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[DSE] Apply initializes attribute to DSE (#107282)
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Apply the initializes attribute to DSE and guard with a flag,
"enable-dse-initializes-attr-improvement".

The attribute support has been landed in:
#84803
The attribute inference will be landed after this PR:
#97373
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@haopliu
haopliu authored Oct 24, 2024
1 parent a905052 commit 089237c
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Showing 2 changed files with 509 additions and 42 deletions.
250 changes: 208 additions & 42 deletions llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -52,6 +52,7 @@
#include "llvm/IR/Argument.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/ConstantRangeList.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfo.h"
Expand Down Expand Up @@ -164,6 +165,11 @@ static cl::opt<bool>
OptimizeMemorySSA("dse-optimize-memoryssa", cl::init(true), cl::Hidden,
cl::desc("Allow DSE to optimize memory accesses."));

// TODO: turn on and remove this flag.
static cl::opt<bool> EnableInitializesImprovement(
"enable-dse-initializes-attr-improvement", cl::init(false), cl::Hidden,
cl::desc("Enable the initializes attr improvement in DSE"));

//===----------------------------------------------------------------------===//
// Helper functions
//===----------------------------------------------------------------------===//
Expand Down Expand Up @@ -809,8 +815,10 @@ bool canSkipDef(MemoryDef *D, bool DefVisibleToCaller) {
// A memory location wrapper that represents a MemoryLocation, `MemLoc`,
// defined by `MemDef`.
struct MemoryLocationWrapper {
MemoryLocationWrapper(MemoryLocation MemLoc, MemoryDef *MemDef)
: MemLoc(MemLoc), MemDef(MemDef) {
MemoryLocationWrapper(MemoryLocation MemLoc, MemoryDef *MemDef,
bool DefByInitializesAttr)
: MemLoc(MemLoc), MemDef(MemDef),
DefByInitializesAttr(DefByInitializesAttr) {
assert(MemLoc.Ptr && "MemLoc should be not null");
UnderlyingObject = getUnderlyingObject(MemLoc.Ptr);
DefInst = MemDef->getMemoryInst();
Expand All @@ -820,20 +828,59 @@ struct MemoryLocationWrapper {
const Value *UnderlyingObject;
MemoryDef *MemDef;
Instruction *DefInst;
bool DefByInitializesAttr = false;
};

// A memory def wrapper that represents a MemoryDef and the MemoryLocation(s)
// defined by this MemoryDef.
struct MemoryDefWrapper {
MemoryDefWrapper(MemoryDef *MemDef, std::optional<MemoryLocation> MemLoc) {
MemoryDefWrapper(MemoryDef *MemDef,
ArrayRef<std::pair<MemoryLocation, bool>> MemLocations) {
DefInst = MemDef->getMemoryInst();
if (MemLoc.has_value())
DefinedLocation = MemoryLocationWrapper(*MemLoc, MemDef);
for (auto &[MemLoc, DefByInitializesAttr] : MemLocations)
DefinedLocations.push_back(
MemoryLocationWrapper(MemLoc, MemDef, DefByInitializesAttr));
}
Instruction *DefInst;
std::optional<MemoryLocationWrapper> DefinedLocation = std::nullopt;
SmallVector<MemoryLocationWrapper, 1> DefinedLocations;
};

bool hasInitializesAttr(Instruction *I) {
CallBase *CB = dyn_cast<CallBase>(I);
return CB && CB->getArgOperandWithAttribute(Attribute::Initializes);
}

struct ArgumentInitInfo {
unsigned Idx;
bool IsDeadOrInvisibleOnUnwind;
ConstantRangeList Inits;
};

// Return the intersected range list of the initializes attributes of "Args".
// "Args" are call arguments that alias to each other.
// If any argument in "Args" doesn't have dead_on_unwind attr and
// "CallHasNoUnwindAttr" is false, return empty.
ConstantRangeList getIntersectedInitRangeList(ArrayRef<ArgumentInitInfo> Args,
bool CallHasNoUnwindAttr) {
if (Args.empty())
return {};

// To address unwind, the function should have nounwind attribute or the
// arguments have dead or invisible on unwind. Otherwise, return empty.
for (const auto &Arg : Args) {
if (!CallHasNoUnwindAttr && !Arg.IsDeadOrInvisibleOnUnwind)
return {};
if (Arg.Inits.empty())
return {};
}

ConstantRangeList IntersectedIntervals = Args.front().Inits;
for (auto &Arg : Args.drop_front())
IntersectedIntervals = IntersectedIntervals.intersectWith(Arg.Inits);

return IntersectedIntervals;
}

struct DSEState {
Function &F;
AliasAnalysis &AA;
Expand Down Expand Up @@ -911,7 +958,8 @@ struct DSEState {

auto *MD = dyn_cast_or_null<MemoryDef>(MA);
if (MD && MemDefs.size() < MemorySSADefsPerBlockLimit &&
(getLocForWrite(&I) || isMemTerminatorInst(&I)))
(getLocForWrite(&I) || isMemTerminatorInst(&I) ||
(EnableInitializesImprovement && hasInitializesAttr(&I))))
MemDefs.push_back(MD);
}
}
Expand Down Expand Up @@ -1147,13 +1195,26 @@ struct DSEState {
return MemoryLocation::getOrNone(I);
}

std::optional<MemoryLocation> getLocForInst(Instruction *I) {
// Returns a list of <MemoryLocation, bool> pairs written by I.
// The bool means whether the write is from Initializes attr.
SmallVector<std::pair<MemoryLocation, bool>, 1>
getLocForInst(Instruction *I, bool ConsiderInitializesAttr) {
SmallVector<std::pair<MemoryLocation, bool>, 1> Locations;
if (isMemTerminatorInst(I)) {
if (auto Loc = getLocForTerminator(I)) {
return Loc->first;
if (auto Loc = getLocForTerminator(I))
Locations.push_back(std::make_pair(Loc->first, false));
return Locations;
}

if (auto Loc = getLocForWrite(I))
Locations.push_back(std::make_pair(*Loc, false));

if (ConsiderInitializesAttr) {
for (auto &MemLoc : getInitializesArgMemLoc(I)) {
Locations.push_back(std::make_pair(MemLoc, true));
}
}
return getLocForWrite(I);
return Locations;
}

/// Assuming this instruction has a dead analyzable write, can we delete
Expand Down Expand Up @@ -1365,7 +1426,8 @@ struct DSEState {
getDomMemoryDef(MemoryDef *KillingDef, MemoryAccess *StartAccess,
const MemoryLocation &KillingLoc, const Value *KillingUndObj,
unsigned &ScanLimit, unsigned &WalkerStepLimit,
bool IsMemTerm, unsigned &PartialLimit) {
bool IsMemTerm, unsigned &PartialLimit,
bool IsInitializesAttrMemLoc) {
if (ScanLimit == 0 || WalkerStepLimit == 0) {
LLVM_DEBUG(dbgs() << "\n ... hit scan limit\n");
return std::nullopt;
Expand Down Expand Up @@ -1602,7 +1664,16 @@ struct DSEState {

// Uses which may read the original MemoryDef mean we cannot eliminate the
// original MD. Stop walk.
if (isReadClobber(MaybeDeadLoc, UseInst)) {
// If KillingDef is a CallInst with "initializes" attribute, the reads in
// the callee would be dominated by initializations, so it should be safe.
bool IsKillingDefFromInitAttr = false;
if (IsInitializesAttrMemLoc) {
if (KillingI == UseInst &&
KillingUndObj == getUnderlyingObject(MaybeDeadLoc.Ptr))
IsKillingDefFromInitAttr = true;
}

if (isReadClobber(MaybeDeadLoc, UseInst) && !IsKillingDefFromInitAttr) {
LLVM_DEBUG(dbgs() << " ... found read clobber\n");
return std::nullopt;
}
Expand Down Expand Up @@ -2171,6 +2242,16 @@ struct DSEState {
return MadeChange;
}

// Return the locations written by the initializes attribute.
// Note that this function considers:
// 1. Unwind edge: use "initializes" attribute only if the callee has
// "nounwind" attribute, or the argument has "dead_on_unwind" attribute,
// or the argument is invisible to caller on unwind. That is, we don't
// perform incorrect DSE on unwind edges in the current function.
// 2. Argument alias: for aliasing arguments, the "initializes" attribute is
// the intersected range list of their "initializes" attributes.
SmallVector<MemoryLocation, 1> getInitializesArgMemLoc(const Instruction *I);

// Try to eliminate dead defs that access `KillingLocWrapper.MemLoc` and are
// killed by `KillingLocWrapper.MemDef`. Return whether
// any changes were made, and whether `KillingLocWrapper.DefInst` was deleted.
Expand All @@ -2182,6 +2263,75 @@ struct DSEState {
bool eliminateDeadDefs(const MemoryDefWrapper &KillingDefWrapper);
};

SmallVector<MemoryLocation, 1>
DSEState::getInitializesArgMemLoc(const Instruction *I) {
const CallBase *CB = dyn_cast<CallBase>(I);
if (!CB)
return {};

// Collect aliasing arguments and their initializes ranges.
SmallMapVector<Value *, SmallVector<ArgumentInitInfo, 2>, 2> Arguments;
for (unsigned Idx = 0, Count = CB->arg_size(); Idx < Count; ++Idx) {
ConstantRangeList Inits;
Attribute InitializesAttr = CB->getParamAttr(Idx, Attribute::Initializes);
if (InitializesAttr.isValid())
Inits = InitializesAttr.getValueAsConstantRangeList();

Value *CurArg = CB->getArgOperand(Idx);
// We don't perform incorrect DSE on unwind edges in the current function,
// and use the "initializes" attribute to kill dead stores if:
// - The call does not throw exceptions, "CB->doesNotThrow()".
// - Or the callee parameter has "dead_on_unwind" attribute.
// - Or the argument is invisible to caller on unwind, and there are no
// unwind edges from this call in the current function (e.g. `CallInst`).
bool IsDeadOrInvisibleOnUnwind =
CB->paramHasAttr(Idx, Attribute::DeadOnUnwind) ||
(isa<CallInst>(CB) && isInvisibleToCallerOnUnwind(CurArg));
ArgumentInitInfo InitInfo{Idx, IsDeadOrInvisibleOnUnwind, Inits};
bool FoundAliasing = false;
for (auto &[Arg, AliasList] : Arguments) {
auto AAR = BatchAA.alias(MemoryLocation::getBeforeOrAfter(Arg),
MemoryLocation::getBeforeOrAfter(CurArg));
if (AAR == AliasResult::NoAlias) {
continue;
} else if (AAR == AliasResult::MustAlias) {
FoundAliasing = true;
AliasList.push_back(InitInfo);
} else {
// For PartialAlias and MayAlias, there is an offset or may be an
// unknown offset between the arguments and we insert an empty init
// range to discard the entire initializes info while intersecting.
FoundAliasing = true;
AliasList.push_back(ArgumentInitInfo{Idx, IsDeadOrInvisibleOnUnwind,
ConstantRangeList()});
}
}
if (!FoundAliasing)
Arguments[CurArg] = {InitInfo};
}

SmallVector<MemoryLocation, 1> Locations;
for (const auto &[_, Args] : Arguments) {
auto IntersectedRanges =
getIntersectedInitRangeList(Args, CB->doesNotThrow());
if (IntersectedRanges.empty())
continue;

for (const auto &Arg : Args) {
for (const auto &Range : IntersectedRanges) {
int64_t Start = Range.getLower().getSExtValue();
int64_t End = Range.getUpper().getSExtValue();
// For now, we only handle locations starting at offset 0.
if (Start == 0)
Locations.push_back(MemoryLocation(CB->getArgOperand(Arg.Idx),
LocationSize::precise(End - Start),
CB->getAAMetadata()));
}
}
}
return Locations;
}

std::pair<bool, bool>
DSEState::eliminateDeadDefs(const MemoryLocationWrapper &KillingLocWrapper) {
bool Changed = false;
Expand All @@ -2208,7 +2358,8 @@ DSEState::eliminateDeadDefs(const MemoryLocationWrapper &KillingLocWrapper) {
std::optional<MemoryAccess *> MaybeDeadAccess = getDomMemoryDef(
KillingLocWrapper.MemDef, Current, KillingLocWrapper.MemLoc,
KillingLocWrapper.UnderlyingObject, ScanLimit, WalkerStepLimit,
isMemTerminatorInst(KillingLocWrapper.DefInst), PartialLimit);
isMemTerminatorInst(KillingLocWrapper.DefInst), PartialLimit,
KillingLocWrapper.DefByInitializesAttr);

if (!MaybeDeadAccess) {
LLVM_DEBUG(dbgs() << " finished walk\n");
Expand All @@ -2231,10 +2382,20 @@ DSEState::eliminateDeadDefs(const MemoryLocationWrapper &KillingLocWrapper) {
}
continue;
}
// We cannot apply the initializes attribute to DeadAccess/DeadDef.
// It would incorrectly consider a call instruction as redundant store
// and remove this call instruction.
// TODO: this conflates the existence of a MemoryLocation with being able
// to delete the instruction. Fix isRemovable() to consider calls with
// side effects that cannot be removed, e.g. calls with the initializes
// attribute, and remove getLocForInst(ConsiderInitializesAttr = false).
MemoryDefWrapper DeadDefWrapper(
cast<MemoryDef>(DeadAccess),
getLocForInst(cast<MemoryDef>(DeadAccess)->getMemoryInst()));
MemoryLocationWrapper &DeadLocWrapper = *DeadDefWrapper.DefinedLocation;
getLocForInst(cast<MemoryDef>(DeadAccess)->getMemoryInst(),
/*ConsiderInitializesAttr=*/false));
assert(DeadDefWrapper.DefinedLocations.size() == 1);
MemoryLocationWrapper &DeadLocWrapper =
DeadDefWrapper.DefinedLocations.front();
LLVM_DEBUG(dbgs() << " (" << *DeadLocWrapper.DefInst << ")\n");
ToCheck.insert(DeadLocWrapper.MemDef->getDefiningAccess());
NumGetDomMemoryDefPassed++;
Expand Down Expand Up @@ -2309,37 +2470,41 @@ DSEState::eliminateDeadDefs(const MemoryLocationWrapper &KillingLocWrapper) {
}

bool DSEState::eliminateDeadDefs(const MemoryDefWrapper &KillingDefWrapper) {
if (!KillingDefWrapper.DefinedLocation.has_value()) {
if (KillingDefWrapper.DefinedLocations.empty()) {
LLVM_DEBUG(dbgs() << "Failed to find analyzable write location for "
<< *KillingDefWrapper.DefInst << "\n");
return false;
}

auto &KillingLocWrapper = *KillingDefWrapper.DefinedLocation;
LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs killed by "
<< *KillingLocWrapper.MemDef << " ("
<< *KillingLocWrapper.DefInst << ")\n");
auto [Changed, DeletedKillingLoc] = eliminateDeadDefs(KillingLocWrapper);

// Check if the store is a no-op.
if (!DeletedKillingLoc && storeIsNoop(KillingLocWrapper.MemDef,
KillingLocWrapper.UnderlyingObject)) {
LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n DEAD: "
<< *KillingLocWrapper.DefInst << '\n');
deleteDeadInstruction(KillingLocWrapper.DefInst);
NumRedundantStores++;
return true;
}
// Can we form a calloc from a memset/malloc pair?
if (!DeletedKillingLoc &&
tryFoldIntoCalloc(KillingLocWrapper.MemDef,
KillingLocWrapper.UnderlyingObject)) {
LLVM_DEBUG(dbgs() << "DSE: Remove memset after forming calloc:\n"
<< " DEAD: " << *KillingLocWrapper.DefInst << '\n');
deleteDeadInstruction(KillingLocWrapper.DefInst);
return true;
bool MadeChange = false;
for (auto &KillingLocWrapper : KillingDefWrapper.DefinedLocations) {
LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs killed by "
<< *KillingLocWrapper.MemDef << " ("
<< *KillingLocWrapper.DefInst << ")\n");
auto [Changed, DeletedKillingLoc] = eliminateDeadDefs(KillingLocWrapper);

// Check if the store is a no-op.
if (!DeletedKillingLoc && storeIsNoop(KillingLocWrapper.MemDef,
KillingLocWrapper.UnderlyingObject)) {
LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n DEAD: "
<< *KillingLocWrapper.DefInst << '\n');
deleteDeadInstruction(KillingLocWrapper.DefInst);
NumRedundantStores++;
MadeChange = true;
continue;
}
// Can we form a calloc from a memset/malloc pair?
if (!DeletedKillingLoc &&
tryFoldIntoCalloc(KillingLocWrapper.MemDef,
KillingLocWrapper.UnderlyingObject)) {
LLVM_DEBUG(dbgs() << "DSE: Remove memset after forming calloc:\n"
<< " DEAD: " << *KillingLocWrapper.DefInst << '\n');
deleteDeadInstruction(KillingLocWrapper.DefInst);
MadeChange = true;
continue;
}
}
return Changed;
return MadeChange;
}

static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
Expand All @@ -2355,7 +2520,8 @@ static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
continue;

MemoryDefWrapper KillingDefWrapper(
KillingDef, State.getLocForInst(KillingDef->getMemoryInst()));
KillingDef, State.getLocForInst(KillingDef->getMemoryInst(),
EnableInitializesImprovement));
MadeChange |= State.eliminateDeadDefs(KillingDefWrapper);
}

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