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//===- DeadStoreElimination.cpp - Fast 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
// This file implements a trivial dead store elimination that only considers
// basic-block local redundant stores.
// FIXME: This should eventually be extended to be a post-dominator tree
// traversal. Doing so would be pretty trivial.
#include "llvm/Transforms/Scalar/DeadStoreElimination.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.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/CaptureTracking.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/OrderedBasicBlock.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <map>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "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");
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"));
// Helper functions
using OverlapIntervalsTy = std::map<int64_t, int64_t>;
using InstOverlapIntervalsTy = DenseMap<Instruction *, OverlapIntervalsTy>;
/// Delete this instruction. Before we do, go through and zero out all the
/// operands of this instruction. If any of them become dead, delete them and
/// the computation tree that feeds them.
/// If ValueSet is non-null, remove any deleted instructions from it as well.
static void
deleteDeadInstruction(Instruction *I, BasicBlock::iterator *BBI,
MemoryDependenceResults &MD, const TargetLibraryInfo &TLI,
InstOverlapIntervalsTy &IOL, OrderedBasicBlock &OBB,
SmallSetVector<const Value *, 16> *ValueSet = nullptr) {
SmallVector<Instruction*, 32> NowDeadInsts;
// Keeping the iterator straight is a pain, so we let this routine tell the
// caller what the next instruction is after we're done mucking about.
BasicBlock::iterator NewIter = *BBI;
// Before we touch this instruction, remove it from memdep!
do {
Instruction *DeadInst = NowDeadInsts.pop_back_val();
// Try to preserve debug information attached to the dead instruction.
// This instruction is dead, zap it, in stages. Start by removing it from
// MemDep, which needs to know the operands and needs it to be in the
// function.
for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
Value *Op = DeadInst->getOperand(op);
DeadInst->setOperand(op, nullptr);
// If this operand just became dead, add it to the NowDeadInsts list.
if (!Op->use_empty()) continue;
if (Instruction *OpI = dyn_cast<Instruction>(Op))
if (isInstructionTriviallyDead(OpI, &TLI))
if (ValueSet) ValueSet->remove(DeadInst);
if (NewIter == DeadInst->getIterator())
NewIter = DeadInst->eraseFromParent();
} while (!NowDeadInsts.empty());
*BBI = NewIter;
/// Does this instruction write some memory? This only returns true for things
/// that we can analyze with other helpers below.
static bool hasAnalyzableMemoryWrite(Instruction *I,
const TargetLibraryInfo &TLI) {
if (isa<StoreInst>(I))
return true;
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
return false;
case Intrinsic::memset:
case Intrinsic::memmove:
case Intrinsic::memcpy:
case Intrinsic::memcpy_element_unordered_atomic:
case Intrinsic::memmove_element_unordered_atomic:
case Intrinsic::memset_element_unordered_atomic:
case Intrinsic::init_trampoline:
case Intrinsic::lifetime_end:
return true;
if (auto CS = CallSite(I)) {
if (Function *F = CS.getCalledFunction()) {
StringRef FnName = F->getName();
if (TLI.has(LibFunc_strcpy) && FnName == TLI.getName(LibFunc_strcpy))
return true;
if (TLI.has(LibFunc_strncpy) && FnName == TLI.getName(LibFunc_strncpy))
return true;
if (TLI.has(LibFunc_strcat) && FnName == TLI.getName(LibFunc_strcat))
return true;
if (TLI.has(LibFunc_strncat) && FnName == TLI.getName(LibFunc_strncat))
return true;
return false;
/// Return a Location stored to by the specified instruction. If isRemovable
/// returns true, this function and getLocForRead completely describe the memory
/// operations for this instruction.
static MemoryLocation getLocForWrite(Instruction *Inst) {
if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
return MemoryLocation::get(SI);
if (auto *MI = dyn_cast<AnyMemIntrinsic>(Inst)) {
// memcpy/memmove/memset.
MemoryLocation Loc = MemoryLocation::getForDest(MI);
return Loc;
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst)) {
switch (II->getIntrinsicID()) {
return MemoryLocation(); // Unhandled intrinsic.
case Intrinsic::init_trampoline:
return MemoryLocation(II->getArgOperand(0));
case Intrinsic::lifetime_end: {
uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
return MemoryLocation(II->getArgOperand(1), Len);
if (auto CS = CallSite(Inst))
// All the supported TLI functions so far happen to have dest as their
// first argument.
return MemoryLocation(CS.getArgument(0));
return MemoryLocation();
/// Return the location read by the specified "hasAnalyzableMemoryWrite"
/// instruction if any.
static MemoryLocation getLocForRead(Instruction *Inst,
const TargetLibraryInfo &TLI) {
assert(hasAnalyzableMemoryWrite(Inst, TLI) && "Unknown instruction case");
// The only instructions that both read and write are the mem transfer
// instructions (memcpy/memmove).
if (auto *MTI = dyn_cast<AnyMemTransferInst>(Inst))
return MemoryLocation::getForSource(MTI);
return MemoryLocation();
/// If the value of this instruction and the memory it writes to is unused, may
/// we delete this instruction?
static bool isRemovable(Instruction *I) {
// Don't remove volatile/atomic stores.
if (StoreInst *SI = dyn_cast<StoreInst>(I))
return SI->isUnordered();
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default: llvm_unreachable("doesn't pass 'hasAnalyzableMemoryWrite' predicate");
case Intrinsic::lifetime_end:
// Never remove dead lifetime_end's, e.g. because it is followed by a
// free.
return false;
case Intrinsic::init_trampoline:
// Always safe to remove init_trampoline.
return true;
case Intrinsic::memset:
case Intrinsic::memmove:
case Intrinsic::memcpy:
// Don't remove volatile memory intrinsics.
return !cast<MemIntrinsic>(II)->isVolatile();
case Intrinsic::memcpy_element_unordered_atomic:
case Intrinsic::memmove_element_unordered_atomic:
case Intrinsic::memset_element_unordered_atomic:
return true;
// note: only get here for calls with analyzable writes - i.e. libcalls
if (auto CS = CallSite(I))
return CS.getInstruction()->use_empty();
return false;
/// 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);
/// Return the pointer that is being written to.
static Value *getStoredPointerOperand(Instruction *I) {
//TODO: factor this to reuse getLocForWrite
MemoryLocation Loc = getLocForWrite(I);
assert(Loc.Ptr &&
"unable to find pointer written for analyzable instruction?");
// TODO: most APIs don't expect const Value *
return const_cast<Value*>(Loc.Ptr);
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
/// Return 'OW_Complete' if a store to the 'Later' location completely
/// overwrites a store to the 'Earlier' location, 'OW_End' if the end of the
/// 'Earlier' location is completely overwritten by 'Later', 'OW_Begin' if the
/// beginning of the 'Earlier' location is overwritten by 'Later'.
/// 'OW_PartialEarlierWithFullLater' means that an earlier (big) store was
/// overwritten by a latter (smaller) store which doesn't write outside the big
/// store's memory locations. Returns 'OW_Unknown' if nothing can be determined.
static OverwriteResult isOverwrite(const MemoryLocation &Later,
const MemoryLocation &Earlier,
const DataLayout &DL,
const TargetLibraryInfo &TLI,
int64_t &EarlierOff, int64_t &LaterOff,
Instruction *DepWrite,
InstOverlapIntervalsTy &IOL,
AliasAnalysis &AA,
const Function *F) {
// FIXME: Vet that this works for size upper-bounds. Seems unlikely that we'll
// get imprecise values here, though (except for unknown sizes).
if (!Later.Size.isPrecise() || !Earlier.Size.isPrecise())
return OW_Unknown;
const uint64_t LaterSize = Later.Size.getValue();
const uint64_t EarlierSize = Earlier.Size.getValue();
const Value *P1 = Earlier.Ptr->stripPointerCasts();
const Value *P2 = Later.Ptr->stripPointerCasts();
// If the start pointers are the same, we just have to compare sizes to see if
// the later store was larger than the earlier store.
if (P1 == P2 || AA.isMustAlias(P1, P2)) {
// Make sure that the Later size is >= the Earlier size.
if (LaterSize >= EarlierSize)
return OW_Complete;
// Check to see if the later store is to the entire object (either a global,
// an alloca, or a byval/inalloca argument). If so, then it clearly
// overwrites any other store to the same object.
const Value *UO1 = GetUnderlyingObject(P1, DL),
*UO2 = GetUnderlyingObject(P2, DL);
// If we can't resolve the same pointers to the same object, then we can't
// analyze them at all.
if (UO1 != UO2)
return OW_Unknown;
// If the "Later" store is to a recognizable object, get its size.
uint64_t ObjectSize = getPointerSize(UO2, DL, TLI, F);
if (ObjectSize != MemoryLocation::UnknownSize)
if (ObjectSize == LaterSize && ObjectSize >= EarlierSize)
return OW_Complete;
// 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.
EarlierOff = 0;
LaterOff = 0;
const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, DL);
const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, DL);
// If the base pointers still differ, we have two completely different stores.
if (BP1 != BP2)
return OW_Unknown;
// The later store completely overlaps the earlier store if:
// 1. Both start at the same offset and the later one's size is greater than
// or equal to the earlier one's, or
// |--earlier--|
// |-- later --|
// 2. The earlier store has an offset greater than the later offset, but which
// still lies completely within the later store.
// |--earlier--|
// |----- later ------|
// We have to be careful here as *Off is signed while *.Size is unsigned.
if (EarlierOff >= LaterOff &&
LaterSize >= EarlierSize &&
uint64_t(EarlierOff - LaterOff) + EarlierSize <= LaterSize)
return OW_Complete;
// We may now overlap, although the overlap is not complete. There might also
// be other incomplete overlaps, and together, they might cover the complete
// earlier write.
// 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 &&
LaterOff < int64_t(EarlierOff + EarlierSize) &&
int64_t(LaterOff + LaterSize) >= EarlierOff) {
// Insert our part of the overlap into the map.
auto &IM = IOL[DepWrite];
LLVM_DEBUG(dbgs() << "DSE: Partial overwrite: Earlier [" << EarlierOff
<< ", " << int64_t(EarlierOff + EarlierSize)
<< ") Later [" << LaterOff << ", "
<< int64_t(LaterOff + LaterSize) << ")\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 LaterIntStart = LaterOff, LaterIntEnd = LaterOff + LaterSize;
// Find any intervals ending at, or after, LaterIntStart which start
// before LaterIntEnd.
auto ILI = IM.lower_bound(LaterIntStart);
if (ILI != IM.end() && ILI->second <= LaterIntEnd) {
// This existing interval is overlapped with the current store somewhere
// in [LaterIntStart, LaterIntEnd]. Merge them by erasing the existing
// intervals and adjusting our start and end.
LaterIntStart = std::min(LaterIntStart, ILI->second);
LaterIntEnd = std::max(LaterIntEnd, ILI->first);
ILI = IM.erase(ILI);
// Continue erasing and adjusting our end in case other previous
// intervals are also overlapped with the current store.
// |--- ealier 1 ---| |--- ealier 2 ---|
// |------- later---------|
while (ILI != IM.end() && ILI->second <= LaterIntEnd) {
assert(ILI->second > LaterIntStart && "Unexpected interval");
LaterIntEnd = std::max(LaterIntEnd, ILI->first);
ILI = IM.erase(ILI);
IM[LaterIntEnd] = LaterIntStart;
ILI = IM.begin();
if (ILI->second <= EarlierOff &&
ILI->first >= int64_t(EarlierOff + EarlierSize)) {
LLVM_DEBUG(dbgs() << "DSE: Full overwrite from partials: Earlier ["
<< EarlierOff << ", "
<< int64_t(EarlierOff + EarlierSize)
<< ") Composite Later [" << ILI->second << ", "
<< ILI->first << ")\n");
return OW_Complete;
// Check for an earlier store which writes to all the memory locations that
// the later store writes to.
if (EnablePartialStoreMerging && LaterOff >= EarlierOff &&
int64_t(EarlierOff + EarlierSize) > LaterOff &&
uint64_t(LaterOff - EarlierOff) + LaterSize <= EarlierSize) {
LLVM_DEBUG(dbgs() << "DSE: Partial overwrite an earlier load ["
<< EarlierOff << ", "
<< int64_t(EarlierOff + EarlierSize)
<< ") by a later store [" << LaterOff << ", "
<< int64_t(LaterOff + LaterSize) << ")\n");
// TODO: Maybe come up with a better name?
return OW_PartialEarlierWithFullLater;
// Another interesting case is if the later store overwrites the end of the
// earlier store.
// |--earlier--|
// |-- later --|
// In this case we may want to trim the size of earlier to avoid generating
// writes to addresses which will definitely be overwritten later
if (!EnablePartialOverwriteTracking &&
(LaterOff > EarlierOff && LaterOff < int64_t(EarlierOff + EarlierSize) &&
int64_t(LaterOff + LaterSize) >= int64_t(EarlierOff + EarlierSize)))
return OW_End;
// Finally, we also need to check if the later store overwrites the beginning
// of the earlier store.
// |--earlier--|
// |-- later --|
// In this case we may want to move the destination address and trim the size
// of earlier to avoid generating writes to addresses which will definitely
// be overwritten later.
if (!EnablePartialOverwriteTracking &&
(LaterOff <= EarlierOff && int64_t(LaterOff + LaterSize) > EarlierOff)) {
assert(int64_t(LaterOff + LaterSize) < int64_t(EarlierOff + EarlierSize) &&
"Expect to be handled as OW_Complete");
return OW_Begin;
// Otherwise, they don't completely overlap.
return OW_Unknown;
/// If 'Inst' might be a self read (i.e. a noop copy of a
/// memory region into an identical pointer) then it doesn't actually make its
/// input dead in the traditional sense. Consider this case:
/// memmove(A <- B)
/// memmove(A <- A)
/// In this case, the second store to A does not make the first store to A dead.
/// The usual situation isn't an explicit A<-A store like this (which can be
/// trivially removed) but a case where two pointers may alias.
/// This function detects when it is unsafe to remove a dependent instruction
/// because the DSE inducing instruction may be a self-read.
static bool isPossibleSelfRead(Instruction *Inst,
const MemoryLocation &InstStoreLoc,
Instruction *DepWrite,
const TargetLibraryInfo &TLI,
AliasAnalysis &AA) {
// Self reads can only happen for instructions that read memory. Get the
// location read.
MemoryLocation InstReadLoc = getLocForRead(Inst, TLI);
if (!InstReadLoc.Ptr)
return false; // Not a reading instruction.
// If the read and written loc obviously don't alias, it isn't a read.
if (AA.isNoAlias(InstReadLoc, InstStoreLoc))
return false;
if (isa<AnyMemCpyInst>(Inst)) {
// LLVM's memcpy overlap semantics are not fully fleshed out (see PR11763)
// but in practice memcpy(A <- B) either means that A and B are disjoint or
// are equal (i.e. there are not partial overlaps). Given that, if we have:
// memcpy/memmove(A <- B) // DepWrite
// memcpy(A <- B) // Inst
// with Inst reading/writing a >= size than DepWrite, we can reason as
// follows:
// - If A == B then both the copies are no-ops, so the DepWrite can be
// removed.
// - If A != B then A and B are disjoint locations in Inst. Since
// Inst.size >= DepWrite.size A and B are disjoint in DepWrite too.
// Therefore DepWrite can be removed.
MemoryLocation DepReadLoc = getLocForRead(DepWrite, TLI);
if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
return false;
// If DepWrite doesn't read memory or if we can't prove it is a must alias,
// then it can't be considered dead.
return true;
/// 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,
AliasAnalysis *AA) {
SmallVector<BasicBlock *, 16> WorkList;
SmallPtrSet<BasicBlock *, 8> Visited;
BasicBlock::iterator FirstBBI(FirstI);
BasicBlock::iterator SecondBBI(SecondI);
BasicBlock *FirstBB = FirstI->getParent();
BasicBlock *SecondBB = SecondI->getParent();
MemoryLocation MemLoc = MemoryLocation::get(SecondI);
// Start checking the store-block.
bool isFirstBlock = true;
// Check all blocks going backward until we reach the load-block.
while (!WorkList.empty()) {
BasicBlock *B = WorkList.pop_back_val();
// Ignore instructions before LI if this is the FirstBB.
BasicBlock::iterator BI = (B == FirstBB ? FirstBBI : B->begin());
BasicBlock::iterator EI;
if (isFirstBlock) {
// Ignore instructions after SI 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 SI.
EI = B->end();
for (; BI != EI; ++BI) {
Instruction *I = &*BI;
if (I->mayWriteToMemory() && I != SecondI)
if (isModSet(AA->getModRefInfo(I, MemLoc)))
return false;
if (B != FirstBB) {
assert(B != &FirstBB->getParent()->getEntryBlock() &&
"Should not hit the entry block because SI must be dominated by LI");
for (auto PredI = pred_begin(B), PE = pred_end(B); PredI != PE; ++PredI) {
if (!Visited.insert(*PredI).second)
return true;
/// Find all blocks that will unconditionally lead to the block BB and append
/// them to F.
static void findUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
BasicBlock *BB, DominatorTree *DT) {
for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
BasicBlock *Pred = *I;
if (Pred == BB) continue;
Instruction *PredTI = Pred->getTerminator();
if (PredTI->getNumSuccessors() != 1)
if (DT->isReachableFromEntry(Pred))
/// Handle frees of entire structures whose dependency is a store
/// to a field of that structure.
static bool handleFree(CallInst *F, AliasAnalysis *AA,
MemoryDependenceResults *MD, DominatorTree *DT,
const TargetLibraryInfo *TLI,
InstOverlapIntervalsTy &IOL, OrderedBasicBlock &OBB) {
bool MadeChange = false;
MemoryLocation Loc = MemoryLocation(F->getOperand(0));
SmallVector<BasicBlock *, 16> Blocks;
const DataLayout &DL = F->getModule()->getDataLayout();
while (!Blocks.empty()) {
BasicBlock *BB = Blocks.pop_back_val();
Instruction *InstPt = BB->getTerminator();
if (BB == F->getParent()) InstPt = F;
MemDepResult Dep =
MD->getPointerDependencyFrom(Loc, false, InstPt->getIterator(), BB);
while (Dep.isDef() || Dep.isClobber()) {
Instruction *Dependency = Dep.getInst();
if (!hasAnalyzableMemoryWrite(Dependency, *TLI) ||
Value *DepPointer =
GetUnderlyingObject(getStoredPointerOperand(Dependency), DL);
// Check for aliasing.
if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
dbgs() << "DSE: Dead Store to soon to be freed memory:\n DEAD: "
<< *Dependency << '\n');
// DCE instructions only used to calculate that store.
BasicBlock::iterator BBI(Dependency);
deleteDeadInstruction(Dependency, &BBI, *MD, *TLI, IOL, OBB);
MadeChange = true;
// Inst's old Dependency is now deleted. Compute the next dependency,
// which may also be dead, as in
// s[0] = 0;
// s[1] = 0; // This has just been deleted.
// free(s);
Dep = MD->getPointerDependencyFrom(Loc, false, BBI, BB);
if (Dep.isNonLocal())
findUnconditionalPreds(Blocks, BB, DT);
return MadeChange;
/// Check to see if the specified location may alias any of the stack objects in
/// the DeadStackObjects set. If so, they become live because the location is
/// being loaded.
static void removeAccessedObjects(const MemoryLocation &LoadedLoc,
SmallSetVector<const Value *, 16> &DeadStackObjects,
const DataLayout &DL, AliasAnalysis *AA,
const TargetLibraryInfo *TLI,
const Function *F) {
const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr, DL);
// A constant can't be in the dead pointer set.
if (isa<Constant>(UnderlyingPointer))
// If the kill pointer can be easily reduced to an alloca, don't bother doing
// extraneous AA queries.
if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
// Remove objects that could alias LoadedLoc.
DeadStackObjects.remove_if([&](const Value *I) {
// See if the loaded location could alias the stack location.
MemoryLocation StackLoc(I, getPointerSize(I, DL, *TLI, F));
return !AA->isNoAlias(StackLoc, LoadedLoc);
/// Remove dead stores to stack-allocated locations in the function end block.
/// Ex:
/// %A = alloca i32
/// ...
/// store i32 1, i32* %A
/// ret void
static bool handleEndBlock(BasicBlock &BB, AliasAnalysis *AA,
MemoryDependenceResults *MD,
const TargetLibraryInfo *TLI,
InstOverlapIntervalsTy &IOL,
OrderedBasicBlock &OBB) {
bool MadeChange = false;
// Keep track of all of the stack objects that are dead at the end of the
// function.
SmallSetVector<const Value*, 16> DeadStackObjects;
// Find all of the alloca'd pointers in the entry block.
BasicBlock &Entry = BB.getParent()->front();
for (Instruction &I : Entry) {
if (isa<AllocaInst>(&I))
// Okay, so these are dead heap objects, but if the pointer never escapes
// then it's leaked by this function anyways.
else if (isAllocLikeFn(&I, TLI) && !PointerMayBeCaptured(&I, true, true))
// Treat byval or inalloca arguments the same, stores to them are dead at the
// end of the function.
for (Argument &AI : BB.getParent()->args())
if (AI.hasByValOrInAllocaAttr())
const DataLayout &DL = BB.getModule()->getDataLayout();
// Scan the basic block backwards
for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
// If we find a store, check to see if it points into a dead stack value.
if (hasAnalyzableMemoryWrite(&*BBI, *TLI) && isRemovable(&*BBI)) {
// See through pointer-to-pointer bitcasts
SmallVector<const Value *, 4> Pointers;
GetUnderlyingObjects(getStoredPointerOperand(&*BBI), Pointers, DL);
// Stores to stack values are valid candidates for removal.
bool AllDead = true;
for (const Value *Pointer : Pointers)
if (!DeadStackObjects.count(Pointer)) {
AllDead = false;
if (AllDead) {
Instruction *Dead = &*BBI;
LLVM_DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
<< *Dead << "\n Objects: ";
for (SmallVectorImpl<const Value *>::iterator I =
E = Pointers.end();
I != E; ++I) {
dbgs() << **I;
if (std::next(I) != E)
dbgs() << ", ";
} dbgs()
<< '\n');
// DCE instructions only used to calculate that store.
deleteDeadInstruction(Dead, &BBI, *MD, *TLI, IOL, OBB,
MadeChange = true;
// Remove any dead non-memory-mutating instructions.
if (isInstructionTriviallyDead(&*BBI, TLI)) {
LLVM_DEBUG(dbgs() << "DSE: Removing trivially dead instruction:\n DEAD: "
<< *&*BBI << '\n');
deleteDeadInstruction(&*BBI, &BBI, *MD, *TLI, IOL, OBB,
MadeChange = true;
if (isa<AllocaInst>(BBI)) {
// Remove allocas from the list of dead stack objects; there can't be
// any references before the definition.
if (auto *Call = dyn_cast<CallBase>(&*BBI)) {
// Remove allocation function calls from the list of dead stack objects;
// there can't be any references before the definition.
if (isAllocLikeFn(&*BBI, TLI))
// If this call does not access memory, it can't be loading any of our
// pointers.
if (AA->doesNotAccessMemory(Call))
// If the call might load from any of our allocas, then any store above
// the call is live.
DeadStackObjects.remove_if([&](const Value *I) {
// See if the call site touches the value.
return isRefSet(AA->getModRefInfo(
Call, I, getPointerSize(I, DL, *TLI, BB.getParent())));
// If all of the allocas were clobbered by the call then we're not going
// to find anything else to process.
if (DeadStackObjects.empty())
// 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>(*BBI))
MemoryLocation LoadedLoc;
// If we encounter a use of the pointer, it is no longer considered dead
if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
if (!L->isUnordered()) // Be conservative with atomic/volatile load
LoadedLoc = MemoryLocation::get(L);
} else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
LoadedLoc = MemoryLocation::get(V);
} else if (!BBI->mayReadFromMemory()) {
// Instruction doesn't read memory. Note that stores that weren't removed
// above will hit this case.
} else {
// Unknown inst; assume it clobbers everything.
// Remove any allocas from the DeadPointer set that are loaded, as this
// makes any stores above the access live.
removeAccessedObjects(LoadedLoc, DeadStackObjects, DL, AA, TLI, BB.getParent());
// If all of the allocas were clobbered by the access then we're not going
// to find anything else to process.
if (DeadStackObjects.empty())
return MadeChange;
static bool tryToShorten(Instruction *EarlierWrite, int64_t &EarlierOffset,
int64_t &EarlierSize, int64_t LaterOffset,
int64_t LaterSize, bool IsOverwriteEnd) {
// TODO: base this on the target vector size so that if the earlier
// store was too small to get vector writes anyway then its likely
// a good idea to shorten it
// Power of 2 vector writes are probably always a bad idea to optimize
// as any store/memset/memcpy is likely using vector instructions so
// shortening it to not vector size is likely to be slower
auto *EarlierIntrinsic = cast<AnyMemIntrinsic>(EarlierWrite);
unsigned EarlierWriteAlign = EarlierIntrinsic->getDestAlignment();
if (!IsOverwriteEnd)
LaterOffset = int64_t(LaterOffset + LaterSize);
if (!(isPowerOf2_64(LaterOffset) && EarlierWriteAlign <= LaterOffset) &&
!((EarlierWriteAlign != 0) && LaterOffset % EarlierWriteAlign == 0))
return false;
int64_t NewLength = IsOverwriteEnd
? LaterOffset - EarlierOffset
: EarlierSize - (LaterOffset - EarlierOffset);
if (auto *AMI = dyn_cast<AtomicMemIntrinsic>(EarlierWrite)) {
// 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 != NewLength % ElementSize)
return false;
LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW "
<< (IsOverwriteEnd ? "END" : "BEGIN") << ": "
<< *EarlierWrite << "\n KILLER (offset " << LaterOffset
<< ", " << EarlierSize << ")\n");
Value *EarlierWriteLength = EarlierIntrinsic->getLength();
Value *TrimmedLength =
ConstantInt::get(EarlierWriteLength->getType(), NewLength);
EarlierSize = NewLength;
if (!IsOverwriteEnd) {
int64_t OffsetMoved = (LaterOffset - EarlierOffset);
Value *Indices[1] = {
ConstantInt::get(EarlierWriteLength->getType(), OffsetMoved)};
GetElementPtrInst *NewDestGEP = GetElementPtrInst::CreateInBounds(
EarlierIntrinsic->getRawDest(), Indices, "", EarlierWrite);
EarlierOffset = EarlierOffset + OffsetMoved;
return true;
static bool tryToShortenEnd(Instruction *EarlierWrite,
OverlapIntervalsTy &IntervalMap,
int64_t &EarlierStart, int64_t &EarlierSize) {
if (IntervalMap.empty() || !isShortenableAtTheEnd(EarlierWrite))
return false;
OverlapIntervalsTy::iterator OII = --IntervalMap.end();
int64_t LaterStart = OII->second;
int64_t LaterSize = OII->first - LaterStart;
if (LaterStart > EarlierStart && LaterStart < EarlierStart + EarlierSize &&
LaterStart + LaterSize >= EarlierStart + EarlierSize) {
if (tryToShorten(EarlierWrite, EarlierStart, EarlierSize, LaterStart,
LaterSize, true)) {
return true;
return false;
static bool tryToShortenBegin(Instruction *EarlierWrite,
OverlapIntervalsTy &IntervalMap,
int64_t &EarlierStart, int64_t &EarlierSize) {
if (IntervalMap.empty() || !isShortenableAtTheBeginning(EarlierWrite))
return false;
OverlapIntervalsTy::iterator OII = IntervalMap.begin();
int64_t LaterStart = OII->second;
int64_t LaterSize = OII->first - LaterStart;
if (LaterStart <= EarlierStart && LaterStart + LaterSize > EarlierStart) {
assert(LaterStart + LaterSize < EarlierStart + EarlierSize &&
"Should have been handled as OW_Complete");
if (tryToShorten(EarlierWrite, EarlierStart, EarlierSize, LaterStart,
LaterSize, false)) {
return true;
return false;
static bool removePartiallyOverlappedStores(AliasAnalysis *AA,
const DataLayout &DL,
InstOverlapIntervalsTy &IOL) {
bool Changed = false;
for (auto OI : IOL) {
Instruction *EarlierWrite = OI.first;
MemoryLocation Loc = getLocForWrite(EarlierWrite);
assert(isRemovable(EarlierWrite) && "Expect only removable instruction");
const Value *Ptr = Loc.Ptr->stripPointerCasts();
int64_t EarlierStart = 0;
int64_t EarlierSize = int64_t(Loc.Size.getValue());
GetPointerBaseWithConstantOffset(Ptr, EarlierStart, DL);
OverlapIntervalsTy &IntervalMap = OI.second;
Changed |=
tryToShortenEnd(EarlierWrite, IntervalMap, EarlierStart, EarlierSize);
if (IntervalMap.empty())
Changed |=
tryToShortenBegin(EarlierWrite, IntervalMap, EarlierStart, EarlierSize);
return Changed;
static bool eliminateNoopStore(Instruction *Inst, BasicBlock::iterator &BBI,
AliasAnalysis *AA, MemoryDependenceResults *MD,
const DataLayout &DL,
const TargetLibraryInfo *TLI,
InstOverlapIntervalsTy &IOL,
OrderedBasicBlock &OBB) {
// Must be a store instruction.
StoreInst *SI = dyn_cast<StoreInst>(Inst);
if (!SI)
return false;
// If we're storing the same value back to a pointer that we just loaded from,
// then the store can be removed.
if (LoadInst *DepLoad = dyn_cast<LoadInst>(SI->getValueOperand())) {
if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
isRemovable(SI) && memoryIsNotModifiedBetween(DepLoad, SI, AA)) {
dbgs() << "DSE: Remove Store Of Load from same pointer:\n LOAD: "
<< *DepLoad << "\n STORE: " << *SI << '\n');
deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL, OBB);
return true;
// Remove null stores into the calloc'ed objects
Constant *StoredConstant = dyn_cast<Constant>(SI->getValueOperand());
if (StoredConstant && StoredConstant->isNullValue() && isRemovable(SI)) {
Instruction *UnderlyingPointer =
dyn_cast<Instruction>(GetUnderlyingObject(SI->getPointerOperand(), DL));
if (UnderlyingPointer && isCallocLikeFn(UnderlyingPointer, TLI) &&
memoryIsNotModifiedBetween(UnderlyingPointer, SI, AA)) {
dbgs() << "DSE: Remove null store to the calloc'ed object:\n DEAD: "
<< *Inst << "\n OBJECT: " << *UnderlyingPointer << '\n');
deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL, OBB);
return true;
return false;
static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
MemoryDependenceResults *MD, DominatorTree *DT,
const TargetLibraryInfo *TLI) {
const DataLayout &DL = BB.getModule()->getDataLayout();
bool MadeChange = false;
OrderedBasicBlock OBB(&BB);
Instruction *LastThrowing = nullptr;
// A map of interval maps representing partially-overwritten value parts.
InstOverlapIntervalsTy IOL;
// Do a top-down walk on the BB.
for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
// Handle 'free' calls specially.
if (CallInst *F = isFreeCall(&*BBI, TLI)) {
MadeChange |= handleFree(F, AA, MD, DT, TLI, IOL, OBB);
// Increment BBI after handleFree has potentially deleted instructions.
// This ensures we maintain a valid iterator.
Instruction *Inst = &*BBI++;
if (Inst->mayThrow()) {
LastThrowing = Inst;
// Check to see if Inst writes to memory. If not, continue.
if (!hasAnalyzableMemoryWrite(Inst, *TLI))
// eliminateNoopStore will update in iterator, if necessary.
if (eliminateNoopStore(Inst, BBI, AA, MD, DL, TLI, IOL, OBB)) {
MadeChange = true;
// If we find something that writes memory, get its memory dependence.
MemDepResult InstDep = MD->getDependency(Inst, &OBB);
// Ignore any store where we can't find a local dependence.
// FIXME: cross-block DSE would be fun. :)
if (!InstDep.isDef() && !InstDep.isClobber())
// Figure out what location is being stored to.
MemoryLocation Loc = getLocForWrite(Inst);
// If we didn't get a useful location, fail.
if (!Loc.Ptr)
// Loop until we find a store we can eliminate or a load that
// invalidates the analysis. Without an upper bound on the number of
// instructions examined, this analysis can become very time-consuming.
// However, the potential gain diminishes as we process more instructions
// without eliminating any of them. Therefore, we limit the number of
// instructions we look at.
auto Limit = MD->getDefaultBlockScanLimit();
while (InstDep.isDef() || InstDep.isClobber()) {
// Get the memory clobbered by the instruction we depend on. MemDep will
// skip any instructions that 'Loc' clearly doesn't interact with. If we
// end up depending on a may- or must-aliased load, then we can't optimize
// away the store and we bail out. However, if we depend on something
// that overwrites the memory location we *can* potentially optimize it.
// Find out what memory location the dependent instruction stores.
Instruction *DepWrite = InstDep.getInst();
if (!hasAnalyzableMemoryWrite(DepWrite, *TLI))
MemoryLocation DepLoc = getLocForWrite(DepWrite);
// If we didn't get a useful location, or if it isn't a size, bail out.
if (!DepLoc.Ptr)
// Make sure we don't look past a call which might throw. This is an
// issue because MemoryDependenceAnalysis works in the wrong direction:
// it finds instructions which dominate the current instruction, rather than
// instructions which are post-dominated by the current instruction.
// If the underlying object is a non-escaping memory allocation, any store
// to it is dead along the unwind edge. Otherwise, we need to preserve
// the store.
if (LastThrowing && OBB.dominates(DepWrite, LastThrowing)) {
const Value* Underlying = GetUnderlyingObject(DepLoc.Ptr, DL);
bool IsStoreDeadOnUnwind = isa<AllocaInst>(Underlying);
if (!IsStoreDeadOnUnwind) {
// We're looking for a call to an allocation function
// where the allocation doesn't escape before the last
// throwing instruction; PointerMayBeCaptured
// reasonably fast approximation.
IsStoreDeadOnUnwind = isAllocLikeFn(Underlying, TLI) &&
!PointerMayBeCaptured(Underlying, false, true);
if (!IsStoreDeadOnUnwind)
// If we find a write that is a) removable (i.e., non-volatile), b) is
// completely obliterated by the store to 'Loc', and c) which we know that
// 'Inst' doesn't load from, then we can remove it.
// Also try to merge two stores if a later one only touches memory written
// to by the earlier one.
if (isRemovable(DepWrite) &&
!isPossibleSelfRead(Inst, Loc, DepWrite, *TLI, *AA)) {
int64_t InstWriteOffset, DepWriteOffset;
OverwriteResult OR = isOverwrite(Loc, DepLoc, DL, *TLI, DepWriteOffset,
InstWriteOffset, DepWrite, IOL, *AA,
if (OR == OW_Complete) {
LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " << *DepWrite
<< "\n KILLER: " << *Inst << '\n');
// Delete the store and now-dead instructions that feed it.
deleteDeadInstruction(DepWrite, &BBI, *MD, *TLI, IOL, OBB);
MadeChange = true;
// We erased DepWrite; start over.
InstDep = MD->getDependency(Inst, &OBB);
} else if ((OR == OW_End && isShortenableAtTheEnd(DepWrite)) ||
((OR == OW_Begin &&
isShortenableAtTheBeginning(DepWrite)))) {
assert(!EnablePartialOverwriteTracking && "Do not expect to perform "
"when partial-overwrite "
"tracking is enabled");
// The overwrite result is known, so these must be known, too.
int64_t EarlierSize = DepLoc.Size.getValue();
int64_t LaterSize = Loc.Size.getValue();
bool IsOverwriteEnd = (OR == OW_End);
MadeChange |= tryToShorten(DepWrite, DepWriteOffset, EarlierSize,
InstWriteOffset, LaterSize, IsOverwriteEnd);
} else if (EnablePartialStoreMerging &&
OR == OW_PartialEarlierWithFullLater) {
auto *Earlier = dyn_cast<StoreInst>(DepWrite);
auto *Later = dyn_cast<StoreInst>(Inst);
if (Earlier && isa<ConstantInt>(Earlier->getValueOperand()) &&
Earlier->getValueOperand()->getType()) &&
Later && isa<ConstantInt>(Later->getValueOperand()) &&
Later->getValueOperand()->getType()) &&
memoryIsNotModifiedBetween(Earlier, Later, AA)) {
// If the store we find is:
// a) partially overwritten by the store to 'Loc'
// b) the later store is fully contained in the earlier one and
// c) they both have a constant value
// d) none of the two stores need padding
// Merge the two stores, replacing the earlier 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 EarlierValue =
APInt LaterValue =
unsigned LaterBits = LaterValue.getBitWidth();
assert(EarlierValue.getBitWidth() > LaterValue.getBitWidth());
LaterValue = LaterValue.zext(EarlierValue.getBitWidth());
// Offset of the smaller store inside the larger store
unsigned BitOffsetDiff = (InstWriteOffset - DepWriteOffset) * 8;
unsigned LShiftAmount =
? EarlierValue.getBitWidth() - BitOffsetDiff - LaterBits
: BitOffsetDiff;
APInt Mask =
APInt::getBitsSet(EarlierValue.getBitWidth(), LShiftAmount,
LShiftAmount + LaterBits);
// Clear the bits we'll be replacing, then OR with the smaller
// store, shifted appropriately.
APInt Merged =
(EarlierValue & ~Mask) | (LaterValue << LShiftAmount);
LLVM_DEBUG(dbgs() << "DSE: Merge Stores:\n Earlier: " << *DepWrite
<< "\n Later: " << *Inst
<< "\n Merged Value: " << Merged << '\n');
auto *SI = new StoreInst(
ConstantInt::get(Earlier->getValueOperand()->getType(), Merged),
Earlier->getPointerOperand(), false,
MaybeAlign(Earlier->getAlignment()), Earlier->getOrdering(),
Earlier->getSyncScopeID(), DepWrite);
unsigned MDToKeep[] = {LLVMContext::MD_dbg, LLVMContext::MD_tbaa,
SI->copyMetadata(*DepWrite, MDToKeep);
// Remove earlier, wider, store
OBB.replaceInstruction(DepWrite, SI);
// Delete the old stores and now-dead instructions that feed them.
deleteDeadInstruction(Inst, &BBI, *MD, *TLI, IOL, OBB);
deleteDeadInstruction(DepWrite, &BBI, *MD, *TLI, IOL, OBB);
MadeChange = true;
// We erased DepWrite and Inst (Loc); start over.
// If this is a may-aliased store that is clobbering the store value, we
// can keep searching past it for another must-aliased pointer that stores
// to the same location. For example, in:
// store -> P
// store -> Q
// store -> P
// we can remove the first store to P even though we don't know if P and Q
// alias.
if (DepWrite == &BB.front()) break;
// Can't look past this instruction if it might read 'Loc'.
if (isRefSet(AA->getModRefInfo(DepWrite, Loc)))
InstDep = MD->getPointerDependencyFrom(Loc, /*isLoad=*/ false,
DepWrite->getIterator(), &BB,
/*QueryInst=*/ nullptr, &Limit);
if (EnablePartialOverwriteTracking)
MadeChange |= removePartiallyOverlappedStores(AA, DL, IOL);
// If this block ends in a return, unwind, or unreachable, all allocas are
// dead at its end, which means stores to them are also dead.
if (BB.getTerminator()->getNumSuccessors() == 0)
MadeChange |= handleEndBlock(BB, AA, MD, TLI, IOL, OBB);
return MadeChange;
static bool eliminateDeadStores(Function &F, AliasAnalysis *AA,
MemoryDependenceResults *MD, DominatorTree *DT,
const TargetLibraryInfo *TLI) {
bool MadeChange = false;
for (BasicBlock &BB : F)
// Only check non-dead blocks. Dead blocks may have strange pointer
// cycles that will confuse alias analysis.
if (DT->isReachableFromEntry(&BB))
MadeChange |= eliminateDeadStores(BB, AA, MD, DT, TLI);
return MadeChange;
// DSE Pass
PreservedAnalyses DSEPass::run(Function &F, FunctionAnalysisManager &AM) {
AliasAnalysis *AA = &AM.getResult<AAManager>(F);
DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);
MemoryDependenceResults *MD = &AM.getResult<MemoryDependenceAnalysis>(F);
const TargetLibraryInfo *TLI = &AM.getResult<TargetLibraryAnalysis>(F);
if (!eliminateDeadStores(F, AA, MD, DT, TLI))
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;
DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
AliasAnalysis *AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
MemoryDependenceResults *MD =
const TargetLibraryInfo *TLI =
return eliminateDeadStores(F, AA, MD, DT, TLI);
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();