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//===- DeadStoreElimination.cpp - MemorySSA Backed Dead Store Elimination -===//
// 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
// The code below implements dead store elimination using MemorySSA. It uses
// the following general approach: given a MemoryDef, walk upwards to find
// clobbering MemoryDefs that may be killed by the starting def. Then check
// that there are no uses that may read the location of the original MemoryDef
// in between both MemoryDefs. A bit more concretely:
// For all MemoryDefs StartDef:
// 1. Get the next dominating clobbering MemoryDef (MaybeDeadAccess) by walking
// upwards.
// 2. Check that there are no reads between MaybeDeadAccess and the StartDef by
// checking all uses starting at MaybeDeadAccess and walking until we see
// StartDef.
// 3. For each found CurrentDef, check that:
// 1. There are no barrier instructions between CurrentDef and StartDef (like
// throws or stores with ordering constraints).
// 2. StartDef is executed whenever CurrentDef is executed.
// 3. StartDef completely overwrites CurrentDef.
// 4. Erase CurrentDef from the function and MemorySSA.
#include "llvm/Transforms/Scalar/DeadStoreElimination.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/MustExecute.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/DebugCounter.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/AssumeBundleBuilder.h"
#include "llvm/Transforms/Utils/BuildLibCalls.h"
#include "llvm/Transforms/Utils/Local.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <iterator>
#include <map>
#include <optional>
#include <utility>
using namespace llvm;
using namespace PatternMatch;
#define DEBUG_TYPE "dse"
STATISTIC(NumRemainingStores, "Number of stores remaining after DSE");
STATISTIC(NumRedundantStores, "Number of redundant stores deleted");
STATISTIC(NumFastStores, "Number of stores deleted");
STATISTIC(NumFastOther, "Number of other instrs removed");
STATISTIC(NumCompletePartials, "Number of stores dead by later partials");
STATISTIC(NumModifiedStores, "Number of stores modified");
STATISTIC(NumCFGChecks, "Number of stores modified");
STATISTIC(NumCFGTries, "Number of stores modified");
STATISTIC(NumCFGSuccess, "Number of stores modified");
"Number of times a valid candidate is returned from getDomMemoryDef");
"Number iterations check for reads in getDomMemoryDef");
DEBUG_COUNTER(MemorySSACounter, "dse-memoryssa",
"Controls which MemoryDefs are eliminated.");
static cl::opt<bool>
cl::init(true), cl::Hidden,
cl::desc("Enable partial-overwrite tracking in DSE"));
static cl::opt<bool>
cl::init(true), cl::Hidden,
cl::desc("Enable partial store merging in DSE"));
static cl::opt<unsigned>
MemorySSAScanLimit("dse-memoryssa-scanlimit", cl::init(150), cl::Hidden,
cl::desc("The number of memory instructions to scan for "
"dead store elimination (default = 150)"));
static cl::opt<unsigned> MemorySSAUpwardsStepLimit(
"dse-memoryssa-walklimit", cl::init(90), cl::Hidden,
cl::desc("The maximum number of steps while walking upwards to find "
"MemoryDefs that may be killed (default = 90)"));
static cl::opt<unsigned> MemorySSAPartialStoreLimit(
"dse-memoryssa-partial-store-limit", cl::init(5), cl::Hidden,
cl::desc("The maximum number candidates that only partially overwrite the "
"killing MemoryDef to consider"
" (default = 5)"));
static cl::opt<unsigned> MemorySSADefsPerBlockLimit(
"dse-memoryssa-defs-per-block-limit", cl::init(5000), cl::Hidden,
cl::desc("The number of MemoryDefs we consider as candidates to eliminated "
"other stores per basic block (default = 5000)"));
static cl::opt<unsigned> MemorySSASameBBStepCost(
"dse-memoryssa-samebb-cost", cl::init(1), cl::Hidden,
"The cost of a step in the same basic block as the killing MemoryDef"
"(default = 1)"));
static cl::opt<unsigned>
MemorySSAOtherBBStepCost("dse-memoryssa-otherbb-cost", cl::init(5),
cl::desc("The cost of a step in a different basic "
"block than the killing MemoryDef"
"(default = 5)"));
static cl::opt<unsigned> MemorySSAPathCheckLimit(
"dse-memoryssa-path-check-limit", cl::init(50), cl::Hidden,
cl::desc("The maximum number of blocks to check when trying to prove that "
"all paths to an exit go through a killing block (default = 50)"));
// This flags allows or disallows DSE to optimize MemorySSA during its
// traversal. Note that DSE optimizing MemorySSA may impact other passes
// downstream of the DSE invocation and can lead to issues not being
// reproducible in isolation (i.e. when MemorySSA is built from scratch). In
// those cases, the flag can be used to check if DSE's MemorySSA optimizations
// impact follow-up passes.
static cl::opt<bool>
OptimizeMemorySSA("dse-optimize-memoryssa", cl::init(true), cl::Hidden,
cl::desc("Allow DSE to optimize memory accesses."));
// Helper functions
using OverlapIntervalsTy = std::map<int64_t, int64_t>;
using InstOverlapIntervalsTy = DenseMap<Instruction *, OverlapIntervalsTy>;
/// Returns true if the end of this instruction can be safely shortened in
/// length.
static bool isShortenableAtTheEnd(Instruction *I) {
// Don't shorten stores for now
if (isa<StoreInst>(I))
return false;
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default: return false;
case Intrinsic::memset:
case Intrinsic::memcpy:
case Intrinsic::memcpy_element_unordered_atomic:
case Intrinsic::memset_element_unordered_atomic:
// Do shorten memory intrinsics.
// FIXME: Add memmove if it's also safe to transform.
return true;
// Don't shorten libcalls calls for now.
return false;
/// Returns true if the beginning of this instruction can be safely shortened
/// in length.
static bool isShortenableAtTheBeginning(Instruction *I) {
// FIXME: Handle only memset for now. Supporting memcpy/memmove should be
// easily done by offsetting the source address.
return isa<AnyMemSetInst>(I);
static uint64_t getPointerSize(const Value *V, const DataLayout &DL,
const TargetLibraryInfo &TLI,
const Function *F) {
uint64_t Size;
ObjectSizeOpts Opts;
Opts.NullIsUnknownSize = NullPointerIsDefined(F);
if (getObjectSize(V, Size, DL, &TLI, Opts))
return Size;
return MemoryLocation::UnknownSize;
namespace {
enum OverwriteResult {
} // end anonymous namespace
/// Check if two instruction are masked stores that completely
/// overwrite one another. More specifically, \p KillingI has to
/// overwrite \p DeadI.
static OverwriteResult isMaskedStoreOverwrite(const Instruction *KillingI,
const Instruction *DeadI,
BatchAAResults &AA) {
const auto *KillingII = dyn_cast<IntrinsicInst>(KillingI);
const auto *DeadII = dyn_cast<IntrinsicInst>(DeadI);
if (KillingII == nullptr || DeadII == nullptr)
return OW_Unknown;
if (KillingII->getIntrinsicID() != DeadII->getIntrinsicID())
return OW_Unknown;
if (KillingII->getIntrinsicID() == Intrinsic::masked_store) {
// Type size.
VectorType *KillingTy =
VectorType *DeadTy = cast<VectorType>(DeadII->getArgOperand(0)->getType());
if (KillingTy->getScalarSizeInBits() != DeadTy->getScalarSizeInBits())
return OW_Unknown;
// Element count.
if (KillingTy->getElementCount() != DeadTy->getElementCount())
return OW_Unknown;
// Pointers.
Value *KillingPtr = KillingII->getArgOperand(1)->stripPointerCasts();
Value *DeadPtr = DeadII->getArgOperand(1)->stripPointerCasts();
if (KillingPtr != DeadPtr && !AA.isMustAlias(KillingPtr, DeadPtr))
return OW_Unknown;
// Masks.
// TODO: check that KillingII's mask is a superset of the DeadII's mask.
if (KillingII->getArgOperand(3) != DeadII->getArgOperand(3))
return OW_Unknown;
return OW_Complete;
return OW_Unknown;
/// Return 'OW_Complete' if a store to the 'KillingLoc' location completely
/// overwrites a store to the 'DeadLoc' location, 'OW_End' if the end of the
/// 'DeadLoc' location is completely overwritten by 'KillingLoc', 'OW_Begin'
/// if the beginning of the 'DeadLoc' location is overwritten by 'KillingLoc'.
/// 'OW_PartialEarlierWithFullLater' means that a dead (big) store was
/// overwritten by a killing (smaller) store which doesn't write outside the big
/// store's memory locations. Returns 'OW_Unknown' if nothing can be determined.
/// NOTE: This function must only be called if both \p KillingLoc and \p
/// DeadLoc belong to the same underlying object with valid \p KillingOff and
/// \p DeadOff.
static OverwriteResult isPartialOverwrite(const MemoryLocation &KillingLoc,
const MemoryLocation &DeadLoc,
int64_t KillingOff, int64_t DeadOff,
Instruction *DeadI,
InstOverlapIntervalsTy &IOL) {
const uint64_t KillingSize = KillingLoc.Size.getValue();
const uint64_t DeadSize = DeadLoc.Size.getValue();
// We may now overlap, although the overlap is not complete. There might also
// be other incomplete overlaps, and together, they might cover the complete
// dead store.
// Note: The correctness of this logic depends on the fact that this function
// is not even called providing DepWrite when there are any intervening reads.
if (EnablePartialOverwriteTracking &&
KillingOff < int64_t(DeadOff + DeadSize) &&
int64_t(KillingOff + KillingSize) >= DeadOff) {
// Insert our part of the overlap into the map.
auto &IM = IOL[DeadI];
LLVM_DEBUG(dbgs() << "DSE: Partial overwrite: DeadLoc [" << DeadOff << ", "
<< int64_t(DeadOff + DeadSize) << ") KillingLoc ["
<< KillingOff << ", " << int64_t(KillingOff + KillingSize)
<< ")\n");
// Make sure that we only insert non-overlapping intervals and combine
// adjacent intervals. The intervals are stored in the map with the ending
// offset as the key (in the half-open sense) and the starting offset as
// the value.
int64_t KillingIntStart = KillingOff;
int64_t KillingIntEnd = KillingOff + KillingSize;
// Find any intervals ending at, or after, KillingIntStart which start
// before KillingIntEnd.
auto ILI = IM.lower_bound(KillingIntStart);
if (ILI != IM.end() && ILI->second <= KillingIntEnd) {
// This existing interval is overlapped with the current store somewhere
// in [KillingIntStart, KillingIntEnd]. Merge them by erasing the existing
// intervals and adjusting our start and end.
KillingIntStart = std::min(KillingIntStart, ILI->second);
KillingIntEnd = std::max(KillingIntEnd, ILI->first);
ILI = IM.erase(ILI);
// Continue erasing and adjusting our end in case other previous
// intervals are also overlapped with the current store.
// |--- dead 1 ---| |--- dead 2 ---|
// |------- killing---------|
while (ILI != IM.end() && ILI->second <= KillingIntEnd) {
assert(ILI->second > KillingIntStart && "Unexpected interval");
KillingIntEnd = std::max(KillingIntEnd, ILI->first);
ILI = IM.erase(ILI);
IM[KillingIntEnd] = KillingIntStart;
ILI = IM.begin();
if (ILI->second <= DeadOff && ILI->first >= int64_t(DeadOff + DeadSize)) {
LLVM_DEBUG(dbgs() << "DSE: Full overwrite from partials: DeadLoc ["
<< DeadOff << ", " << int64_t(DeadOff + DeadSize)
<< ") Composite KillingLoc [" << ILI->second << ", "
<< ILI->first << ")\n");
return OW_Complete;
// Check for a dead store which writes to all the memory locations that
// the killing store writes to.
if (EnablePartialStoreMerging && KillingOff >= DeadOff &&
int64_t(DeadOff + DeadSize) > KillingOff &&
uint64_t(KillingOff - DeadOff) + KillingSize <= DeadSize) {
LLVM_DEBUG(dbgs() << "DSE: Partial overwrite a dead load [" << DeadOff
<< ", " << int64_t(DeadOff + DeadSize)
<< ") by a killing store [" << KillingOff << ", "
<< int64_t(KillingOff + KillingSize) << ")\n");
// TODO: Maybe come up with a better name?
return OW_PartialEarlierWithFullLater;
// Another interesting case is if the killing store overwrites the end of the
// dead store.
// |--dead--|
// |-- killing --|
// In this case we may want to trim the size of dead store to avoid
// generating stores to addresses which will definitely be overwritten killing
// store.
if (!EnablePartialOverwriteTracking &&
(KillingOff > DeadOff && KillingOff < int64_t(DeadOff + DeadSize) &&
int64_t(KillingOff + KillingSize) >= int64_t(DeadOff + DeadSize)))
return OW_End;
// Finally, we also need to check if the killing store overwrites the
// beginning of the dead store.
// |--dead--|
// |-- killing --|
// In this case we may want to move the destination address and trim the size
// of dead store to avoid generating stores to addresses which will definitely
// be overwritten killing store.
if (!EnablePartialOverwriteTracking &&
(KillingOff <= DeadOff && int64_t(KillingOff + KillingSize) > DeadOff)) {
assert(int64_t(KillingOff + KillingSize) < int64_t(DeadOff + DeadSize) &&
"Expect to be handled as OW_Complete");
return OW_Begin;
// Otherwise, they don't completely overlap.
return OW_Unknown;
/// Returns true if the memory which is accessed by the second instruction is not
/// modified between the first and the second instruction.
/// Precondition: Second instruction must be dominated by the first
/// instruction.
static bool
memoryIsNotModifiedBetween(Instruction *FirstI, Instruction *SecondI,
BatchAAResults &AA, const DataLayout &DL,
DominatorTree *DT) {
// Do a backwards scan through the CFG from SecondI to FirstI. Look for
// instructions which can modify the memory location accessed by SecondI.
// While doing the walk keep track of the address to check. It might be
// different in different basic blocks due to PHI translation.
using BlockAddressPair = std::pair<BasicBlock *, PHITransAddr>;
SmallVector<BlockAddressPair, 16> WorkList;
// Keep track of the address we visited each block with. Bail out if we
// visit a block with different addresses.
DenseMap<BasicBlock *, Value *> Visited;
BasicBlock::iterator FirstBBI(FirstI);
BasicBlock::iterator SecondBBI(SecondI);
BasicBlock *FirstBB = FirstI->getParent();
BasicBlock *SecondBB = SecondI->getParent();
MemoryLocation MemLoc;
if (auto *MemSet = dyn_cast<MemSetInst>(SecondI))
MemLoc = MemoryLocation::getForDest(MemSet);
MemLoc = MemoryLocation::get(SecondI);
auto *MemLocPtr = const_cast<Value *>(MemLoc.Ptr);
// Start checking the SecondBB.
std::make_pair(SecondBB, PHITransAddr(MemLocPtr, DL, nullptr)));
bool isFirstBlock = true;
// Check all blocks going backward until we reach the FirstBB.
while (!WorkList.empty()) {
BlockAddressPair Current = WorkList.pop_back_val();
BasicBlock *B = Current.first;
PHITransAddr &Addr = Current.second;
Value *Ptr = Addr.getAddr();
// Ignore instructions before FirstI if this is the FirstBB.
BasicBlock::iterator BI = (B == FirstBB ? FirstBBI : B->begin());
BasicBlock::iterator EI;
if (isFirstBlock) {
// Ignore instructions after SecondI if this is the first visit of SecondBB.
assert(B == SecondBB && "first block is not the store block");
EI = SecondBBI;
isFirstBlock = false;
} else {
// It's not SecondBB or (in case of a loop) the second visit of SecondBB.
// In this case we also have to look at instructions after SecondI.
EI = B->end();
for (; BI != EI; ++BI) {
Instruction *I = &*BI;
if (I->mayWriteToMemory() && I != SecondI)
if (isModSet(AA.getModRefInfo(I, MemLoc.getWithNewPtr(Ptr))))
return false;
if (B != FirstBB) {
assert(B != &FirstBB->getParent()->getEntryBlock() &&
"Should not hit the entry block because SI must be dominated by LI");
for (BasicBlock *Pred : predecessors(B)) {
PHITransAddr PredAddr = Addr;
if (PredAddr.NeedsPHITranslationFromBlock(B)) {
if (!PredAddr.IsPotentiallyPHITranslatable())
return false;
if (PredAddr.PHITranslateValue(B, Pred, DT, false))
return false;
Value *TranslatedPtr = PredAddr.getAddr();
auto Inserted = Visited.insert(std::make_pair(Pred, TranslatedPtr));
if (!Inserted.second) {
// We already visited this block before. If it was with a different
// address - bail out!
if (TranslatedPtr != Inserted.first->second)
return false;
// ... otherwise just skip it.
WorkList.push_back(std::make_pair(Pred, PredAddr));
return true;
static void shortenAssignment(Instruction *Inst, uint64_t OldOffsetInBits,
uint64_t OldSizeInBits, uint64_t NewSizeInBits,
bool IsOverwriteEnd) {
DIExpression::FragmentInfo DeadFragment;
DeadFragment.SizeInBits = OldSizeInBits - NewSizeInBits;
DeadFragment.OffsetInBits =
OldOffsetInBits + (IsOverwriteEnd ? NewSizeInBits : 0);
auto CreateDeadFragExpr = [Inst, DeadFragment]() {
// FIXME: This should be using the DIExpression in the Alloca's dbg.assign
// for the variable, since that could also contain a fragment?
return *DIExpression::createFragmentExpression(
DIExpression::get(Inst->getContext(), std::nullopt),
DeadFragment.OffsetInBits, DeadFragment.SizeInBits);
// A DIAssignID to use so that the inserted dbg.assign intrinsics do not
// link to any instructions. Created in the loop below (once).
DIAssignID *LinkToNothing = nullptr;
// Insert an unlinked dbg.assign intrinsic for the dead fragment after each
// overlapping dbg.assign intrinsic.
for (auto *DAI : at::getAssignmentMarkers(Inst)) {
if (auto FragInfo = DAI->getExpression()->getFragmentInfo()) {
if (!DIExpression::fragmentsOverlap(*FragInfo, DeadFragment))
// Fragments overlap: insert a new dbg.assign for this dead part.
auto *NewAssign = cast<DbgAssignIntrinsic>(DAI->clone());
if (!LinkToNothing)
LinkToNothing = DIAssignID::getDistinct(Inst->getContext());
static bool tryToShorten(Instruction *DeadI, int64_t &DeadStart,
uint64_t &DeadSize, int64_t KillingStart,
uint64_t KillingSize, bool IsOverwriteEnd) {
auto *DeadIntrinsic = cast<AnyMemIntrinsic>(DeadI);
Align PrefAlign = DeadIntrinsic->getDestAlign().valueOrOne();
// We assume that memet/memcpy operates in chunks of the "largest" native
// type size and aligned on the same value. That means optimal start and size
// of memset/memcpy should be modulo of preferred alignment of that type. That
// is it there is no any sense in trying to reduce store size any further
// since any "extra" stores comes for free anyway.
// On the other hand, maximum alignment we can achieve is limited by alignment
// of initial store.
// TODO: Limit maximum alignment by preferred (or abi?) alignment of the
// "largest" native type.
// Note: What is the proper way to get that value?
// Should TargetTransformInfo::getRegisterBitWidth be used or anything else?
// PrefAlign = std::min(DL.getPrefTypeAlign(LargestType), PrefAlign);
int64_t ToRemoveStart = 0;
uint64_t ToRemoveSize = 0;
// Compute start and size of the region to remove. Make sure 'PrefAlign' is
// maintained on the remaining store.
if (IsOverwriteEnd) {
// Calculate required adjustment for 'KillingStart' in order to keep
// remaining store size aligned on 'PerfAlign'.
uint64_t Off =
offsetToAlignment(uint64_t(KillingStart - DeadStart), PrefAlign);
ToRemoveStart = KillingStart + Off;
if (DeadSize <= uint64_t(ToRemoveStart - DeadStart))
return false;
ToRemoveSize = DeadSize - uint64_t(ToRemoveStart - DeadStart);
} else {
ToRemoveStart = DeadStart;
assert(KillingSize >= uint64_t(DeadStart - KillingStart) &&
"Not overlapping accesses?");
ToRemoveSize = KillingSize - uint64_t(DeadStart - KillingStart);
// Calculate required adjustment for 'ToRemoveSize'in order to keep
// start of the remaining store aligned on 'PerfAlign'.
uint64_t Off = offsetToAlignment(ToRemoveSize, PrefAlign);
if (Off != 0) {
if (ToRemoveSize <= (PrefAlign.value() - Off))
return false;
ToRemoveSize -= PrefAlign.value() - Off;
assert(isAligned(PrefAlign, ToRemoveSize) &&
"Should preserve selected alignment");
assert(ToRemoveSize > 0 && "Shouldn't reach here if nothing to remove");
assert(DeadSize > ToRemoveSize && "Can't remove more than original size");
uint64_t NewSize = DeadSize - ToRemoveSize;
if (auto *AMI = dyn_cast<AtomicMemIntrinsic>(DeadI)) {
// When shortening an atomic memory intrinsic, the newly shortened
// length must remain an integer multiple of the element size.
const uint32_t ElementSize = AMI->getElementSizeInBytes();
if (0 != NewSize % ElementSize)
return false;
LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW "
<< (IsOverwriteEnd ? "END" : "BEGIN") << ": " << *DeadI
<< "\n KILLER [" << ToRemoveStart << ", "
<< int64_t(ToRemoveStart + ToRemoveSize) << ")\n");
Value *DeadWriteLength = DeadIntrinsic->getLength();
Value *TrimmedLength = ConstantInt::get(DeadWriteLength->getType(), NewSize);
if (!IsOverwriteEnd) {
Value *OrigDest = DeadIntrinsic->getRawDest();
Type *Int8PtrTy =
Value *Dest = OrigDest;
if (OrigDest->getType() != Int8PtrTy)
Dest = CastInst::CreatePointerCast(OrigDest, Int8PtrTy, "", DeadI);
Value *Indices[1] = {
ConstantInt::get(DeadWriteLength->getType(), ToRemoveSize)};
Instruction *NewDestGEP = GetElementPtrInst::CreateInBounds(
Type::getInt8Ty(DeadIntrinsic->getContext()), Dest, Indices, "", DeadI);
if (NewDestGEP->getType() != OrigDest->getType())
NewDestGEP = CastInst::CreatePointerCast(NewDestGEP, OrigDest->getType(),
"", DeadI);
// Update attached dbg.assign intrinsics. Assume 8-bit byte.
shortenAssignment(DeadI, DeadStart * 8, DeadSize * 8, NewSize * 8,
// Finally update start and size of dead access.
if (!IsOverwriteEnd)
DeadStart += ToRemoveSize;
DeadSize = NewSize;
return true;
static bool tryToShortenEnd(Instruction *DeadI, OverlapIntervalsTy &IntervalMap,
int64_t &DeadStart, uint64_t &DeadSize) {
if (IntervalMap.empty() || !isShortenableAtTheEnd(DeadI))
return false;
OverlapIntervalsTy::iterator OII = --IntervalMap.end();
int64_t KillingStart = OII->second;
uint64_t KillingSize = OII->first - KillingStart;
assert(OII->first - KillingStart >= 0 && "Size expected to be positive");
if (KillingStart > DeadStart &&
// Note: "KillingStart - KillingStart" is known to be positive due to
// preceding check.
(uint64_t)(KillingStart - DeadStart) < DeadSize &&
// Note: "DeadSize - (uint64_t)(KillingStart - DeadStart)" is known to
// be non negative due to preceding checks.
KillingSize >= DeadSize - (uint64_t)(KillingStart - DeadStart)) {
if (tryToShorten(DeadI, DeadStart, DeadSize, KillingStart, KillingSize,
true)) {
return true;
return false;
static bool tryToShortenBegin(Instruction *DeadI,
OverlapIntervalsTy &IntervalMap,
int64_t &DeadStart, uint64_t &DeadSize) {
if (IntervalMap.empty() || !isShortenableAtTheBeginning(DeadI))
return false;
OverlapIntervalsTy::iterator OII = IntervalMap.begin();
int64_t KillingStart = OII->second;
uint64_t KillingSize = OII->first - KillingStart;
assert(OII->first - KillingStart >= 0 && "Size expected to be positive");
if (KillingStart <= DeadStart &&
// Note: "DeadStart - KillingStart" is known to be non negative due to
// preceding check.
KillingSize > (uint64_t)(DeadStart - KillingStart)) {
// Note: "KillingSize - (uint64_t)(DeadStart - DeadStart)" is known to
// be positive due to preceding checks.
assert(KillingSize - (uint64_t)(DeadStart - KillingStart) < DeadSize &&
"Should have been handled as OW_Complete");
if (tryToShorten(DeadI, DeadStart, DeadSize, KillingStart, KillingSize,
false)) {
return true;
return false;
static Constant *
tryToMergePartialOverlappingStores(StoreInst *KillingI, StoreInst *DeadI,
int64_t KillingOffset, int64_t DeadOffset,
const DataLayout &DL, BatchAAResults &AA,
DominatorTree *DT) {
if (DeadI && isa<ConstantInt>(DeadI->getValueOperand()) &&
DL.typeSizeEqualsStoreSize(DeadI->getValueOperand()->getType()) &&
KillingI && isa<ConstantInt>(KillingI->getValueOperand()) &&
DL.typeSizeEqualsStoreSize(KillingI->getValueOperand()->getType()) &&
memoryIsNotModifiedBetween(DeadI, KillingI, AA, DL, DT)) {
// If the store we find is:
// a) partially overwritten by the store to 'Loc'
// b) the killing store is fully contained in the dead one and
// c) they both have a constant value
// d) none of the two stores need padding
// Merge the two stores, replacing the dead store's value with a
// merge of both values.
// TODO: Deal with other constant types (vectors, etc), and probably
// some mem intrinsics (if needed)
APInt DeadValue = cast<ConstantInt>(DeadI->getValueOperand())->getValue();
APInt KillingValue =
unsigned KillingBits = KillingValue.getBitWidth();
assert(DeadValue.getBitWidth() > KillingValue.getBitWidth());
KillingValue = KillingValue.zext(DeadValue.getBitWidth());
// Offset of the smaller store inside the larger store
unsigned BitOffsetDiff = (KillingOffset - DeadOffset) * 8;
unsigned LShiftAmount =
DL.isBigEndian() ? DeadValue.getBitWidth() - BitOffsetDiff - KillingBits
: BitOffsetDiff;
APInt Mask = APInt::getBitsSet(DeadValue.getBitWidth(), LShiftAmount,
LShiftAmount + KillingBits);
// Clear the bits we'll be replacing, then OR with the smaller
// store, shifted appropriately.
APInt Merged = (DeadValue & ~Mask) | (KillingValue << LShiftAmount);
LLVM_DEBUG(dbgs() << "DSE: Merge Stores:\n Dead: " << *DeadI
<< "\n Killing: " << *KillingI
<< "\n Merged Value: " << Merged << '\n');
return ConstantInt::get(DeadI->getValueOperand()->getType(), Merged);
return nullptr;
namespace {
// Returns true if \p I is an intrisnic that does not read or write memory.
bool isNoopIntrinsic(Instruction *I) {
if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
case Intrinsic::lifetime_start:
case Intrinsic::lifetime_end:
case Intrinsic::invariant_end:
case Intrinsic::launder_invariant_group:
case Intrinsic::assume:
return true;
case Intrinsic::dbg_addr:
case Intrinsic::dbg_declare:
case Intrinsic::dbg_label:
case Intrinsic::dbg_value:
llvm_unreachable("Intrinsic should not be modeled in MemorySSA");
return false;
return false;
// Check if we can ignore \p D for DSE.
bool canSkipDef(MemoryDef *D, bool DefVisibleToCaller) {
Instruction *DI = D->getMemoryInst();
// Calls that only access inaccessible memory cannot read or write any memory
// locations we consider for elimination.
if (auto *CB = dyn_cast<CallBase>(DI))
if (CB->onlyAccessesInaccessibleMemory())
return true;
// We can eliminate stores to locations not visible to the caller across
// throwing instructions.
if (DI->mayThrow() && !DefVisibleToCaller)
return true;
// We can remove the dead stores, irrespective of the fence and its ordering
// (release/acquire/seq_cst). Fences only constraints the ordering of
// already visible stores, it does not make a store visible to other
// threads. So, skipping over a fence does not change a store from being
// dead.
if (isa<FenceInst>(DI))
return true;
// Skip intrinsics that do not really read or modify memory.
if (isNoopIntrinsic(DI))
return true;
return false;
struct DSEState {
Function &F;
AliasAnalysis &AA;
EarliestEscapeInfo EI;
/// The single BatchAA instance that is used to cache AA queries. It will
/// not be invalidated over the whole run. This is safe, because:
/// 1. Only memory writes are removed, so the alias cache for memory
/// locations remains valid.
/// 2. No new instructions are added (only instructions removed), so cached
/// information for a deleted value cannot be accessed by a re-used new
/// value pointer.
BatchAAResults BatchAA;
MemorySSA &MSSA;
DominatorTree &DT;
PostDominatorTree &PDT;
const TargetLibraryInfo &TLI;
const DataLayout &DL;
const LoopInfo &LI;
// Whether the function contains any irreducible control flow, useful for
// being accurately able to detect loops.
bool ContainsIrreducibleLoops;
// All MemoryDefs that potentially could kill other MemDefs.
SmallVector<MemoryDef *, 64> MemDefs;
// Any that should be skipped as they are already deleted
SmallPtrSet<MemoryAccess *, 4> SkipStores;
// Keep track whether a given object is captured before return or not.
DenseMap<const Value *, bool> CapturedBeforeReturn;
// Keep track of all of the objects that are invisible to the caller after
// the function returns.
DenseMap<const Value *, bool> InvisibleToCallerAfterRet;
// Keep track of blocks with throwing instructions not modeled in MemorySSA.
SmallPtrSet<BasicBlock *, 16> ThrowingBlocks;
// Post-order numbers for each basic block. Used to figure out if memory
// accesses are executed before another access.
DenseMap<BasicBlock *, unsigned> PostOrderNumbers;
// Values that are only used with assumes. Used to refine pointer escape
// analysis.
SmallPtrSet<const Value *, 32> EphValues;
/// Keep track of instructions (partly) overlapping with killing MemoryDefs per
/// basic block.
MapVector<BasicBlock *, InstOverlapIntervalsTy> IOLs;
// Check if there are root nodes that are terminated by UnreachableInst.
// Those roots pessimize post-dominance queries. If there are such roots,
// fall back to CFG scan starting from all non-unreachable roots.
bool AnyUnreachableExit;
// Whether or not we should iterate on removing dead stores at the end of the
// function due to removing a store causing a previously captured pointer to
// no longer be captured.
bool ShouldIterateEndOfFunctionDSE;
// Class contains self-reference, make sure it's not copied/moved.
DSEState(const DSEState &) = delete;
DSEState &operator=(const DSEState &) = delete;
DSEState(Function &F, AliasAnalysis &AA, MemorySSA &MSSA, DominatorTree &DT,
PostDominatorTree &PDT, AssumptionCache &AC,
const TargetLibraryInfo &TLI, const LoopInfo &LI)
: F(F), AA(AA), EI(DT, LI, EphValues), BatchAA(AA, &EI), MSSA(MSSA),
DT(DT), PDT(PDT), TLI(TLI), DL(F.getParent()->getDataLayout()), LI(LI) {
// Collect blocks with throwing instructions not modeled in MemorySSA and
// alloc-like objects.
unsigned PO = 0;
for (BasicBlock *BB : post_order(&F)) {
PostOrderNumbers[BB] = PO++;
for (Instruction &I : *BB) {
MemoryAccess *MA = MSSA.getMemoryAccess(&I);
if (I.mayThrow() && !MA)
auto *MD = dyn_cast_or_null<MemoryDef>(MA);
if (MD && MemDefs.size() < MemorySSADefsPerBlockLimit &&
(getLocForWrite(&I) || isMemTerminatorInst(&I)))
// Treat byval or inalloca arguments the same as Allocas, stores to them are
// dead at the end of the function.
for (Argument &AI : F.args())
if (AI.hasPassPointeeByValueCopyAttr())
InvisibleToCallerAfterRet.insert({&AI, true});
// Collect whether there is any irreducible control flow in the function.
ContainsIrreducibleLoops = mayContainIrreducibleControl(F, &LI);
AnyUnreachableExit = any_of(PDT.roots(), [](const BasicBlock *E) {
return isa<UnreachableInst>(E->getTerminator());
CodeMetrics::collectEphemeralValues(&F, &AC, EphValues);
LocationSize strengthenLocationSize(const Instruction *I,
LocationSize Size) const {
if (auto *CB = dyn_cast<CallBase>(I)) {
LibFunc F;
if (TLI.getLibFunc(*CB, F) && TLI.has(F) &&
(F == LibFunc_memset_chk || F == LibFunc_memcpy_chk)) {
// Use the precise location size specified by the 3rd argument
// for determining KillingI overwrites DeadLoc if it is a memset_chk
// instruction. memset_chk will write either the amount specified as 3rd
// argument or the function will immediately abort and exit the program.
// NOTE: AA may determine NoAlias if it can prove that the access size
// is larger than the allocation size due to that being UB. To avoid
// returning potentially invalid NoAlias results by AA, limit the use of
// the precise location size to isOverwrite.
if (const auto *Len = dyn_cast<ConstantInt>(CB->getArgOperand(2)))
return LocationSize::precise(Len->getZExtValue());
return Size;
/// Return 'OW_Complete' if a store to the 'KillingLoc' location (by \p
/// KillingI instruction) completely overwrites a store to the 'DeadLoc'
/// location (by \p DeadI instruction).
/// Return OW_MaybePartial if \p KillingI does not completely overwrite
/// \p DeadI, but they both write to the same underlying object. In that
/// case, use isPartialOverwrite to check if \p KillingI partially overwrites
/// \p DeadI. Returns 'OR_None' if \p KillingI is known to not overwrite the
/// \p DeadI. Returns 'OW_Unknown' if nothing can be determined.
OverwriteResult isOverwrite(const Instruction *KillingI,
const Instruction *DeadI,
const MemoryLocation &KillingLoc,
const MemoryLocation &DeadLoc,
int64_t &KillingOff, int64_t &DeadOff) {
// AliasAnalysis does not always account for loops. Limit overwrite checks
// to dependencies for which we can guarantee they are independent of any
// loops they are in.
if (!isGuaranteedLoopIndependent(DeadI, KillingI, DeadLoc))
return OW_Unknown;
LocationSize KillingLocSize =
strengthenLocationSize(KillingI, KillingLoc.Size);
const Value *DeadPtr = DeadLoc.Ptr->stripPointerCasts();
const Value *KillingPtr = KillingLoc.Ptr->stripPointerCasts();
const Value *DeadUndObj = getUnderlyingObject(DeadPtr);
const Value *KillingUndObj = getUnderlyingObject(KillingPtr);
// Check whether the killing store overwrites the whole object, in which
// case the size/offset of the dead store does not matter.
if (DeadUndObj == KillingUndObj && KillingLocSize.isPrecise()) {
uint64_t KillingUndObjSize = getPointerSize(KillingUndObj, DL, TLI, &F);
if (KillingUndObjSize != MemoryLocation::UnknownSize &&
KillingUndObjSize == KillingLocSize.getValue())
return OW_Complete;
// FIXME: Vet that this works for size upper-bounds. Seems unlikely that we'll
// get imprecise values here, though (except for unknown sizes).
if (!KillingLocSize.isPrecise() || !DeadLoc.Size.isPrecise()) {
// In case no constant size is known, try to an IR values for the number
// of bytes written and check if they match.
const auto *KillingMemI = dyn_cast<MemIntrinsic>(KillingI);
const auto *DeadMemI = dyn_cast<MemIntrinsic>(DeadI);
if (KillingMemI && DeadMemI) {
const Value *KillingV = KillingMemI->getLength();
const Value *DeadV = DeadMemI->getLength();
if (KillingV == DeadV && BatchAA.isMustAlias(DeadLoc, KillingLoc))
return OW_Complete;
// Masked stores have imprecise locations, but we can reason about them
// to some extent.
return isMaskedStoreOverwrite(KillingI, DeadI, BatchAA);
const uint64_t KillingSize = KillingLocSize.getValue();
const uint64_t DeadSize = DeadLoc.Size.getValue();
// Query the alias information
AliasResult AAR = BatchAA.alias(KillingLoc, DeadLoc);
// If the start pointers are the same, we just have to compare sizes to see if
// the killing store was larger than the dead store.
if (AAR == AliasResult::MustAlias) {
// Make sure that the KillingSize size is >= the DeadSize size.
if (KillingSize >= DeadSize)
return OW_Complete;
// If we hit a partial alias we may have a full overwrite
if (AAR == AliasResult::PartialAlias && AAR.hasOffset()) {
int32_t Off = AAR.getOffset();
if (Off >= 0 && (uint64_t)Off + DeadSize <= KillingSize)
return OW_Complete;
// If we can't resolve the same pointers to the same object, then we can't
// analyze them at all.
if (DeadUndObj != KillingUndObj) {
// Non aliasing stores to different objects don't overlap. Note that
// if the killing store is known to overwrite whole object (out of
// bounds access overwrites whole object as well) then it is assumed to
// completely overwrite any store to the same object even if they don't
// actually alias (see next check).
if (AAR == AliasResult::NoAlias)
return OW_None;
return OW_Unknown;
// Okay, we have stores to two completely different pointers. Try to
// decompose the pointer into a "base + constant_offset" form. If the base
// pointers are equal, then we can reason about the two stores.
DeadOff = 0;
KillingOff = 0;
const Value *DeadBasePtr =
GetPointerBaseWithConstantOffset(DeadPtr, DeadOff, DL);
const Value *KillingBasePtr =
GetPointerBaseWithConstantOffset(KillingPtr, KillingOff, DL);
// If the base pointers still differ, we have two completely different
// stores.
if (DeadBasePtr != KillingBasePtr)
return OW_Unknown;
// The killing access completely overlaps the dead store if and only if
// both start and end of the dead one is "inside" the killing one:
// |<->|--dead--|<->|
// |-----killing------|
// Accesses may overlap if and only if start of one of them is "inside"
// another one:
// |<->|--dead--|<-------->|
// |-------killing--------|
// OR
// |-------dead-------|
// |<->|---killing---|<----->|
// We have to be careful here as *Off is signed while *.Size is unsigned.
// Check if the dead access starts "not before" the killing one.
if (DeadOff >= KillingOff) {
// If the dead access ends "not after" the killing access then the
// dead one is completely overwritten by the killing one.
if (uint64_t(DeadOff - KillingOff) + DeadSize <= KillingSize)
return OW_Complete;
// If start of the dead access is "before" end of the killing access
// then accesses overlap.
else if ((uint64_t)(DeadOff - KillingOff) < KillingSize)
return OW_MaybePartial;
// If start of the killing access is "before" end of the dead access then
// accesses overlap.
else if ((uint64_t)(KillingOff - DeadOff) < DeadSize) {
return OW_MaybePartial;
// Can reach here only if accesses are known not to overlap.
return OW_None;
bool isInvisibleToCallerAfterRet(const Value *V) {
if (isa<AllocaInst>(V))
return true;
auto I = InvisibleToCallerAfterRet.insert({V, false});
if (I.second) {
if (!isInvisibleToCallerOnUnwind(V)) {
I.first->second = false;
} else if (isNoAliasCall(V)) {
I.first->second = !PointerMayBeCaptured(V, true, false, EphValues);
return I.first->second;
bool isInvisibleToCallerOnUnwind(const Value *V) {
bool RequiresNoCaptureBeforeUnwind;
if (!isNotVisibleOnUnwind(V, RequiresNoCaptureBeforeUnwind))
return false;
if (!RequiresNoCaptureBeforeUnwind)
return true;
auto I = CapturedBeforeReturn.insert({V, true});
if (I.second)
// NOTE: This could be made more precise by PointerMayBeCapturedBefore
// with the killing MemoryDef. But we refrain from doing so for now to
// limit compile-time and this does not cause any changes to the number
// of stores removed on a large test set in practice.
I.first->second = PointerMayBeCaptured(V, false, true, EphValues);
return !I.first->second;
std::optional<MemoryLocation> getLocForWrite(Instruction *I) const {
if (!I->mayWriteToMemory())
return std::nullopt;
if (auto *CB = dyn_cast<CallBase>(I))
return MemoryLocation::getForDest(CB, TLI);
return MemoryLocation::getOrNone(I);
/// Assuming this instruction has a dead analyzable write, can we delete
/// this instruction?
bool isRemovable(Instruction *I) {
assert(getLocForWrite(I) && "Must have analyzable write");
// Don't remove volatile/atomic stores.
if (StoreInst *SI = dyn_cast<StoreInst>(I))
return SI->isUnordered();
if (auto *CB = dyn_cast<CallBase>(I)) {
// Don't remove volatile memory intrinsics.
if (auto *MI = dyn_cast<MemIntrinsic>(CB))
return !MI->isVolatile();
// Never remove dead lifetime intrinsics, e.g. because they are followed
// by a free.
if (CB->isLifetimeStartOrEnd())
return false;
return CB->use_empty() && CB->willReturn() && CB->doesNotThrow() &&
return false;
/// Returns true if \p UseInst completely overwrites \p DefLoc
/// (stored by \p DefInst).
bool isCompleteOverwrite(const MemoryLocation &DefLoc, Instruction *DefInst,
Instruction *UseInst) {
// UseInst has a MemoryDef associated in MemorySSA. It's possible for a
// MemoryDef to not write to memory, e.g. a volatile load is modeled as a
// MemoryDef.
if (!UseInst->mayWriteToMemory())
return false;
if (auto *CB = dyn_cast<CallBase>(UseInst))
if (CB->onlyAccessesInaccessibleMemory())
return false;
int64_t InstWriteOffset, DepWriteOffset;
if (auto CC = getLocForWrite(UseInst))
return isOverwrite(UseInst, DefInst, *CC, DefLoc, InstWriteOffset,
DepWriteOffset) == OW_Complete;
return false;
/// Returns true if \p Def is not read before returning from the function.
bool isWriteAtEndOfFunction(MemoryDef *Def) {
LLVM_DEBUG(dbgs() << " Check if def " << *Def << " ("
<< *Def->getMemoryInst()
<< ") is at the end the function \n");
auto MaybeLoc = getLocForWrite(Def->getMemoryInst());
if (!MaybeLoc) {
LLVM_DEBUG(dbgs() << " ... could not get location for write.\n");
return false;
SmallVector<MemoryAccess *, 4> WorkList;
SmallPtrSet<MemoryAccess *, 8> Visited;
auto PushMemUses = [&WorkList, &Visited](MemoryAccess *Acc) {
if (!Visited.insert(Acc).second)
for (Use &U : Acc->uses())
for (unsigned I = 0; I < WorkList.size(); I++) {
if (WorkList.size() >= MemorySSAScanLimit) {
LLVM_DEBUG(dbgs() << " ... hit exploration limit.\n");
return false;
MemoryAccess *UseAccess = WorkList[I];
if (isa<MemoryPhi>(UseAccess)) {
// AliasAnalysis does not account for loops. Limit elimination to
// candidates for which we can guarantee they always store to the same
// memory location.
if (!isGuaranteedLoopInvariant(MaybeLoc->Ptr))
return false;
// TODO: Checking for aliasing is expensive. Consider reducing the amount
// of times this is called and/or caching it.
Instruction *UseInst = cast<MemoryUseOrDef>(UseAccess)->getMemoryInst();
if (isReadClobber(*MaybeLoc, UseInst)) {
LLVM_DEBUG(dbgs() << " ... hit read clobber " << *UseInst << ".\n");
return false;
if (MemoryDef *UseDef = dyn_cast<MemoryDef>(UseAccess))
return true;
/// If \p I is a memory terminator like llvm.lifetime.end or free, return a
/// pair with the MemoryLocation terminated by \p I and a boolean flag
/// indicating whether \p I is a free-like call.
std::optional<std::pair<MemoryLocation, bool>>
getLocForTerminator(Instruction *I) const {
uint64_t Len;
Value *Ptr;
if (match(I, m_Intrinsic<Intrinsic::lifetime_end>(m_ConstantInt(Len),
return {std::make_pair(MemoryLocation(Ptr, Len), false)};
if (auto *CB = dyn_cast<CallBase>(I)) {
if (Value *FreedOp = getFreedOperand(CB, &TLI))
return {std::make_pair(MemoryLocation::getAfter(FreedOp), true)};
return std::nullopt;
/// Returns true if \p I is a memory terminator instruction like
/// llvm.lifetime.end or free.
bool isMemTerminatorInst(Instruction *I) const {
auto *CB = dyn_cast<CallBase>(I);
return CB && (CB->getIntrinsicID() == Intrinsic::lifetime_end ||
getFreedOperand(CB, &TLI) != nullptr);
/// Returns true if \p MaybeTerm is a memory terminator for \p Loc from
/// instruction \p AccessI.
bool isMemTerminator(const MemoryLocation &Loc, Instruction *AccessI,
Instruction *MaybeTerm) {
std::optional<std::pair<MemoryLocation, bool>> MaybeTermLoc =
if (!MaybeTermLoc)
return false;
// If the terminator is a free-like call, all accesses to the underlying
// object can be considered terminated.
if (getUnderlyingObject(Loc.Ptr) !=
return false;
auto TermLoc = MaybeTermLoc->first;
if (MaybeTermLoc->second) {
const Value *LocUO = getUnderlyingObject(Loc.Ptr);
return BatchAA.isMustAlias(TermLoc.Ptr, LocUO);
int64_t InstWriteOffset = 0;
int64_t DepWriteOffset = 0;
return isOverwrite(MaybeTerm, AccessI, TermLoc, Loc, InstWriteOffset,
DepWriteOffset) == OW_Complete;
// Returns true if \p Use may read from \p DefLoc.
bool isReadClobber(const MemoryLocation &DefLoc, Instruction *UseInst) {
if (isNoopIntrinsic(UseInst))
return false;
// Monotonic or weaker atomic stores can be re-ordered and do not need to be
// treated as read clobber.
if (auto SI = dyn_cast<StoreInst>(UseInst))
return isStrongerThan(SI->getOrdering(), AtomicOrdering::Monotonic);
if (!UseInst->mayReadFromMemory())
return false;
if (auto *CB = dyn_cast<CallBase>(UseInst))
if (CB->onlyAccessesInaccessibleMemory())
return false;
return isRefSet(BatchAA.getModRefInfo(UseInst, DefLoc));
/// Returns true if a dependency between \p Current and \p KillingDef is
/// guaranteed to be loop invariant for the loops that they are in. Either
/// because they are known to be in the same block, in the same loop level or
/// by guaranteeing that \p CurrentLoc only references a single MemoryLocation
/// during execution of the containing function.
bool isGuaranteedLoopIndependent(const Instruction *Current,
const Instruction *KillingDef,
const MemoryLocation &CurrentLoc) {
// If the dependency is within the same block or loop level (being careful
// of irreducible loops), we know that AA will return a valid result for the
// memory dependency. (Both at the function level, outside of any loop,
// would also be valid but we currently disable that to limit compile time).
if (Current->getParent() == KillingDef->getParent())
return true;
const Loop *CurrentLI = LI.getLoopFor(Current->getParent());
if (!ContainsIrreducibleLoops && CurrentLI &&
CurrentLI == LI.getLoopFor(KillingDef->getParent()))
return true;
// Otherwise check the memory location is invariant to any loops.
return isGuaranteedLoopInvariant(CurrentLoc.Ptr);
/// Returns true if \p Ptr is guaranteed to be loop invariant for any possible
/// loop. In particular, this guarantees that it only references a single
/// MemoryLocation during execution of the containing function.
bool isGuaranteedLoopInvariant(const Value *Ptr) {
Ptr = Ptr->stripPointerCasts();
if (auto *GEP = dyn_cast<GEPOperator>(Ptr))
if (GEP->hasAllConstantIndices())
Ptr = GEP->getPointerOperand()->stripPointerCasts();
if (auto *I = dyn_cast<Instruction>(Ptr)) {
return I->getParent()->isEntryBlock() ||
(!ContainsIrreducibleLoops && !LI.getLoopFor(I->getParent()));
return true;
// Find a MemoryDef writing to \p KillingLoc and dominating \p StartAccess,
// with no read access between them or on any other path to a function exit
// block if \p KillingLoc is not accessible after the function returns. If
// there is no such MemoryDef, return std::nullopt. The returned value may not
// (completely) overwrite \p KillingLoc. Currently we bail out when we
// encounter an aliasing MemoryUse (read).
std::optional<MemoryAccess *>
getDomMemoryDef(MemoryDef *KillingDef, MemoryAccess *StartAccess,
const MemoryLocation &KillingLoc, const Value *KillingUndObj,
unsigned &ScanLimit, unsigned &WalkerStepLimit,
bool IsMemTerm, unsigned &PartialLimit) {
if (ScanLimit == 0 || WalkerStepLimit == 0) {
LLVM_DEBUG(dbgs() << "\n ... hit scan limit\n");
return std::nullopt;
MemoryAccess *Current = StartAccess;
Instruction *KillingI = KillingDef->getMemoryInst();
LLVM_DEBUG(dbgs() << " trying to get dominating access\n");
// Only optimize defining access of KillingDef when directly starting at its
// defining access. The defining access also must only access KillingLoc. At
// the moment we only support instructions with a single write location, so
// it should be sufficient to disable optimizations for instructions that
// also read from memory.
bool CanOptimize = OptimizeMemorySSA &&
KillingDef->getDefiningAccess() == StartAccess &&
// Find the next clobbering Mod access for DefLoc, starting at StartAccess.
std::optional<MemoryLocation> CurrentLoc;
for (;; Current = cast<MemoryDef>(Current)->getDefiningAccess()) {
dbgs() << " visiting " << *Current;
if (!MSSA.isLiveOnEntryDef(Current) && isa<MemoryUseOrDef>(Current))
dbgs() << " (" << *cast<MemoryUseOrDef>(Current)->getMemoryInst()
<< ")";
dbgs() << "\n";
// Reached TOP.
if (MSSA.isLiveOnEntryDef(Current)) {
LLVM_DEBUG(dbgs() << " ... found LiveOnEntryDef\n");
if (CanOptimize && Current != KillingDef->getDefiningAccess())
// The first clobbering def is... none.
return std::nullopt;
// Cost of a step. Accesses in the same block are more likely to be valid
// candidates for elimination, hence consider them cheaper.
unsigned StepCost = KillingDef->getBlock() == Current->getBlock()
? MemorySSASameBBStepCost
: MemorySSAOtherBBStepCost;
if (WalkerStepLimit <= StepCost) {
LLVM_DEBUG(dbgs() << " ... hit walker step limit\n");
return std::nullopt;
WalkerStepLimit -= StepCost;
// Return for MemoryPhis. They cannot be eliminated directly and the
// caller is responsible for traversing them.
if (isa<MemoryPhi>(Current)) {
LLVM_DEBUG(dbgs() << " ... found MemoryPhi\n");
return Current;
// Below, check if CurrentDef is a valid candidate to be eliminated by
// KillingDef. If it is not, check the next candidate.
MemoryDef *CurrentDef = cast<MemoryDef>(Current);
Instruction *CurrentI = CurrentDef->getMemoryInst();
if (canSkipDef(CurrentDef, !isInvisibleToCallerOnUnwind(KillingUndObj))) {
CanOptimize = false;
// Before we try to remove anything, check for any extra throwing
// instructions that block us from DSEing
if (mayThrowBetween(KillingI, CurrentI, KillingUndObj)) {
LLVM_DEBUG(dbgs() << " ... skip, may throw!\n");
return std::nullopt;
// Check for anything that looks like it will be a barrier to further
// removal
if (isDSEBarrier(KillingUndObj, CurrentI)) {
LLVM_DEBUG(dbgs() << " ... skip, barrier\n");
return std::nullopt;
// If Current is known to be on path that reads DefLoc or is a read
// clobber, bail out, as the path is not profitable. We skip this check
// for intrinsic calls, because the code knows how to handle memcpy
// intrinsics.
if (!isa<IntrinsicInst>(CurrentI) && isReadClobber(KillingLoc, CurrentI))
return std::nullopt;
// Quick check if there are direct uses that are read-clobbers.
if (any_of(Current->uses(), [this, &KillingLoc, StartAccess](Use &U) {
if (auto *UseOrDef = dyn_cast<MemoryUseOrDef>(U.getUser()))
return !MSSA.dominates(StartAccess, UseOrDef) &&
isReadClobber(KillingLoc, UseOrDef->getMemoryInst());
return false;
})) {
LLVM_DEBUG(dbgs() << " ... found a read clobber\n");
return std::nullopt;
// If Current does not have an analyzable write location or is not
// removable, skip it.
CurrentLoc = getLocForWrite(CurrentI);
if (!CurrentLoc || !isRemovable(CurrentI)) {
CanOptimize = false;
// AliasAnalysis does not account for loops. Limit elimination to
// candidates for which we can guarantee they always store to the same
// memory location and not located in different loops.
if (!isGuaranteedLoopIndependent(CurrentI, KillingI, *CurrentLoc)) {
LLVM_DEBUG(dbgs() << " ... not guaranteed loop independent\n");
CanOptimize = false;
if (IsMemTerm) {
// If the killing def is a memory terminator (e.g. lifetime.end), check
// the next candidate if the current Current does not write the same
// underlying object as the terminator.
if (!isMemTerminator(*CurrentLoc, CurrentI, KillingI)) {
CanOptimize = false;
} else {
int64_t KillingOffset = 0;
int64_t DeadOffset = 0;
auto OR = isOverwrite(KillingI, CurrentI, KillingLoc, *CurrentLoc,
KillingOffset, DeadOffset);
if (CanOptimize) {
// CurrentDef is the earliest write clobber of KillingDef. Use it as
// optimized access. Do not optimize if CurrentDef is already the
// defining access of KillingDef.
if (CurrentDef != KillingDef->getDefiningAccess() &&
(OR == OW_Complete || OR == OW_MaybePartial))
// Once a may-aliasing def is encountered do not set an optimized
// access.
if (OR != OW_None)
CanOptimize = false;
// If Current does not write to the same object as KillingDef, check
// the next candidate.
if (OR == OW_Unknown || OR == OW_None)
else if (OR == OW_MaybePartial) {
// If KillingDef only partially overwrites Current, check the next
// candidate if the partial step limit is exceeded. This aggressively
// limits the number of candidates for partial store elimination,
// which are less likely to be removable in the end.
if (PartialLimit <= 1) {
WalkerStepLimit -= 1;
LLVM_DEBUG(dbgs() << " ... reached partial limit ... continue with next access\n");
PartialLimit -= 1;
// Accesses to objects accessible after the function returns can only be
// eliminated if the access is dead along all paths to the exit. Collect
// the blocks with killing (=completely overwriting MemoryDefs) and check if
// they cover all paths from MaybeDeadAccess to any function exit.
SmallPtrSet<Instruction *, 16> KillingDefs;
MemoryAccess *MaybeDeadAccess = Current;
MemoryLocation MaybeDeadLoc = *CurrentLoc;
Instruction *MaybeDeadI = cast<MemoryDef>(MaybeDeadAccess)->getMemoryInst();
LLVM_DEBUG(dbgs() << " Checking for reads of " << *MaybeDeadAccess << " ("
<< *MaybeDeadI << ")\n");
SmallSetVector<MemoryAccess *, 32> WorkList;
auto PushMemUses = [&WorkList](MemoryAccess *Acc) {
for (Use &U : Acc->uses())
// Check if DeadDef may be read.
for (unsigned I = 0; I < WorkList.size(); I++) {
MemoryAccess *UseAccess = WorkList[I];
LLVM_DEBUG(dbgs() << " " << *UseAccess);
// Bail out if the number of accesses to check exceeds the scan limit.
if (ScanLimit < (WorkList.size() - I)) {
LLVM_DEBUG(dbgs() << "\n ... hit scan limit\n");
return std::nullopt;
if (isa<MemoryPhi>(UseAccess)) {
if (any_of(KillingDefs, [this, UseAccess](Instruction *KI) {
return DT.properlyDominates(KI->getParent(),
})) {
LLVM_DEBUG(dbgs() << " ... skipping, dominated by killing block\n");
LLVM_DEBUG(dbgs() << "\n ... adding PHI uses\n");
Instruction *UseInst = cast<MemoryUseOrDef>(UseAccess)->getMemoryInst();
LLVM_DEBUG(dbgs() << " (" << *UseInst << ")\n");
if (any_of(KillingDefs, [this, UseInst](Instruction *KI) {
return DT.dominates(KI, UseInst);
})) {
LLVM_DEBUG(dbgs() << " ... skipping, dominated by killing def\n");
// A memory terminator kills all preceeding MemoryDefs and all succeeding
// MemoryAccesses. We do not have to check it's users.
if (isMemTerminator(MaybeDeadLoc, MaybeDeadI, UseInst)) {
<< " ... skipping, memterminator invalidates following accesses\n");
if (isNoopIntrinsic(cast<MemoryUseOrDef>(UseAccess)->getMemoryInst())) {
LLVM_DEBUG(dbgs() << " ... adding uses of intrinsic\n");
if (UseInst->mayThrow() && !isInvisibleToCallerOnUnwind(KillingUndObj)) {
LLVM_DEBUG(dbgs() << " ... found throwing instruction\n");
return std::nullopt;
// Uses which may read the original MemoryDef mean we cannot eliminate the
// original MD. Stop walk.
if (isReadClobber(MaybeDeadLoc, UseInst)) {
LLVM_DEBUG(dbgs() << " ... found read clobber\n");
return std::nullopt;
// If this worklist walks back to the original memory access (and the
// pointer is not guarenteed loop invariant) then we cannot assume that a
// store kills itself.
if (MaybeDeadAccess == UseAccess &&
!isGuaranteedLoopInvariant(MaybeDeadLoc.Ptr)) {
LLVM_DEBUG(dbgs() << " ... found not loop invariant self access\n");
return std::nullopt;
// Otherwise, for the KillingDef and MaybeDeadAccess we only have to check
// if it reads the memory location.
// TODO: It would probably be better to check for self-reads before
// calling the function.
if (KillingDef == UseAccess || MaybeDeadAccess == UseAccess) {
LLVM_DEBUG(dbgs() << " ... skipping killing def/dom access\n");
// Check all uses for MemoryDefs, except for defs completely overwriting
// the original location. Otherwise we have to check uses of *all*
// MemoryDefs we discover, including non-aliasing ones. Otherwise we might
// miss cases like the following
// 1 = Def(LoE) ; <----- DeadDef stores [0,1]
// 2 = Def(1) ; (2, 1) = NoAlias, stores [2,3]
// Use(2) ; MayAlias 2 *and* 1, loads [0, 3].
// (The Use points to the *first* Def it may alias)
// 3 = Def(1) ; <---- Current (3, 2) = NoAlias, (3,1) = MayAlias,
// stores [0,1]
if (MemoryDef *UseDef = dyn_cast<MemoryDef>(UseAccess)) {
if (isCompleteOverwrite(MaybeDeadLoc, MaybeDeadI, UseInst)) {
BasicBlock *MaybeKillingBlock = UseInst->getParent();
if (PostOrderNumbers.find(MaybeKillingBlock)->second <
PostOrderNumbers.find(MaybeDeadAccess->getBlock())->second) {
if (!isInvisibleToCallerAfterRet(KillingUndObj)) {
<< " ... found killing def " << *UseInst << "\n");
} else {
<< " ... found preceeding def " << *UseInst << "\n");
return std::nullopt;
} else
// For accesses to locations visible after the function returns, make sure
// that the location is dead (=overwritten) along all paths from
// MaybeDeadAccess to the exit.
if (!isInvisibleToCallerAfterRet(KillingUndObj)) {
SmallPtrSet<BasicBlock *, 16> KillingBlocks;
for (Instruction *KD : KillingDefs)
assert(!KillingBlocks.empty() &&
"Expected at least a single killing block");
// Find the common post-dominator of all killing blocks.
BasicBlock *CommonPred = *KillingBlocks.begin();
for (BasicBlock *BB : llvm::drop_begin(KillingBlocks)) {
if (!CommonPred)
CommonPred = PDT.findNearestCommonDominator(CommonPred, BB);
// If the common post-dominator does not post-dominate MaybeDeadAccess,
// there is a path from MaybeDeadAccess to an exit not going through a
// killing block.
if (!PDT.dominates(CommonPred, MaybeDeadAccess->getBlock())) {
if (!AnyUnreachableExit)
return std::nullopt;
// Fall back to CFG scan starting at all non-unreachable roots if not
// all paths to the exit go through CommonPred.
CommonPred = nullptr;
// If CommonPred itself is in the set of killing blocks, we're done.
if (KillingBlocks.count(CommonPred))
return {MaybeDeadAccess};
SetVector<BasicBlock *> WorkList;
// If CommonPred is null, there are multiple exits from the function.
// They all have to be added to the worklist.
if (CommonPred)
for (BasicBlock *R : PDT.roots()) {
if (!isa<UnreachableInst>(R->getTerminator()))
// Check if all paths starting from an exit node go through one of the
// killing blocks before reaching MaybeDeadAccess.
for (unsigned I = 0; I < WorkList.size(); I++) {
BasicBlock *Current = WorkList[I];
if (KillingBlocks.count(Current))
if (Current == MaybeDeadAccess->getBlock())
return std::nullopt;
// MaybeDeadAccess is reachable from the entry, so we don't have to
// explore unreachable blocks further.
if (!DT.isReachableFromEntry(Current))
for (BasicBlock *Pred : predecessors(Current))
if (WorkList.size() >= MemorySSAPathCheckLimit)
return std::nullopt;
// No aliasing MemoryUses of MaybeDeadAccess found, MaybeDeadAccess is
// potentially dead.
return {MaybeDeadAccess};
// Delete dead memory defs
void deleteDeadInstruction(Instruction *SI) {
MemorySSAUpdater Updater(&MSSA);
SmallVector<Instruction *, 32> NowDeadInsts;
while (!NowDeadInsts.empty()) {
Instruction *DeadInst = NowDeadInsts.pop_back_val();
// Try to preserve debug information attached to the dead instruction.
// Remove the Instruction from MSSA.
if (MemoryAccess *MA = MSSA.getMemoryAccess(DeadInst)) {
if (MemoryDef *MD = dyn_cast<MemoryDef>(MA)) {
if (auto *SI = dyn_cast<StoreInst>(MD->getMemoryInst())) {
if (SI->getValueOperand()->getType()->isPointerTy()) {
const Value *UO = getUnderlyingObject(SI->getValueOperand());
if (CapturedBeforeReturn.erase(UO))
ShouldIterateEndOfFunctionDSE = true;
auto I = IOLs.find(DeadInst->getParent());
if (I != IOLs.end())
// Remove its operands
for (Use &O : DeadInst->operands())
if (Instruction *OpI = dyn_cast<Instruction>(O)) {
O = nullptr;
if (isInstructionTriviallyDead(OpI, &TLI))
// Check for any extra throws between \p KillingI and \p DeadI that block
// DSE. This only checks extra maythrows (those that aren't MemoryDef's).
// MemoryDef that may throw are handled during the walk from one def to the
// next.
bool mayThrowBetween(Instruction *KillingI, Instruction *DeadI,
const Value *KillingUndObj) {
// First see if we can ignore it by using the fact that KillingI is an
// alloca/alloca like object that is not visible to the caller during
// execution of the function.
if (KillingUndObj && isInvisibleToCallerOnUnwind(KillingUndObj))
return false;
if (KillingI->getParent() == DeadI->getParent())
return ThrowingBlocks.count(KillingI->getParent());
return !ThrowingBlocks.empty();
// Check if \p DeadI acts as a DSE barrier for \p KillingI. The following
// instructions act as barriers:
// * A memory instruction that may throw and \p KillingI accesses a non-stack
// object.
// * Atomic stores stronger that monotonic.
bool isDSEBarrier(const Value *KillingUndObj, Instruction *DeadI) {
// If DeadI may throw it acts as a barrier, unless we are to an
// alloca/alloca like object that does not escape.
if (DeadI->mayThrow() && !isInvisibleToCallerOnUnwind(KillingUndObj))
return true;
// If DeadI is an atomic load/store stronger than monotonic, do not try to
// eliminate/reorder it.
if (DeadI->isAtomic()) {
if (auto *LI = dyn_cast<LoadInst>(DeadI))
return isStrongerThanMonotonic(LI->getOrdering());
if (auto *SI = dyn_cast<StoreInst>(DeadI))
return isStrongerThanMonotonic(SI->getOrdering());
if (auto *ARMW = dyn_cast<AtomicRMWInst>(DeadI))
return isStrongerThanMonotonic(ARMW->getOrdering());
if (auto *CmpXchg = dyn_cast<AtomicCmpXchgInst>(DeadI))
return isStrongerThanMonotonic(CmpXchg->getSuccessOrdering()) ||
llvm_unreachable("other instructions should be skipped in MemorySSA");
return false;
/// Eliminate writes to objects that are not visible in the caller and are not
/// accessed before returning from the function.
bool eliminateDeadWritesAtEndOfFunction() {
bool MadeChange = false;
<< "Trying to eliminate MemoryDefs at the end of the function\n");
do {
ShouldIterateEndOfFunctionDSE = false;
for (MemoryDef *Def : llvm::reverse(MemDefs)) {
if (SkipStores.contains(Def))
Instruction *DefI = Def->getMemoryInst();
auto DefLoc = getLocForWrite(DefI);
if (!DefLoc || !isRemovable(DefI))
// NOTE: Currently eliminating writes at the end of a function is
// limited to MemoryDefs with a single underlying object, to save
// compile-time. In practice it appears the case with multiple
// underlying objects is very uncommon. If it turns out to be important,
// we can use getUnderlyingObjects here instead.
const Value *UO = getUnderlyingObject(DefLoc->Ptr);
if (!isInvisibleToCallerAfterRet(UO))
if (isWriteAtEndOfFunction(Def)) {
// See through pointer-to-pointer bitcasts
LLVM_DEBUG(dbgs() << " ... MemoryDef is not accessed until the end "
"of the function\n");
MadeChange = true;
} while (ShouldIterateEndOfFunctionDSE);
return MadeChange;
/// If we have a zero initializing memset following a call to malloc,
/// try folding it into a call to calloc.
bool tryFoldIntoCalloc(MemoryDef *Def, const Value *DefUO) {
Instruction *DefI = Def->getMemoryInst();
MemSetInst *MemSet = dyn_cast<MemSetInst>(DefI);
if (!MemSet)
// TODO: Could handle zero store to small allocation as well.
return false;
Constant *StoredConstant = dyn_cast<Constant>(MemSet->getValue());
if (!StoredConstant || !StoredConstant->isNullValue())
return false;
if (!isRemovable(DefI))
// The memset might be volatile..
return false;
if (F.hasFnAttribute(Attribute::SanitizeMemory) ||
F.hasFnAttribute(Attribute::SanitizeAddress) ||
F.hasFnAttribute(Attribute::SanitizeHWAddress) ||
F.getName() == "calloc")
return false;
auto *Malloc = const_cast<CallInst *>(dyn_cast<CallInst>(DefUO));
if (!Malloc)
return false;
auto *InnerCallee = Malloc->getCalledFunction();
if (!InnerCallee)
return false;
LibFunc Func;
if (!TLI.getLibFunc(*InnerCallee, Func) || !TLI.has(Func) ||
Func != LibFunc_malloc)
return false;
auto shouldCreateCalloc = [](CallInst *Malloc, CallInst *Memset) {
// Check for br(icmp ptr, null), truebb, falsebb) pattern at the end
// of malloc block
auto *MallocBB = Malloc->getParent(),
*MemsetBB = Memset->getParent();
if (MallocBB == MemsetBB)
return true;
auto *Ptr = Memset->getArgOperand(0);
auto *TI = MallocBB->getTerminator();
ICmpInst::Predicate Pred;
BasicBlock *TrueBB, *FalseBB;
if (!match(TI, m_Br(m_ICmp(Pred, m_Specific(Ptr), m_Zero()), TrueBB,
return false;
if (Pred != ICmpInst::ICMP_EQ || MemsetBB != FalseBB)
return false;
return true;
if (Malloc->getOperand(0) != MemSet->getLength())
return false;
if (!shouldCreateCalloc(Malloc, MemSet) ||
!DT.dominates(Malloc, MemSet) ||
!memoryIsNotModifiedBetween(Malloc, MemSet, BatchAA, DL, &DT))
return false;
IRBuilder<> IRB(Malloc);
Type *SizeTTy = Malloc->getArgOperand(0)->getType();
auto *Calloc = emitCalloc(ConstantInt::get(SizeTTy, 1),
Malloc->getArgOperand(0), IRB, TLI);
if (!Calloc)
return false;
MemorySSAUpdater Updater(&MSSA);
auto *LastDef =
auto *NewAccess =
Updater.createMemoryAccessAfter(cast<Instruction>(Calloc), LastDef,
auto *NewAccessMD = cast<MemoryDef>(NewAccess);
Updater.insertDef(NewAccessMD, /*RenameUses=*/true);
return true;
/// \returns true if \p Def is a no-op store, either because it
/// directly stores back a loaded value or stores zero to a calloced object.
bool storeIsNoop(MemoryDef *Def, const Value *DefUO) {
Instruction *DefI = Def->getMemoryInst();
StoreInst *Store = dyn_cast<StoreInst>(DefI);
MemSetInst *MemSet = dyn_cast<MemSetInst>(DefI);
Constant *StoredConstant = nullptr;
if (Store)
StoredConstant = dyn_cast<Constant>(Store->getOperand(0));
else if (MemSet)
StoredConstant = dyn_cast<Constant>(MemSet->getValue());
return false;
if (!isRemovable(DefI))
return false;
if (StoredConstant) {
Constant *InitC =
getInitialValueOfAllocation(DefUO, &TLI, StoredConstant->getType());
// If the clobbering access is LiveOnEntry, no instructions between them
// can modify the memory location.
if (InitC && InitC == StoredConstant)
return MSSA.isLiveOnEntryDef(
MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def, BatchAA));
if (!Store)
return false;
if (auto *LoadI = dyn_cast<LoadInst>(Store->getOperand(0))) {
if (LoadI->getPointerOperand() == Store->getOperand(1)) {
// Get the defining access for the load.
auto *LoadAccess = MSSA.getMemoryAccess(LoadI)->getDefiningAccess();
// Fast path: the defining accesses are the same.
if (LoadAccess == Def->getDefiningAccess())
return true;
// Look through phi accesses. Recursively scan all phi accesses by
// adding them to a worklist. Bail when we run into a memory def that
// does not match LoadAccess.
SetVector<MemoryAccess *> ToCheck;
MemoryAccess *Current =
MSSA.getWalker()->getClobberingMemoryAccess(Def, BatchAA);
// We don't want to bail when we run into the store memory def. But,
// the phi access may point to it. So, pretend like we've already
// checked it.
// Start at current (1) to simulate already having checked Def.
for (unsigned I = 1; I < ToCheck.size(); ++I) {
Current = ToCheck[I];
if (auto PhiAccess = dyn_cast<MemoryPhi>(Current)) {
// Check all the operands.
for (auto &Use : PhiAccess->incoming_values())
// If we found a memory def, bail. This happens when we have an
// unrelated write in between an otherwise noop store.
assert(isa<MemoryDef>(Current) &&
"Only MemoryDefs should reach here.");
// TODO: Skip no alias MemoryDefs that have no aliasing reads.
// We are searching for the definition of the store's destination.
// So, if that is the same definition as the load, then this is a
// noop. Otherwise, fail.
if (LoadAccess != Current)
return false;
return true;
return false;
bool removePartiallyOverlappedStores(InstOverlapIntervalsTy &IOL) {
bool Changed = false;
for (auto OI : IOL) {
Instruction *DeadI = OI.first;
MemoryLocation Loc = *getLocForWrite(DeadI);
assert(isRemovable(DeadI) && "Expect only removable instruction");
const Value *Ptr = Loc.Ptr->stripPointerCasts();
int64_t DeadStart = 0;
uint64_t DeadSize = Loc.Size.getValue();
GetPointerBaseWithConstantOffset(Ptr, DeadStart, DL);
OverlapIntervalsTy &IntervalMap = OI.second;
Changed |= tryToShortenEnd(DeadI, IntervalMap, DeadStart, DeadSize);
if (IntervalMap.empty())
Changed |= tryToShortenBegin(DeadI, IntervalMap, DeadStart, DeadSize);
return Changed;
/// Eliminates writes to locations where the value that is being written
/// is already stored at the same location.
bool eliminateRedundantStoresOfExistingValues() {
bool MadeChange = false;
LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs that write the "
"already existing value\n");
for (auto *Def : MemDefs) {
if (SkipStores.contains(Def) || MSSA.isLiveOnEntryDef(Def))
Instruction *DefInst = Def->getMemoryInst();
auto MaybeDefLoc = getLocForWrite(DefInst);
if (!MaybeDefLoc || !isRemovable(DefInst))
MemoryDef *UpperDef;
// To conserve compile-time, we avoid walking to the next clobbering def.
// Instead, we just try to get the optimized access, if it exists. DSE
// will try to optimize defs during the earlier traversal.
if (Def->isOptimized())
UpperDef = dyn_cast<MemoryDef>(Def->getOptimized());
UpperDef = dyn_cast<MemoryDef>(Def->getDefiningAccess());
if (!UpperDef || MSSA.isLiveOnEntryDef(UpperDef))
Instruction *UpperInst = UpperDef->getMemoryInst();
auto IsRedundantStore = [&]() {
if (DefInst->isIdenticalTo(UpperInst))
return true;
if (auto *MemSetI = dyn_cast<MemSetInst>(UpperInst)) {
if (auto *SI = dyn_cast<StoreInst>(DefInst)) {
// MemSetInst must have a write location.
MemoryLocation UpperLoc = *getLocForWrite(UpperInst);
int64_t InstWriteOffset = 0;
int64_t DepWriteOffset = 0;
auto OR = isOverwrite(UpperInst, DefInst, UpperLoc, *MaybeDefLoc,
InstWriteOffset, DepWriteOffset);
Value *StoredByte = isBytewiseValue(SI->getValueOperand(), DL);
return StoredByte && StoredByte == MemSetI->getOperand(1) &&
OR == OW_Complete;
return false;
if (!IsRedundantStore() || isReadClobber(*MaybeDefLoc, DefInst))
LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n DEAD: " << *DefInst
<< '\n');
MadeChange = true;
return MadeChange;
static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
DominatorTree &DT, PostDominatorTree &PDT,
AssumptionCache &AC,
const TargetLibraryInfo &TLI,
const LoopInfo &LI) {
bool MadeChange = false;
DSEState State(F, AA, MSSA, DT, PDT, AC, TLI, LI);
// For each store:
for (unsigned I = 0; I < State.MemDefs.size(); I++) {
MemoryDef *KillingDef = State.MemDefs[I];
if (State.SkipStores.count(KillingDef))
Instruction *KillingI = KillingDef->getMemoryInst();
std::optional<MemoryLocation> MaybeKillingLoc;
if (State.isMemTerminatorInst(KillingI)) {
if (auto KillingLoc = State.getLocForTerminator(KillingI))
MaybeKillingLoc = KillingLoc->first;
} else {
MaybeKillingLoc = State.getLocForWrite(KillingI);
if (!MaybeKillingLoc) {
LLVM_DEBUG(dbgs() << "Failed to find analyzable write location for "
<< *KillingI << "\n");
MemoryLocation KillingLoc = *MaybeKillingLoc;
assert(KillingLoc.Ptr && "KillingLoc should not be null");
const Value *KillingUndObj = getUnderlyingObject(KillingLoc.Ptr);
LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs killed by "
<< *KillingDef << " (" << *KillingI << ")\n");
unsigned ScanLimit = MemorySSAScanLimit;
unsigned WalkerStepLimit = MemorySSAUpwardsStepLimit;
unsigned PartialLimit = MemorySSAPartialStoreLimit;
// Worklist of MemoryAccesses that may be killed by KillingDef.
SetVector<MemoryAccess *> ToCheck;
bool Shortend = false;
bool IsMemTerm = State.isMemTerminatorInst(KillingI);
// Check if MemoryAccesses in the worklist are killed by KillingDef.
for (unsigned I = 0; I < ToCheck.size(); I++) {
MemoryAccess *Current = ToCheck[I];
if (State.SkipStores.count(Current))
std::optional<MemoryAccess *> MaybeDeadAccess = State.getDomMemoryDef(
KillingDef, Current, KillingLoc, KillingUndObj, ScanLimit,
WalkerStepLimit, IsMemTerm, PartialLimit);
if (!MaybeDeadAccess) {
LLVM_DEBUG(dbgs() << " finished walk\n");
MemoryAccess *DeadAccess = *MaybeDeadAccess;
LLVM_DEBUG(dbgs() << " Checking if we can kill " << *DeadAccess);
if (isa<MemoryPhi>(DeadAccess)) {
LLVM_DEBUG(dbgs() << "\n ... adding incoming values to worklist\n");
for (Value *V : cast<MemoryPhi>(DeadAccess)->incoming_values()) {
MemoryAccess *IncomingAccess = cast<MemoryAccess>(V);
BasicBlock *IncomingBlock = IncomingAccess->getBlock();
BasicBlock *PhiBlock = DeadAccess->getBlock();
// We only consider incoming MemoryAccesses that come before the
// MemoryPhi. Otherwise we could discover candidates that do not
// strictly dominate our starting def.
if (State.PostOrderNumbers[IncomingBlock] >
auto *DeadDefAccess = cast<MemoryDef>(DeadAccess);
Instruction *DeadI = DeadDefAccess->getMemoryInst();
LLVM_DEBUG(dbgs() << " (" << *DeadI << ")\n");
if (!DebugCounter::shouldExecute(MemorySSACounter))
MemoryLocation DeadLoc = *State.getLocForWrite(DeadI);
if (IsMemTerm) {
const Value *DeadUndObj = getUnderlyingObject(DeadLoc.Ptr);
if (KillingUndObj != DeadUndObj)
LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " << *DeadI
<< "\n KILLER: " << *KillingI << '\n');
MadeChange = true;
} else {
// Check if DeadI overwrites KillingI.
int64_t KillingOffset = 0;
int64_t DeadOffset = 0;
OverwriteResult OR = State.isOverwrite(
KillingI, DeadI, KillingLoc, DeadLoc, KillingOffset, DeadOffset);
if (OR == OW_MaybePartial) {
auto Iter = State.IOLs.insert(
std::make_pair<BasicBlock *, InstOverlapIntervalsTy>(
DeadI->getParent(), InstOverlapIntervalsTy()));
auto &IOL = Iter.first->second;
OR = isPartialOverwrite(KillingLoc, DeadLoc, KillingOffset,
DeadOffset, DeadI, IOL);
if (EnablePartialStoreMerging && OR == OW_PartialEarlierWithFullLater) {
auto *DeadSI = dyn_cast<StoreInst>(DeadI);
auto *KillingSI = dyn_cast<StoreInst>(KillingI);
// We are re-using tryToMergePartialOverlappingStores, which requires
// DeadSI to dominate DeadSI.
// TODO: implement tryToMergeParialOverlappingStores using MemorySSA.
if (DeadSI && KillingSI && DT.dominates(DeadSI, KillingSI)) {
if (Constant *Merged = tryToMergePartialOverlappingStores(
KillingSI, DeadSI, KillingOffset, DeadOffset, State.DL,
State.BatchAA, &DT)) {
// Update stored value of earlier store to merged constant.
DeadSI->setOperand(0, Merged);
MadeChange = true;
Shortend = true;
// Remove killing store and remove any outstanding overlap
// intervals for the updated store.
auto I = State.IOLs.find(DeadSI->getParent());
if (I != State.IOLs.end())
if (OR == OW_Complete) {
LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " << *DeadI
<< "\n KILLER: " << *KillingI << '\n');
MadeChange = true;
// Check if the store is a no-op.
if (!Shortend && State.storeIsNoop(KillingDef, KillingUndObj)) {
LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n DEAD: " << *KillingI
<< '\n');
MadeChange = true;
// Can we form a calloc from a memset/malloc pair?
if (!Shortend && State.tryFoldIntoCalloc(KillingDef, KillingUndObj)) {
LLVM_DEBUG(dbgs() << "DSE: Remove memset after forming calloc:\n"
<< " DEAD: " << *KillingI << '\n');
MadeChange = true;
if (EnablePartialOverwriteTracking)
for (auto &KV : State.IOLs)
MadeChange |= State.removePartiallyOverlappedStores(KV.second);
MadeChange |= State.eliminateRedundantStoresOfExistingValues();
MadeChange |= State.eliminateDeadWritesAtEndOfFunction();
return MadeChange;
} // end anonymous namespace
// DSE Pass
PreservedAnalyses DSEPass::run(Function &F, FunctionAnalysisManager &AM) {
AliasAnalysis &AA = AM.getResult<AAManager>(F);
const TargetLibraryInfo &TLI = AM.getResult<TargetLibraryAnalysis>(F);
DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
MemorySSA &MSSA = AM.getResult<MemorySSAAnalysis>(F).getMSSA();
PostDominatorTree &PDT = AM.getResult<PostDominatorTreeAnalysis>(F);
AssumptionCache &AC = AM.getResult<AssumptionAnalysis>(F);
LoopInfo &LI = AM.getResult<LoopAnalysis>(F);
bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, AC, TLI, LI);
if (AreStatisticsEnabled())
for (auto &I : instructions(F))
NumRemainingStores += isa<StoreInst>(&I);
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
namespace {
/// A legacy pass for the legacy pass manager that wraps \c DSEPass.
class DSELegacyPass : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
DSELegacyPass() : FunctionPass(ID) {
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
AliasAnalysis &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
const TargetLibraryInfo &TLI =
MemorySSA &MSSA = getAnalysis<MemorySSAWrapperPass>().getMSSA();
PostDominatorTree &PDT =
AssumptionCache &AC =
LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, AC, TLI, LI);
if (AreStatisticsEnabled())
for (auto &I : instructions(F))
NumRemainingStores += isa<StoreInst>(&I);
return Changed;
void getAnalysisUsage(AnalysisUsage &AU) const override {
} // end anonymous namespace
char DSELegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(DSELegacyPass, "dse", "Dead Store Elimination", false,
INITIALIZE_PASS_END(DSELegacyPass, "dse", "Dead Store Elimination", false,
FunctionPass *llvm::createDeadStoreEliminationPass() {
return new DSELegacyPass();