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[SelectionDAG] Preserve CTTZ_ELTS lane count during expansion #217982

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[SelectionDAG] Preserve CTTZ_ELTS lane count during expansion #217982

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5 changes: 3 additions & 2 deletions llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -13859,6 +13859,8 @@ SDValue TargetLowering::expandVECTOR_COMPRESS(SDNode *Node,
SDValue TargetLowering::expandCttzElts(SDNode *Node, SelectionDAG &DAG) const {
SDLoc DL(Node);
EVT VT = Node->getValueType(0);
ElementCount EC =
Node->getOperand(0).getValueType().getVectorElementCount();

@RKSimon RKSimon Aug 22, 2026

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We have a lot of Node->getOperand(0) calls - pull out?


bool ZeroIsPoison = Node->getOpcode() == ISD::CTTZ_ELTS_ZERO_POISON;
auto [Mask, StepVec] =
Expand Down Expand Up @@ -13893,8 +13895,7 @@ SDValue TargetLowering::expandCttzElts(SDNode *Node, SelectionDAG &DAG) const {
if (getTypeAction(StepVT.getSimpleVT()) == TypePromoteInteger)
StepVT = getTypeToTransformTo(*DAG.getContext(), StepVT);

SDValue VL =
DAG.getElementCount(DL, StepVT, StepVecVT.getVectorElementCount());
SDValue VL = DAG.getElementCount(DL, StepVT, EC);
SDValue SplatVL = DAG.getSplat(StepVecVT, DL, VL);
StepVec = DAG.getNode(ISD::SUB, DL, StepVecVT, SplatVL, StepVec);
SDValue Zeroes = DAG.getConstant(0, DL, StepVecVT);
Expand Down
17 changes: 17 additions & 0 deletions llvm/test/CodeGen/X86/intrinsic-cttz-elts.ll
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@@ -1,4 +1,5 @@
; RUN: llc -mtriple=x86_64-unknown-unknown < %s | FileCheck %s
; RUN: llc -mtriple=x86_64-unknown-unknown -mattr=+avx512f < %s | FileCheck %s --check-prefix=AVX512

define i8 @ctz_v8i16(<8 x i16> %a) {
; CHECK-LABEL: .LCPI0_0:
Expand Down Expand Up @@ -101,5 +102,21 @@ define i8 @ctz_v8i16_poison(<8 x i16> %a) {
ret i8 %res
}

define i32 @ctz_zero_v4i1() {
; AVX512-LABEL: ctz_zero_v4i1:
; AVX512: addb $5, %al
%res = call i32 @llvm.experimental.cttz.elts.i32.v4i1(<4 x i1> zeroinitializer, i1 false)
ret i32 %res
}

define i32 @ctz_zero_v8i1() {
; AVX512-LABEL: ctz_zero_v8i1:
; AVX512: addb $9, %al
%res = call i32 @llvm.experimental.cttz.elts.i32.v8i1(<8 x i1> zeroinitializer, i1 false)
ret i32 %res
}

@RKSimon RKSimon Aug 22, 2026

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These require CHECK coverage as well


declare i8 @llvm.experimental.cttz.elts.i8.v8i16(<8 x i16>, i1)
declare i16 @llvm.experimental.cttz.elts.i16.v4i32(<4 x i32>, i1)
declare i32 @llvm.experimental.cttz.elts.i32.v4i1(<4 x i1>, i1)
declare i32 @llvm.experimental.cttz.elts.i32.v8i1(<8 x i1>, i1)

@RKSimon RKSimon Aug 22, 2026

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delete all 4 of these - we don't need intrinsic forward declarations anymore