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[VectorCombine] Preserve scoped alias metadata #153714

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Merged
merged 9 commits into from
Aug 18, 2025
Merged

[VectorCombine] Preserve scoped alias metadata #153714

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25cbc66
Make VectorCombine Pass Alias Info
knwng Aug 15, 2025
3c3d2b8
Merge remote-tracking branch 'upstream/main' into fix_vector_combine
knwng Aug 15, 2025
adce747
add test; address comments
knwng Aug 15, 2025
2148978
address comments
knwng Aug 16, 2025
6574cf4
fix lint
knwng Aug 17, 2025
9f4a0a0
fix lint and test
knwng Aug 17, 2025
8dd4467
fix lint
knwng Aug 17, 2025
91f0c8d
Merge remote-tracking branch 'upstream/main' into fix_vector_combine
knwng Aug 18, 2025
c2a88cf
Merge remote-tracking branch 'upstream/main' into fix_vector_combine
knwng Aug 18, 2025
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12 changes: 12 additions & 0 deletions llvm/lib/Transforms/Vectorize/VectorCombine.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1811,6 +1811,10 @@ bool VectorCombine::scalarizeLoadExtract(Instruction &I) {
// erased in the correct order.
Worklist.push(LI);

LLVMContext &ctx = LI->getContext();
unsigned aliasScopeKind = ctx.getMDKindID("alias.scope");
unsigned noAliasKind = ctx.getMDKindID("noalias");

// Replace extracts with narrow scalar loads.
for (User *U : LI->users()) {
auto *EI = cast<ExtractElementInst>(U);
Expand All @@ -1831,6 +1835,14 @@ bool VectorCombine::scalarizeLoadExtract(Instruction &I) {
LI->getAlign(), VecTy->getElementType(), Idx, *DL);
NewLoad->setAlignment(ScalarOpAlignment);

if (MDNode *aliasScope = LI->getMetadata(aliasScopeKind)) {
NewLoad->setMetadata(aliasScopeKind, aliasScope);
}

if (MDNode *noAlias = LI->getMetadata(noAliasKind)) {
NewLoad->setMetadata(noAliasKind, noAlias);
}
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@nikic nikic Aug 15, 2025

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If possible, it would be better to use getAAMetadata() and adjustForAccess() here.

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@knwng knwng Aug 15, 2025

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Hi @nikic, thank you for the advice. I know it's a more thoughtful solution. But I'm not familiar with AA in LLVM. Can you guide me on how to use adjustForAccess(), like how to correctly calculate offset and type? Thanks!

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@nikic nikic Aug 16, 2025

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The type is VecTy->getElementType(). The offset would be Idx * the type size like here:

llvm-project/llvm/lib/Transforms/Vectorize/VectorCombine.cpp

Lines 1652 to 1654 in adce747

if (auto *C = dyn_cast<ConstantInt>(Idx))
return commonAlignment(VectorAlignment,
C->getZExtValue() * DL.getTypeStoreSize(ScalarType));

The problem is if Idx is a variable. In that case the offset is unknown. So I would make the whole logic conditional on the dyn_cast<ConstantInt>(Idx) case.


replaceValue(*EI, *NewLoad);
}

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