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[X86] Use GFNI for LZCNT vXi8 ops #141888

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Merged
merged 4 commits into from
Jun 4, 2025
Merged

[X86] Use GFNI for LZCNT vXi8 ops #141888

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157a532
[X86][GFNI] Add lowering support for CTLZ vXi8 using GF2P8AFFINEQB
houngkoungting May 31, 2025
e998750
Merge branch 'main' into main
houngkoungting May 31, 2025
e789fab
[X86][GFNI] Lower vXi8 ctlz using GF2P8AFFINEQB
houngkoungting Jun 1, 2025
c087674
[X86][GFNI] Fix style and logic for CTLZ vXi8 lowering
houngkoungting Jun 2, 2025
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27 changes: 27 additions & 0 deletions llvm/lib/Target/X86/X86ISelLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -28999,6 +28999,30 @@ static SDValue LowerVectorCTLZ(SDValue Op, const SDLoc &DL,
return LowerVectorCTLZInRegLUT(Op, DL, Subtarget, DAG);
}

static SDValue LowerVectorCTLZ_GFNI(SDValue Op, const SDLoc &DL,
SelectionDAG &DAG,
const X86Subtarget &Subtarget) {
MVT VT = Op.getSimpleValueType();
SDValue Input = Op.getOperand(0);

assert(VT.isVector() && VT.getVectorElementType() == MVT::i8 &&
"Expected vXi8 input for GFNI-based CTLZ lowering");

SDValue Reversed = DAG.getNode(ISD::BITREVERSE, DL, VT, Input);

SDValue Neg = DAG.getNegative(Reversed, DL, VT);
SDValue Filtered = DAG.getNode(ISD::AND, DL, VT, Reversed, Neg);

MVT VT64 = MVT::getVectorVT(MVT::i64, VT.getSizeInBits() / 64);
SDValue CTTZConst = DAG.getConstant(0xAACCF0FF00000000ULL, DL, VT64);
SDValue CTTZMatrix = DAG.getBitcast(VT, CTTZConst);

SDValue LZCNT =
DAG.getNode(X86ISD::GF2P8AFFINEQB, DL, VT, Filtered, CTTZMatrix,
DAG.getTargetConstant(8, DL, MVT::i8));
return LZCNT;
}

static SDValue LowerCTLZ(SDValue Op, const X86Subtarget &Subtarget,
SelectionDAG &DAG) {
MVT VT = Op.getSimpleValueType();
Expand All @@ -29007,6 +29031,9 @@ static SDValue LowerCTLZ(SDValue Op, const X86Subtarget &Subtarget,
SDLoc dl(Op);
unsigned Opc = Op.getOpcode();

if (VT.isVector() && VT.getScalarType() == MVT::i8 && Subtarget.hasGFNI())
return LowerVectorCTLZ_GFNI(Op, dl, DAG, Subtarget);

if (VT.isVector())
return LowerVectorCTLZ(Op, dl, Subtarget, DAG);

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