because they are needed in some of the
// iterators.
inline bool isOptimized() const;
inline MemoryAccess *getOptimized() const;
inline void setOptimized(MemoryAccess *);
// Retrieve AliasResult type of the optimized access. Ideally this would be
// returned by the caching walker and may go away in the future.
Optional<AliasResult> getOptimizedAccessType() const {
return OptimizedAccessAlias;
/// Reset the ID of what this MemoryUse was optimized to, causing it to
/// be rewalked by the walker if necessary.
/// This really should only be called by tests.
inline void resetOptimized();
friend class MemorySSA;
friend class MemorySSAUpdater;
MemoryUseOrDef(LLVMContext &C, MemoryAccess *DMA, unsigned Vty,
DeleteValueTy DeleteValue, Instruction *MI, BasicBlock *BB,
unsigned NumOperands)
: MemoryAccess(C, Vty, DeleteValue, BB, NumOperands),
MemoryInstruction(MI), OptimizedAccessAlias(MayAlias) {
// Use deleteValue() to delete a generic MemoryUseOrDef.
~MemoryUseOrDef() = default;
void setOptimizedAccessType(Optional<AliasResult> AR) {
OptimizedAccessAlias = AR;
void setDefiningAccess(MemoryAccess *DMA, bool Optimized = false,
Optional<AliasResult> AR = MayAlias) {
if (!Optimized) {
setOperand(0, DMA);
Instruction *MemoryInstruction;
Optional<AliasResult> OptimizedAccessAlias;
/// Represents read-only accesses to memory
/// In particular, the set of Instructions that will be represented by
/// MemoryUse's is exactly the set of Instructions for which
/// AliasAnalysis::getModRefInfo returns "Ref".
class MemoryUse final : public MemoryUseOrDef {
MemoryUse(LLVMContext &C, MemoryAccess *DMA, Instruction *MI, BasicBlock *BB)
: MemoryUseOrDef(C, DMA, MemoryUseVal, deleteMe, MI, BB,
/*NumOperands=*/1) {}
// allocate space for exactly one operand
void *operator new(size_t s) { return User::operator new(s, 1); }
static bool classof(const Value *MA) {
return MA->getValueID() == MemoryUseVal;
void print(raw_ostream &OS) const;
void setOptimized(MemoryAccess *DMA) {
OptimizedID = DMA->getID();
setOperand(0, DMA);
bool isOptimized() const {
return getDefiningAccess() && OptimizedID == getDefiningAccess()->getID();
MemoryAccess *getOptimized() const {
return getDefiningAccess();
void resetOptimized() {
friend class MemorySSA;
static void deleteMe(DerivedUser *Self);
template <>
struct OperandTraits<MemoryUse> : public FixedNumOperandTraits<MemoryUse, 1> {};
/// Represents a read-write access to memory, whether it is a must-alias,
/// or a may-alias.
/// In particular, the set of Instructions that will be represented by
/// MemoryDef's is exactly the set of Instructions for which
/// AliasAnalysis::getModRefInfo returns "Mod" or "ModRef".
/// Note that, in order to provide def-def chains, all defs also have a use
/// associated with them. This use points to the nearest reaching
/// MemoryDef/MemoryPhi.
class MemoryDef final : public MemoryUseOrDef {
friend class MemorySSA;
MemoryDef(LLVMContext &C, MemoryAccess *DMA, Instruction *MI, BasicBlock *BB,
unsigned Ver)
: MemoryUseOrDef(C, DMA, MemoryDefVal, deleteMe, MI, BB,
ID(Ver) {}
// allocate space for exactly two operands
void *operator new(size_t s) { return User::operator new(s, 2); }
static bool classof(const Value *MA) {
return MA->getValueID() == MemoryDefVal;
void setOptimized(MemoryAccess *MA) {
setOperand(1, MA);
OptimizedID = MA->getID();
MemoryAccess *getOptimized() const {
return cast_or_null<MemoryAccess>(getOperand(1));
bool isOptimized() const {
return getOptimized() && OptimizedID == getOptimized()->getID();
void resetOptimized() {
setOperand(1, nullptr);
void print(raw_ostream &OS) const;
unsigned getID() const { return ID; }
static void deleteMe(DerivedUser *Self);
const unsigned ID;
template <>
struct OperandTraits<MemoryDef> : public FixedNumOperandTraits<MemoryDef, 2> {};
template <>
struct OperandTraits<MemoryUseOrDef> {
static Use *op_begin(MemoryUseOrDef *MUD) {
if (auto *MU = dyn_cast<MemoryUse>(MUD))
return OperandTraits<MemoryUse>::op_begin(MU);
return OperandTraits<MemoryDef>::op_begin(cast<MemoryDef>(MUD));
static Use *op_end(MemoryUseOrDef *MUD) {
if (auto *MU = dyn_cast<MemoryUse>(MUD))
return OperandTraits<MemoryUse>::op_end(MU);
return OperandTraits<MemoryDef>::op_end(cast<MemoryDef>(MUD));
static unsigned operands(const MemoryUseOrDef *MUD) {
if (const auto *MU = dyn_cast<MemoryUse>(MUD))
return OperandTraits<MemoryUse>::operands(MU);
return OperandTraits<MemoryDef>::operands(cast<MemoryDef>(MUD));
/// Represents phi nodes for memory accesses.
/// These have the same semantic as regular phi nodes, with the exception that
/// only one phi will ever exist in a given basic block.
/// Guaranteeing one phi per block means guaranteeing there is only ever one
/// valid reaching MemoryDef/MemoryPHI along each path to the phi node.
/// This is ensured by not allowing disambiguation of the RHS of a MemoryDef or
/// a MemoryPhi's operands.
/// That is, given
/// if (a) {
/// store %a
/// store %b
/// }
/// it *must* be transformed into
/// if (a) {
/// 1 = MemoryDef(liveOnEntry)
/// store %a
/// 2 = MemoryDef(1)
/// store %b
/// }
/// and *not*
/// if (a) {
/// 1 = MemoryDef(liveOnEntry)
/// store %a
/// 2 = MemoryDef(liveOnEntry)
/// store %b
/// }
/// even if the two stores do not conflict. Otherwise, both 1 and 2 reach the
/// end of the branch, and if there are not two phi nodes, one will be
/// disconnected completely from the SSA graph below that point.
/// Because MemoryUse's do not generate new definitions, they do not have this
/// issue.
class MemoryPhi final : public MemoryAccess {
// allocate space for exactly zero operands
void *operator new(size_t s) { return User::operator new(s); }
/// Provide fast operand accessors
MemoryPhi(LLVMContext &C, BasicBlock *BB, unsigned Ver, unsigned NumPreds = 0)
: MemoryAccess(C, MemoryPhiVal, deleteMe, BB, 0), ID(Ver),
ReservedSpace(NumPreds) {
// Block iterator interface. This provides access to the list of incoming
// basic blocks, which parallels the list of incoming values.
using block_iterator = BasicBlock **;
using const_block_iterator = BasicBlock *const *;
block_iterator block_begin() {
auto *Ref = reinterpret_cast<Use::UserRef *>(op_begin() + ReservedSpace);
return reinterpret_cast<block_iterator>(Ref + 1);
const_block_iterator block_begin() const {
const auto *Ref =
reinterpret_cast<const Use::UserRef *>(op_begin() + ReservedSpace);
return reinterpret_cast<const_block_iterator>(Ref + 1);
block_iterator block_end() { return block_begin() + getNumOperands(); }
const_block_iterator block_end() const {
return block_begin() + getNumOperands();
iterator_range<block_iterator> blocks() {
return make_range(block_begin(), block_end());
iterator_range<const_block_iterator> blocks() const {
return make_range(block_begin(), block_end());
op_range incoming_values() { return operands(); }
const_op_range incoming_values() const { return operands(); }
/// Return the number of incoming edges
unsigned getNumIncomingValues() const { return getNumOperands(); }
/// Return incoming value number x
MemoryAccess *getIncomingValue(unsigned I) const { return getOperand(I); }
void setIncomingValue(unsigned I, MemoryAccess *V) {
assert(V && "PHI node got a null value!");
setOperand(I, V);
static unsigned getOperandNumForIncomingValue(unsigned I) { return I; }
static unsigned getIncomingValueNumForOperand(unsigned I) { return I; }
/// Return incoming basic block number @p i.
BasicBlock *getIncomingBlock(unsigned I) const { return block_begin()[I]; }
/// Return incoming basic block corresponding
/// to an operand of the PHI.
BasicBlock *getIncomingBlock(const Use &U) const {
assert(this == U.getUser() && "Iterator doesn't point to PHI's Uses?");
return getIncomingBlock(unsigned(&U - op_begin()));
/// Return incoming basic block corresponding
/// to value use iterator.
BasicBlock *getIncomingBlock(MemoryAccess::const_user_iterator I) const {
return getIncomingBlock(I.getUse());
void setIncomingBlock(unsigned I, BasicBlock *BB) {
assert(BB && "PHI node got a null basic block!");
block_begin()[I] = BB;
/// Add an incoming value to the end of the PHI list
void addIncoming(MemoryAccess *V, BasicBlock *BB) {
if (getNumOperands() == ReservedSpace)
growOperands(); // Get more space!
// Initialize some new operands.
setNumHungOffUseOperands(getNumOperands() + 1);
setIncomingValue(getNumOperands() - 1, V);
setIncomingBlock(getNumOperands() - 1, BB);
/// Return the first index of the specified basic
/// block in the value list for this PHI. Returns -1 if no instance.
int getBasicBlockIndex(const BasicBlock *BB) const {
for (unsigned I = 0, E = getNumOperands(); I != E; ++I)
if (block_begin()[I] == BB)
return I;
return -1;
MemoryAccess *getIncomingValueForBlock(const BasicBlock *BB) const {
int Idx = getBasicBlockIndex(BB);
assert(Idx >= 0 && "Invalid basic block argument!");
return getIncomingValue(Idx);
// After deleting incoming position I, the order of incoming may be changed.
void unorderedDeleteIncoming(unsigned I) {
unsigned E = getNumOperands();
assert(I < E && "Cannot remove out of bounds Phi entry.");
// MemoryPhi must have at least two incoming values, otherwise the MemoryPhi
// itself should be deleted.
assert(E >= 2 && "Cannot only remove incoming values in MemoryPhis with "
"at least 2 values.");
setIncomingValue(I, getIncomingValue(E - 1));
setIncomingBlock(I, block_begin()[E - 1]);
setOperand(E - 1, nullptr);
block_begin()[E - 1] = nullptr;
setNumHungOffUseOperands(getNumOperands() - 1);
// After deleting entries that satisfy Pred, remaining entries may have
// changed order.
template <typename Fn> void unorderedDeleteIncomingIf(Fn &&Pred) {
for (unsigned I = 0, E = getNumOperands(); I != E; ++I)
if (Pred(getIncomingValue(I), getIncomingBlock(I))) {
E = getNumOperands();
assert(getNumOperands() >= 1 &&
"Cannot remove all incoming blocks in a MemoryPhi.");
// After deleting incoming block BB, the incoming blocks order may be changed.
void unorderedDeleteIncomingBlock(const BasicBlock *BB) {
[&](const MemoryAccess *, const BasicBlock *B) { return BB == B; });
// After deleting incoming memory access MA, the incoming accesses order may
// be changed.
void unorderedDeleteIncomingValue(const MemoryAccess *MA) {
[&](const MemoryAccess *M, const BasicBlock *) { return MA == M; });
static bool classof(const Value *V) {
return V->getValueID() == MemoryPhiVal;
void print(raw_ostream &OS) const;
unsigned getID() const { return ID; }
friend class MemorySSA;
/// this is more complicated than the generic
/// User::allocHungoffUses, because we have to allocate Uses for the incoming
/// values and pointers to the incoming blocks, all in one allocation.
void allocHungoffUses(unsigned N) {
User::allocHungoffUses(N, /* IsPhi */ true);
// For debugging only
const unsigned ID;
unsigned ReservedSpace;
/// This grows the operand list in response to a push_back style of
/// operation. This grows the number of ops by 1.5 times.
void growOperands() {
unsigned E = getNumOperands();
// 2 op PHI nodes are VERY common, so reserve at least enough for that.
ReservedSpace = std::max(E + E / 2, 2u);
growHungoffUses(ReservedSpace, /* IsPhi */ true);
static void deleteMe(DerivedUser *Self);
inline unsigned MemoryAccess::getID() const {
assert((isa<MemoryDef>(this) || isa<MemoryPhi>(this)) &&
"only memory defs and phis have ids");
if (const auto *MD = dyn_cast<MemoryDef>(this))
return MD->getID();
return cast<MemoryPhi>(this)->getID();
inline bool MemoryUseOrDef::isOptimized() const {
if (const auto *MD = dyn_cast<MemoryDef>(this))
return MD->isOptimized();
return cast<MemoryUse>(this)->isOptimized();
inline MemoryAccess *MemoryUseOrDef::getOptimized() const {
if (const auto *MD = dyn_cast<MemoryDef>(this))
return MD->getOptimized();
return cast<MemoryUse>(this)->getOptimized();
inline void MemoryUseOrDef::setOptimized(MemoryAccess *MA) {
if (auto *MD = dyn_cast<MemoryDef>(this))
inline void MemoryUseOrDef::resetOptimized() {
if (auto *MD = dyn_cast<MemoryDef>(this))
template <> struct OperandTraits<MemoryPhi> : public HungoffOperandTraits<2> {};
/// Encapsulates MemorySSA, including all data associated with memory
/// accesses.
class MemorySSA {
MemorySSA(Function &, AliasAnalysis *, DominatorTree *);
// MemorySSA must remain where it's constructed; Walkers it creates store
// pointers to it.
MemorySSA(MemorySSA &&) = delete;
MemorySSAWalker *getWalker();
MemorySSAWalker *getSkipSelfWalker();
/// Given a memory Mod/Ref'ing instruction, get the MemorySSA
/// access associated with it. If passed a basic block gets the memory phi
/// node that exists for that block, if there is one. Otherwise, this will get
/// a MemoryUseOrDef.
MemoryUseOrDef *getMemoryAccess(const Instruction *I) const {
return cast_or_null<MemoryUseOrDef>(ValueToMemoryAccess.lookup(I));
MemoryPhi *getMemoryAccess(const BasicBlock *BB) const {
return cast_or_null<MemoryPhi>(ValueToMemoryAccess.lookup(cast<Value>(BB)));
void dump() const;
void print(raw_ostream &) const;
/// Return true if \p MA represents the live on entry value
/// Loads and stores from pointer arguments and other global values may be
/// defined by memory operations that do not occur in the current function, so
/// they may be live on entry to the function. MemorySSA represents such
/// memory state by the live on entry definition, which is guaranteed to occur
/// before any other memory access in the function.
inline bool isLiveOnEntryDef(const MemoryAccess *MA) const {
return MA == LiveOnEntryDef.get();
inline MemoryAccess *getLiveOnEntryDef() const {
return LiveOnEntryDef.get();
// Sadly, iplists, by default, owns and deletes pointers added to the
// list. It's not currently possible to have two iplists for the same type,
// where one owns the pointers, and one does not. This is because the traits
// are per-type, not per-tag. If this ever changes, we should make the
// DefList an iplist.
using AccessList = iplist<MemoryAccess, ilist_tag<MSSAHelpers::AllAccessTag>>;
using DefsList =
simple_ilist<MemoryAccess, ilist_tag<MSSAHelpers::DefsOnlyTag>>;
/// Return the list of MemoryAccess's for a given basic block.
/// This list is not modifiable by the user.
const AccessList *getBlockAccesses(const BasicBlock *BB) const {
return getWritableBlockAccesses(BB);
/// Return the list of MemoryDef's and MemoryPhi's for a given basic
/// block.
/// This list is not modifiable by the user.
const DefsList *getBlockDefs(const BasicBlock *BB) const {
return getWritableBlockDefs(BB);
/// Given two memory accesses in the same basic block, determine
/// whether MemoryAccess \p A dominates MemoryAccess \p B.
bool locallyDominates(const MemoryAccess *A, const MemoryAccess *B) const;
/// Given two memory accesses in potentially different blocks,
/// determine whether MemoryAccess \p A dominates MemoryAccess \p B.
bool dominates(const MemoryAccess *A, const MemoryAccess *B) const;
/// Given a MemoryAccess and a Use, determine whether MemoryAccess \p A
/// dominates Use \p B.
bool dominates(const MemoryAccess *A, const Use &B) const;
/// Verify that MemorySSA is self consistent (IE definitions dominate
/// all uses, uses appear in the right places). This is used by unit tests.
void verifyMemorySSA() const;
/// Used in various insertion functions to specify whether we are talking
/// about the beginning or end of a block.
enum InsertionPlace { Beginning, End };
// Used by Memory SSA annotater, dumpers, and wrapper pass
friend class MemorySSAAnnotatedWriter;
friend class MemorySSAPrinterLegacyPass;
friend class MemorySSAUpdater;
void verifyDefUses(Function &F) const;
void verifyDomination(Function &F) const;
void verifyOrdering(Function &F) const;
void verifyDominationNumbers(const Function &F) const;
// This is used by the use optimizer and updater.
AccessList *getWritableBlockAccesses(const BasicBlock *BB) const {
auto It = PerBlockAccesses.find(BB);
return It == PerBlockAccesses.end() ? nullptr : It->second.get();
// This is used by the use optimizer and updater.
DefsList *getWritableBlockDefs(const BasicBlock *BB) const {
auto It = PerBlockDefs.find(BB);
return It == PerBlockDefs.end() ? nullptr : It->second.get();
// These is used by the updater to perform various internal MemorySSA
// machinsations. They do not always leave the IR in a correct state, and
// relies on the updater to fixup what it breaks, so it is not public.
void moveTo(MemoryUseOrDef *What, BasicBlock *BB, AccessList::iterator Where);
void moveTo(MemoryAccess *What, BasicBlock *BB, InsertionPlace Point);
// Rename the dominator tree branch rooted at BB.
void renamePass(BasicBlock *BB, MemoryAccess *IncomingVal,
SmallPtrSetImpl<BasicBlock *> &Visited) {
renamePass(DT->getNode(BB), IncomingVal, Visited, true, true);
void removeFromLookups(MemoryAccess *);
void removeFromLists(MemoryAccess *, bool ShouldDelete = true);
void insertIntoListsForBlock(MemoryAccess *, const BasicBlock *,
void insertIntoListsBefore(MemoryAccess *, const BasicBlock *,
MemoryUseOrDef *createDefinedAccess(Instruction *, MemoryAccess *,
const MemoryUseOrDef *Template = nullptr);
template <class AliasAnalysisType> class ClobberWalkerBase;
template <class AliasAnalysisType> class CachingWalker;
template <class AliasAnalysisType> class SkipSelfWalker;
class OptimizeUses;
CachingWalker<AliasAnalysis> *getWalkerImpl();
void buildMemorySSA(BatchAAResults &BAA);
void optimizeUses();
void prepareForMoveTo(MemoryAccess *, BasicBlock *);
void verifyUseInDefs(MemoryAccess *, MemoryAccess *) const;
using AccessMap = DenseMap<const BasicBlock *, std::unique_ptr<AccessList>>;
using DefsMap = DenseMap<const BasicBlock *, std::unique_ptr<DefsList>>;
determineInsertionPoint(const SmallPtrSetImpl<BasicBlock *> &DefiningBlocks);
void markUnreachableAsLiveOnEntry(BasicBlock *BB);
bool dominatesUse(const MemoryAccess *, const MemoryAccess *) const;
MemoryPhi *createMemoryPhi(BasicBlock *BB);
template <typename AliasAnalysisType>
MemoryUseOrDef *createNewAccess(Instruction *, AliasAnalysisType *,
const MemoryUseOrDef *Template = nullptr);
MemoryAccess *findDominatingDef(BasicBlock *, enum InsertionPlace);
void placePHINodes(const SmallPtrSetImpl<BasicBlock *> &);
MemoryAccess *renameBlock(BasicBlock *, MemoryAccess *, bool);
void renameSuccessorPhis(BasicBlock *, MemoryAccess *, bool);
void renamePass(DomTreeNode *, MemoryAccess *IncomingVal,
SmallPtrSetImpl<BasicBlock *> &Visited,
bool SkipVisited = false, bool RenameAllUses = false);
AccessList *getOrCreateAccessList(const BasicBlock *);
DefsList *getOrCreateDefsList(const BasicBlock *);
void renumberBlock(const BasicBlock *) const;
AliasAnalysis *AA;
DominatorTree *DT;
Function &F;
// Memory SSA mappings
DenseMap<const Value *, MemoryAccess *> ValueToMemoryAccess;
// These two mappings contain the main block to access/def mappings for
// MemorySSA. The list contained in PerBlockAccesses really owns all the
// MemoryAccesses.
// Both maps maintain the invariant that if a block is found in them, the
// corresponding list is not empty, and if a block is not found in them, the
// corresponding list is empty.
AccessMap PerBlockAccesses;
DefsMap PerBlockDefs;
std::unique_ptr<MemoryAccess, ValueDeleter> LiveOnEntryDef;
// Domination mappings
// Note that the numbering is local to a block, even though the map is
// global.
mutable SmallPtrSet<const BasicBlock *, 16> BlockNumberingValid;
mutable DenseMap<const MemoryAccess *, unsigned long> BlockNumbering;
// Memory SSA building info
std::unique_ptr<ClobberWalkerBase<AliasAnalysis>> WalkerBase;
std::unique_ptr<CachingWalker<AliasAnalysis>> Walker;
std::unique_ptr<SkipSelfWalker<AliasAnalysis>> SkipWalker;
unsigned NextID;
// Internal MemorySSA utils, for use by MemorySSA classes and walkers
class MemorySSAUtil {
friend class GVNHoist;
friend class MemorySSAWalker;
// This function should not be used by new passes.
static bool defClobbersUseOrDef(MemoryDef *MD, const MemoryUseOrDef *MU,
AliasAnalysis &AA);
// This pass does eager building and then printing of MemorySSA. It is used by
// the tests to be able to build, dump, and verify Memory SSA.
class MemorySSAPrinterLegacyPass : public FunctionPass {
bool runOnFunction(Function &) override;
void getAnalysisUsage(AnalysisUsage &AU) const override;
static char ID;
/// An analysis that produces \c MemorySSA for a function.
class MemorySSAAnalysis : public AnalysisInfoMixin<MemorySSAAnalysis> {
friend AnalysisInfoMixin<MemorySSAAnalysis>;
static AnalysisKey Key;
// Wrap MemorySSA result to ensure address stability of internal MemorySSA
// pointers after construction. Use a wrapper class instead of plain
// unique_ptr<MemorySSA> to avoid build breakage on MSVC.
struct Result {
Result(std::unique_ptr<MemorySSA> &&MSSA) : MSSA(std::move(MSSA)) {}
MemorySSA &getMSSA() { return *MSSA.get(); }
std::unique_ptr<MemorySSA> MSSA;
Result run(Function &F, FunctionAnalysisManager &AM);
/// Printer pass for \c MemorySSA.
class MemorySSAPrinterPass : public PassInfoMixin<MemorySSAPrinterPass> {
raw_ostream &OS;
explicit MemorySSAPrinterPass(raw_ostream &OS) : OS(OS) {}
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
/// Verifier pass for \c MemorySSA.
struct MemorySSAVerifierPass : PassInfoMixin<MemorySSAVerifierPass> {
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
/// Legacy analysis pass which computes \c MemorySSA.
class MemorySSAWrapperPass : public FunctionPass {
static char ID;
bool runOnFunction(Function &) override;
void releaseMemory() override;
MemorySSA &getMSSA() { return *MSSA; }
const MemorySSA &getMSSA() const { return *MSSA; }
void getAnalysisUsage(AnalysisUsage &AU) const override;
void verifyAnalysis() const override;
void print(raw_ostream &OS, const Module *M = nullptr) const override;
std::unique_ptr<MemorySSA> MSSA;
/// This is the generic walker interface for walkers of MemorySSA.
/// Walkers are used to be able to further disambiguate the def-use chains
/// MemorySSA gives you, or otherwise produce better info than MemorySSA gives
/// you.
/// In particular, while the def-use chains provide basic information, and are
/// guaranteed to give, for example, the nearest may-aliasing MemoryDef for a
/// MemoryUse as AliasAnalysis considers it, a user mant want better or other
/// information. In particular, they may want to use SCEV info to further
/// disambiguate memory accesses, or they may want the nearest dominating
/// may-aliasing MemoryDef for a call or a store. This API enables a
/// standardized interface to getting and using that info.
class MemorySSAWalker {
MemorySSAWalker(MemorySSA *);
virtual ~MemorySSAWalker() = default;
using MemoryAccessSet = SmallVector<MemoryAccess *, 8>;
/// Given a memory Mod/Ref/ModRef'ing instruction, calling this
/// will give you the nearest dominating MemoryAccess that Mod's the location
/// the instruction accesses (by skipping any def which AA can prove does not
/// alias the location(s) accessed by the instruction given).
/// Note that this will return a single access, and it must dominate the
/// Instruction, so if an operand of a MemoryPhi node Mod's the instruction,
/// this will return the MemoryPhi, not the operand. This means that
/// given:
/// if (a) {
/// 1 = MemoryDef(liveOnEntry)
/// store %a
/// } else {
/// 2 = MemoryDef(liveOnEntry)
/// store %b
/// }
/// 3 = MemoryPhi(2, 1)
/// MemoryUse(3)
/// load %a
/// calling this API on load(%a) will return the MemoryPhi, not the MemoryDef
/// in the if (a) branch.
MemoryAccess *getClobberingMemoryAccess(const Instruction *I) {
MemoryAccess *MA = MSSA->getMemoryAccess(I);
assert(MA && "Handed an instruction that MemorySSA doesn't recognize?");
return getClobberingMemoryAccess(MA);
/// Does the same thing as getClobberingMemoryAccess(const Instruction *I),
/// but takes a MemoryAccess instead of an Instruction.
virtual MemoryAccess *getClobberingMemoryAccess(MemoryAccess *) = 0;
/// Given a potentially clobbering memory access and a new location,
/// calling this will give you the nearest dominating clobbering MemoryAccess
/// (by skipping non-aliasing def links).
/// This version of the function is mainly used to disambiguate phi translated
/// pointers, where the value of a pointer may have changed from the initial
/// memory access. Note that this expects to be handed either a MemoryUse,
/// or an already potentially clobbering access. Unlike the above API, if
/// given a MemoryDef that clobbers the pointer as the starting access, it
/// will return that MemoryDef, whereas the above would return the clobber
/// starting from the use side of the memory def.
virtual MemoryAccess *getClobberingMemoryAccess(MemoryAccess *,
const MemoryLocation &) = 0;
/// Given a memory access, invalidate anything this walker knows about
/// that access.
/// This API is used by walkers that store information to perform basic cache
/// invalidation. This will be called by MemorySSA at appropriate times for
/// the walker it uses or returns.
virtual void invalidateInfo(MemoryAccess *) {}
friend class MemorySSA; // For updating MSSA pointer in MemorySSA move
// constructor.
MemorySSA *MSSA;
/// A MemorySSAWalker that does no alias queries, or anything else. It
/// simply returns the links as they were constructed by the builder.
class DoNothingMemorySSAWalker final : public MemorySSAWalker {
// Keep the overrides below from hiding the Instruction overload of
// getClobberingMemoryAccess.
using MemorySSAWalker::getClobberingMemoryAccess;
MemoryAccess *getClobberingMemoryAccess(MemoryAccess *) override;
MemoryAccess *getClobberingMemoryAccess(MemoryAccess *,
const MemoryLocation &) override;
using MemoryAccessPair = std::pair<MemoryAccess *, MemoryLocation>;
using ConstMemoryAccessPair = std::pair<const MemoryAccess *, MemoryLocation>;
/// Iterator base class used to implement const and non-const iterators
/// over the defining accesses of a MemoryAccess.
template <class T>
class memoryaccess_def_iterator_base
: public iterator_facade_base<memoryaccess_def_iterator_base<T>,
std::forward_iterator_tag, T, ptrdiff_t, T *,
T *> {
using BaseT = typename memoryaccess_def_iterator_base::iterator_facade_base;
memoryaccess_def_iterator_base(T *Start) : Access(Start) {}
memoryaccess_def_iterator_base() = default;
bool operator==(const memoryaccess_def_iterator_base &Other) const {
return Access == Other.Access && (!Access || ArgNo == Other.ArgNo);
// This is a bit ugly, but for MemoryPHI's, unlike PHINodes, you can't get the
// block from the operand in constant time (In a PHINode, the uselist has
// both, so it's just subtraction). We provide it as part of the
// iterator to avoid callers having to linear walk to get the block.
// If the operation becomes constant time on MemoryPHI's, this bit of
// abstraction breaking should be removed.
BasicBlock *getPhiArgBlock() const {
MemoryPhi *MP = dyn_cast<MemoryPhi>(Access);
assert(MP && "Tried to get phi arg block when not iterating over a PHI");
return MP->getIncomingBlock(ArgNo);
typename BaseT::iterator::pointer operator*() const {
assert(Access && "Tried to access past the end of our iterator");
// Go to the first argument for phis, and the defining access for everything
// else.
if (MemoryPhi *MP = dyn_cast<MemoryPhi>(Access))
return MP->getIncomingValue(ArgNo);
return cast<MemoryUseOrDef>(Access)->getDefiningAccess();
using BaseT::operator++;
memoryaccess_def_iterator &operator++() {
assert(Access && "Hit end of iterator");
if (MemoryPhi *MP = dyn_cast<MemoryPhi>(Access)) {
if (++ArgNo >= MP->getNumIncomingValues()) {
ArgNo = 0;
Access = nullptr;
} else {
Access = nullptr;
return *this;
T *Access = nullptr;
unsigned ArgNo = 0;
inline memoryaccess_def_iterator MemoryAccess::defs_begin() {
return memoryaccess_def_iterator(this);
inline const_memoryaccess_def_iterator MemoryAccess::defs_begin() const {
return const_memoryaccess_def_iterator(this);
inline memoryaccess_def_iterator MemoryAccess::defs_end() {
return memoryaccess_def_iterator();
inline const_memoryaccess_def_iterator MemoryAccess::defs_end() const {
return const_memoryaccess_def_iterator();
/// GraphTraits for a MemoryAccess, which walks defs in the normal case,
/// and uses in the inverse case.
template <> struct GraphTraits<MemoryAccess *> {
using NodeRef = MemoryAccess *;
using ChildIteratorType = memoryaccess_def_iterator;
static NodeRef getEntryNode(NodeRef N) { return N; }
static ChildIteratorType child_begin(NodeRef N) { return N->defs_begin(); }
static ChildIteratorType child_end(NodeRef N) { return N->defs_end(); }
template <> struct GraphTraits<Inverse<MemoryAccess *>> {
using NodeRef = MemoryAccess *;
using ChildIteratorType = MemoryAccess::iterator;
static NodeRef getEntryNode(NodeRef N) { return N; }
static ChildIteratorType child_begin(NodeRef N) { return N->user_begin(); }
static ChildIteratorType child_end(NodeRef N) { return N->user_end(); }
/// Provide an iterator that walks defs, giving both the memory access,
/// and the current pointer location, updating the pointer location as it
/// changes due to phi node translation.
/// This iterator, while somewhat specialized, is what most clients actually
/// want when walking upwards through MemorySSA def chains. It takes a pair of
/// <MemoryAccess,MemoryLocation>, and walks defs, properly translating the
/// memory location through phi nodes for the user.
class upward_defs_iterator
: public iterator_facade_base<upward_defs_iterator,
const MemoryAccessPair> {
using BaseT = upward_defs_iterator::iterator_facade_base;
upward_defs_iterator(const MemoryAccessPair &Info)
: DefIterator(Info.first), Location(Info.second),
OriginalAccess(Info.first) {
CurrentPair.first = nullptr;
WalkingPhi = Info.first && isa<MemoryPhi>(Info.first);
upward_defs_iterator() { CurrentPair.first = nullptr; }
bool operator==(const upward_defs_iterator &Other) const {
return DefIterator == Other.DefIterator;
BaseT::iterator::reference operator*() const {
assert(DefIterator != OriginalAccess->defs_end() &&
"Tried to access past the end of our iterator");
return CurrentPair;
using BaseT::operator++;
upward_defs_iterator &operator++() {
assert(DefIterator != OriginalAccess->defs_end() &&
"Tried to access past the end of the iterator");
if (DefIterator != OriginalAccess->defs_end())
return *this;
BasicBlock *getPhiArgBlock() const { return DefIterator.getPhiArgBlock(); }
void fillInCurrentPair() {
CurrentPair.first = *DefIterator;
if (WalkingPhi && Location.Ptr) {
PHITransAddr Translator(
const_cast<Value *>(Location.Ptr),
OriginalAccess->getBlock()->getModule()->getDataLayout(), nullptr);
if (!Translator.PHITranslateValue(OriginalAccess->getBlock(),
DefIterator.getPhiArgBlock(), nullptr,
if (Translator.getAddr() != Location.Ptr) {
CurrentPair.second = Location.getWithNewPtr(Translator.getAddr());
CurrentPair.second = Location;
MemoryAccessPair CurrentPair;
memoryaccess_def_iterator DefIterator;
MemoryLocation Location;
MemoryAccess *OriginalAccess = nullptr;
bool WalkingPhi = false;
inline upward_defs_iterator upward_defs_begin(const MemoryAccessPair &Pair) {
return upward_defs_iterator(Pair);
inline upward_defs_iterator upward_defs_end() { return upward_defs_iterator(); }
inline iterator_range<upward_defs_iterator>
upward_defs(const MemoryAccessPair &Pair) {
return make_range(upward_defs_begin(Pair), upward_defs_end());
/// Walks the defining accesses of MemoryDefs. Stops after we hit something that
/// has no defining use (e.g. a MemoryPhi or liveOnEntry). Note that, when
/// comparing against a null def_chain_iterator, this will compare equal only
/// after walking said Phi/liveOnEntry.
/// The UseOptimizedChain flag specifies whether to walk the clobbering
/// access chain, or all the accesses.
/// Normally, MemoryDef are all just def/use linked together, so a def_chain on
/// a MemoryDef will walk all MemoryDefs above it in the program until it hits
/// a phi node. The optimized chain walks the clobbering access of a store.
/// So if you are just trying to find, given a store, what the next
/// thing that would clobber the same memory is, you want the optimized chain.
template <class T, bool UseOptimizedChain = false>
struct def_chain_iterator
: public iterator_facade_base<def_chain_iterator<T, UseOptimizedChain>,
std::forward_iterator_tag, MemoryAccess *> {
def_chain_iterator() : MA(nullptr) {}
def_chain_iterator(T MA) : MA(MA) {}
T operator*() const { return MA; }
def_chain_iterator &operator++() {
// N.B. liveOnEntry has a null defining access.
if (auto *MUD = dyn_cast<MemoryUseOrDef>(MA)) {
if (UseOptimizedChain && MUD->isOptimized())
MA = MUD->getOptimized();
MA = MUD->getDefiningAccess();
} else {
MA = nullptr;
return *this;
bool operator==(const def_chain_iterator &O) const { return MA == O.MA; }
template <class T>
inline iterator_range<def_chain_iterator<T>>
def_chain(T MA, MemoryAccess *UpTo = nullptr) {
assert((!UpTo || find(def_chain(MA), UpTo) != def_chain_iterator<T>()) &&
"UpTo isn't in the def chain!");
return make_range(def_chain_iterator<T>(MA), def_chain_iterator<T>(UpTo));
template <class T>
inline iterator_range<def_chain_iterator<T, true>> optimized_def_chain(T MA) {
return make_range(def_chain_iterator<T, true>(MA),
def_chain_iterator<T, true>(nullptr));
} // end namespace llvm