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

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merged 4 commits into from
Aug 1, 2025
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[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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9 changes: 8 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 @@ -419,6 +419,7 @@ Instruction *InstCombinerImpl::visitExtractElementInst(ExtractElementInst &EI) {
// If extracting a specified index from the vector, see if we can recursively
// find a previously computed scalar that was inserted into the vector.
auto *IndexC = dyn_cast<ConstantInt>(Index);
auto *II = dyn_cast<IntrinsicInst>(SrcVec);
bool HasKnownValidIndex = false;
if (IndexC) {
// Canonicalize type of constant indices to i64 to simplify CSE
Expand All @@ -429,7 +430,7 @@ Instruction *InstCombinerImpl::visitExtractElementInst(ExtractElementInst &EI) {
unsigned NumElts = EC.getKnownMinValue();
HasKnownValidIndex = IndexC->getValue().ult(NumElts);

if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(SrcVec)) {
if (II) {
Intrinsic::ID IID = II->getIntrinsicID();
// Index needs to be lower than the minimum size of the vector, because
// for scalable vector, the vector size is known at run time.
Expand Down Expand Up @@ -462,6 +463,12 @@ Instruction *InstCombinerImpl::visitExtractElementInst(ExtractElementInst &EI) {
return ScalarPHI;
}

// If SrcVec is a subvector starting at index 0, extract from the
// wider source vector
if (II && II->getIntrinsicID() == Intrinsic::vector_extract)
if (cast<ConstantInt>(II->getArgOperand(1))->isZero())
return ExtractElementInst::Create(II->getArgOperand(0), Index);
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@paulwalker-arm paulwalker-arm Jul 31, 2025

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Up to you but using PatternMatch would remove the need for II and casts. For example:

Value *V;                                                                         
if (match(SrcVec, m_Intrinsic<Intrinsic::vector_extract>(m_Value(V), m_Zero())))  
  return ExtractElementInst::Create(V, Index);


// TODO come up with a n-ary matcher that subsumes both unary and
// binary matchers.
UnaryOperator *UO;
Expand Down
36 changes: 36 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 +1,36 @@
; 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 @extract_const_idx(<vscale x 4 x i1> %a) {
; CHECK-LABEL: define i1 @extract_const_idx(
; 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
}

define float @extract_variable_idx(<vscale x 4 x float> %a, i32 %idx) {
; CHECK-LABEL: define float @extract_variable_idx(
; CHECK-SAME: <vscale x 4 x float> [[A:%.*]], i32 [[IDX:%.*]]) {
; CHECK-NEXT: [[ELT:%.*]] = extractelement <vscale x 4 x float> [[A]], i32 [[IDX]]
; CHECK-NEXT: ret float [[ELT]]
;
%subvec = call <vscale x 2 x float> @llvm.vector.extract.nxv2f32.nxv4f32.i64(<vscale x 4 x float> %a, i64 0)
%elt = extractelement <vscale x 2 x float> %subvec, i32 %idx
ret float %elt
}

define float @negative_test(<vscale x 4 x float> %a) {
; CHECK-LABEL: define float @negative_test(
; CHECK-SAME: <vscale x 4 x float> [[A:%.*]]) {
; CHECK-NEXT: [[SUBVEC:%.*]] = call <vscale x 2 x float> @llvm.vector.extract.nxv2f32.nxv4f32(<vscale x 4 x float> [[A]], i64 2)
; CHECK-NEXT: [[ELT:%.*]] = extractelement <vscale x 2 x float> [[SUBVEC]], i64 1
; CHECK-NEXT: ret float [[ELT]]
;
%subvec = call <vscale x 2 x float> @llvm.vector.extract.nxv2f32.nxv4f32.i64(<vscale x 4 x float> %a, i64 2)
%elt = extractelement <vscale x 2 x float> %subvec, i32 1
ret float %elt
}