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[DSE] Delay deleting non-memory-defs until end of DSE. #83411

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Merged
merged 7 commits into from
Mar 2, 2024
Merged

[DSE] Delay deleting non-memory-defs until end of DSE. #83411

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f29e3f7
[DSE] Delay deleting non-memory-defs until end of DSE.
fhahn Feb 29, 2024
952b51b
Merge remote-tracking branch 'origin/main' into perf/dse-delay-delete
fhahn Feb 29, 2024
33f9631
!fixup fix build failure.
fhahn Feb 29, 2024
1d036f4
Merge remote-tracking branch 'origin/main' into perf/dse-delay-delete
fhahn Mar 1, 2024
be3aaa1
!fixup remove assert, directly delete MemDefs with void ty.
fhahn Mar 1, 2024
8fc503c
Merge remote-tracking branch 'origin/main' into perf/dse-delay-delete
fhahn Mar 2, 2024
c839e00
!fixup add extra tests.
fhahn Mar 2, 2024
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31 changes: 26 additions & 5 deletions llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -857,6 +857,9 @@ struct DSEState {
// no longer be captured.
bool ShouldIterateEndOfFunctionDSE;

/// Dead instructions to be removed at the end of DSE.
SmallVector<Instruction *> ToRemove;

// Class contains self-reference, make sure it's not copied/moved.
DSEState(const DSEState &) = delete;
DSEState &operator=(const DSEState &) = delete;
Expand Down Expand Up @@ -1692,7 +1695,8 @@ struct DSEState {
return {MaybeDeadAccess};
}

// Delete dead memory defs
/// Delete dead memory defs and recursively add their operands to ToRemove if
/// they became dead.
void deleteDeadInstruction(Instruction *SI) {
MemorySSAUpdater Updater(&MSSA);
SmallVector<Instruction *, 32> NowDeadInsts;
Expand All @@ -1708,8 +1712,10 @@ struct DSEState {
salvageKnowledge(DeadInst);

// Remove the Instruction from MSSA.
if (MemoryAccess *MA = MSSA.getMemoryAccess(DeadInst)) {
if (MemoryDef *MD = dyn_cast<MemoryDef>(MA)) {
MemoryAccess *MA = MSSA.getMemoryAccess(DeadInst);
bool IsMemDef = MA && isa<MemoryDef>(MA);
if (MA) {
if (IsMemDef) {
SkipStores.insert(MD);
if (auto *SI = dyn_cast<StoreInst>(MD->getMemoryInst())) {
if (SI->getValueOperand()->getType()->isPointerTy()) {
Expand All @@ -1730,13 +1736,21 @@ struct DSEState {
// Remove its operands
for (Use &O : DeadInst->operands())
if (Instruction *OpI = dyn_cast<Instruction>(O)) {
O = nullptr;
O.set(PoisonValue::get(O->getType()));
if (isInstructionTriviallyDead(OpI, &TLI))
NowDeadInsts.push_back(OpI);
}

EI.removeInstruction(DeadInst);
DeadInst->eraseFromParent();
// Remove memory defs directly, but only queue other dead instructions for
// later removal. They may have been used as memory locations that have
// been cached by BatchAA. Removing them here may lead to newly created
// instructions to be allocated at the same address, yielding stale cache
// entries.
if (IsMemDef)
DeadInst->eraseFromParent();
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Not sure if there could be cases where a MemoryDef also loads a value that is then used as pointer of MemoryLocation

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Is it safer to just delay the deletion for them too?

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+1 to this question, I'm not clear if this is possible. MemoryDefs could be volatile loads, but this is not the case here since the pass is only looking at store/writing instructions. A memcpy does also load, so a memcpy to a location that is later overwritten, could be deleted here, while copying from a location that's already in the cache.

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At this time I'm inclined to say this cannot happen. So let's have this fix in for the existing issue.

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Deleting a memcpy wouldn't be a problem, as long as its pointer operands aren't deleted.

But we might remove something like a malloc call, which is a memory def, and the result of the malloc could be used as address and be in the cache. Updated the code to remove memory-defs that do not produce a value (i.e. have type void)

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Is it possible to add more tests to cover the void/non-void type cases mentioned?

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Added a couple of extra tests based on the original test.

else
ToRemove.push_back(DeadInst);
}
}

Expand Down Expand Up @@ -2287,6 +2301,13 @@ static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,

MadeChange |= State.eliminateRedundantStoresOfExistingValues();
MadeChange |= State.eliminateDeadWritesAtEndOfFunction();

while (!State.ToRemove.empty()) {
Instruction *DeadInst = State.ToRemove.pop_back_val();
assert(!MSSA.getMemoryAccess(DeadInst));
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Nit: Perhaps an assertion comment along the lines of "Dead MemDefs are erased right away"?

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I went ahead and dropped the assert, as it was incorrect and never triggered, due to memory accesses always being removed from MemorySSA.

Also, in the latest version, we may remove memory-defs here, if they produce values.

DeadInst->eraseFromParent();
}

return MadeChange;
}
} // end anonymous namespace
Expand Down
42 changes: 42 additions & 0 deletions llvm/test/Transforms/DeadStoreElimination/batchaa-caching-new-pointers.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,42 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
; RUN: opt -S -passes=dse < %s | FileCheck %s
;
; DSE kills `store i32 44, ptr %struct.byte.4, align 4` but should not kill
; `call void @llvm.memset.p0.i64(...)` because it has a clobber read:
; `%ret = load ptr, ptr %struct.byte.8`


%struct.type = type { ptr, ptr }

define ptr @foo(ptr noundef %ptr) {
; CHECK-LABEL: define ptr @foo(
; CHECK-SAME: ptr noundef [[PTR:%.*]]) {
; CHECK-NEXT: [[STRUCT_ALLOCA:%.*]] = alloca [[STRUCT_TYPE:%.*]], align 8
; CHECK-NEXT: call void @llvm.lifetime.start.p0(i64 56, ptr nonnull [[STRUCT_ALLOCA]]) #[[ATTR2:[0-9]+]]
; CHECK-NEXT: [[STRUCT_BYTE_8:%.*]] = getelementptr inbounds i8, ptr [[STRUCT_ALLOCA]], i64 8
; CHECK-NEXT: [[TMP1:%.*]] = getelementptr inbounds i8, ptr [[STRUCT_BYTE_8]], i64 4
; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr noundef nonnull align 4 [[TMP1]], i8 42, i64 4, i1 false)
; CHECK-NEXT: store i32 43, ptr [[STRUCT_BYTE_8]], align 4
; CHECK-NEXT: [[RET:%.*]] = load ptr, ptr [[STRUCT_BYTE_8]], align 8
; CHECK-NEXT: call void @llvm.lifetime.end.p0(i64 56, ptr nonnull [[STRUCT_ALLOCA]]) #[[ATTR2]]
; CHECK-NEXT: ret ptr [[RET]]
;
%struct.alloca = alloca %struct.type, align 8
call void @llvm.lifetime.start.p0(i64 56, ptr nonnull %struct.alloca) nounwind
%struct.byte.8 = getelementptr inbounds i8, ptr %struct.alloca, i64 8
; Set %struct.alloca[8, 16) to 42.
call void @llvm.memset.p0.i64(ptr noundef nonnull align 4 %struct.byte.8, i8 42, i64 8, i1 false)
; Set %struct.alloca[8, 12) to 43.
store i32 43, ptr %struct.byte.8, align 4
; Set %struct.alloca[4, 8) to 44.
%struct.byte.4 = getelementptr inbounds i8, ptr %struct.alloca, i64 4
store i32 44, ptr %struct.byte.4, align 4
; Return %struct.alloca[8, 16).
%ret = load ptr, ptr %struct.byte.8
call void @llvm.lifetime.end.p0(i64 56, ptr nonnull %struct.alloca) nounwind
ret ptr %ret
}

declare void @llvm.memset.p0.i64(ptr nocapture writeonly, i8, i64, i1 immarg)
declare void @llvm.lifetime.end.p0(i64 immarg, ptr nocapture)
declare void @llvm.lifetime.start.p0(i64 immarg, ptr nocapture)