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[Instcombine] Combine extractelement from a vector_extract at index 0 #151491

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Merged
merged 4 commits into from
Aug 1, 2025
Merged

[Instcombine] Combine extractelement from a vector_extract at index 0 #151491

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0341313
- Add a test for extractelement from scalable vector_extract at index 0
kmclaughlin-arm Jul 17, 2025
d93bdea
[Instcombine] Combine extractelement from a vector_extract at index 0
kmclaughlin-arm Jul 17, 2025
d9b7aa9
- Add variable index & negative tests
kmclaughlin-arm Jul 31, 2025
c43f883
- Use PatternMatch
kmclaughlin-arm Jul 31, 2025
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6 changes: 5 additions & 1 deletion llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -444,7 +444,11 @@ Instruction *InstCombinerImpl::visitExtractElementInst(ExtractElementInst &EI) {
else
Idx = PoisonValue::get(Ty);
return replaceInstUsesWith(EI, Idx);
}
} else if (IID == Intrinsic::vector_extract)
// If II is a subvector starting at index 0, extract from the wider
// source vector
if (cast<ConstantInt>(II->getArgOperand(1))->getZExtValue() == 0)
return ExtractElementInst::Create(II->getArgOperand(0), Index);
}

// InstSimplify should handle cases where the index is invalid.
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13 changes: 13 additions & 0 deletions llvm/test/Transforms/InstCombine/scalable-extract-subvec-elt.ll
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@nikic nikic Jul 31, 2025

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Add a negative test with non-zero vector.extract?

Also test a variable index extract?

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@kmclaughlin-arm kmclaughlin-arm Jul 31, 2025

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Thank you @nikic, while adding a variable index test I realised this case wasn't handled.

Original file line number Diff line number Diff line change
@@ -0,0 +1,13 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
; RUN: opt -S -passes=instcombine < %s | FileCheck %s

define i1 @scalable_test(<vscale x 4 x i1> %a) {
; CHECK-LABEL: define i1 @scalable_test(
; CHECK-SAME: <vscale x 4 x i1> [[A:%.*]]) {
; CHECK-NEXT: [[ELT:%.*]] = extractelement <vscale x 4 x i1> [[A]], i64 1
; CHECK-NEXT: ret i1 [[ELT]]
;
%subvec = call <vscale x 2 x i1> @llvm.vector.extract.nxv2i1.nxv4i1.i64(<vscale x 4 x i1> %a, i64 0)
%elt = extractelement <vscale x 2 x i1> %subvec, i32 1
ret i1 %elt
}