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[DAG] ComputeNumSignBits - use computeKnownBits instead of isConstOrConstSplat to detect legalised constants #152991

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[DAG] ComputeNumSignBits - use computeKnownBits instead of isConstOrConstSplat to detect legalised constants #152991

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[DAG] ComputeNumSignBits - use computeKnownBits instead of isConstOrC…
RKSimon Aug 11, 2025
c73e808
Merge branch 'main' into dag-signbit-clamp-knowbits
RKSimon Aug 21, 2025
f5f1e2f
Merge branch 'main' into dag-signbit-clamp-knowbits
RKSimon Aug 22, 2025
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Merge branch 'main' into dag-signbit-clamp-knowbits
RKSimon Aug 27, 2025
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19 changes: 10 additions & 9 deletions llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4940,17 +4940,18 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
// If we have a clamp pattern, we know that the number of sign bits will be
// the minimum of the clamp min/max range.
bool IsMax = (Opcode == ISD::SMAX);
ConstantSDNode *CstLow = nullptr, *CstHigh = nullptr;
if ((CstLow = isConstOrConstSplat(Op.getOperand(1), DemandedElts)))
KnownBits KnownLow, KnownHigh;
KnownLow = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
if (KnownLow.isConstant())
if (Op.getOperand(0).getOpcode() == (IsMax ? ISD::SMIN : ISD::SMAX))
CstHigh =
isConstOrConstSplat(Op.getOperand(0).getOperand(1), DemandedElts);
if (CstLow && CstHigh) {
KnownHigh = computeKnownBits(Op.getOperand(0).getOperand(1),
DemandedElts, Depth + 2);
if (KnownLow.isConstant() && KnownHigh.isConstant()) {
if (!IsMax)
std::swap(CstLow, CstHigh);
if (CstLow->getAPIntValue().sle(CstHigh->getAPIntValue())) {
Tmp = CstLow->getAPIntValue().getNumSignBits();
Tmp2 = CstHigh->getAPIntValue().getNumSignBits();
std::swap(KnownLow, KnownHigh);
if (KnownLow.getConstant().sle(KnownHigh.getConstant())) {
Tmp = KnownLow.getConstant().getNumSignBits();
Tmp2 = KnownHigh.getConstant().getNumSignBits();
return std::min(Tmp, Tmp2);
}
}
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