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[DSE] Support redundant stores eliminated by memset. 

This patch adds support to remove stores that write the same value
as earlier memesets.

It uses isOverwrite to check that a memset completely overwrites a later
store. The candidate store must store the same bytewise value as the
byte stored by the memset.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D112321
This commit is contained in:
Florian Hahn 2021-10-29 21:58:42 +01:00
parent e6b323379e
commit 274a9b0f0b
No known key found for this signature in database
GPG Key ID: 60242B98015C34AE
2 changed files with 25 additions and 5 deletions
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28
llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
View File

@ -1955,10 +1955,32 @@ struct DSEState {
auto *UpperDef = dyn_cast<MemoryDef>(Def->getDefiningAccess());
if (!UpperDef || MSSA.isLiveOnEntryDef(UpperDef))
continue;
auto *DefInst = Def->getMemoryInst();
auto *UpperInst = UpperDef->getMemoryInst();
Instruction *DefInst = Def->getMemoryInst();
Instruction *UpperInst = UpperDef->getMemoryInst();
auto IsRedundantStore = [this, DefInst,
UpperInst](MemoryLocation UpperLoc) {
if (DefInst->isIdenticalTo(UpperInst))
return true;
if (auto *MemSetI = dyn_cast<MemSetInst>(UpperInst)) {
if (auto *SI = dyn_cast<StoreInst>(DefInst)) {
auto MaybeDefLoc = getLocForWriteEx(DefInst);
if (!MaybeDefLoc)
return false;
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;
};
auto MaybeUpperLoc = getLocForWriteEx(UpperInst);
if (!MaybeUpperLoc || !DefInst->isIdenticalTo(UpperInst) ||
if (!MaybeUpperLoc || !IsRedundantStore(*MaybeUpperLoc) ||
isReadClobber(*MaybeUpperLoc, DefInst))
continue;
LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n DEAD: " << *DefInst

2
llvm/test/Transforms/DeadStoreElimination/stores-of-existing-values.ll
View File

@ -403,8 +403,6 @@ declare void @llvm.memset.p0i8.i64(i8* nocapture writeonly, i8, i64, i1 immarg)
define void @test12_memset_simple(i8* %ptr) {
; CHECK-LABEL: @test12_memset_simple(
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* [[PTR:%.*]], i8 0, i64 10, i1 false)
; CHECK-NEXT: [[PTR_5:%.*]] = getelementptr i8, i8* [[PTR]], i64 4
; CHECK-NEXT: store i8 0, i8* [[PTR_5]], align 1
; CHECK-NEXT: ret void
;
call void @llvm.memset.p0i8.i64(i8* %ptr, i8 0, i64 10, i1 false)