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[RISCV] Prefer (select (x < 0), y, z) -> x >> (XLEN - 1) & (y - z) + z even with Zicond. #125772

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Merged
merged 1 commit into from
Feb 6, 2025
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[RISCV] Prefer (select (x < 0), y, z) -> x >> (XLEN - 1) & (y - z) + z even with Zicond. #125772

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27 changes: 27 additions & 0 deletions llvm/lib/Target/RISCV/RISCVISelLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -8430,6 +8430,33 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
if (isa<ConstantSDNode>(TrueV) && isa<ConstantSDNode>(FalseV)) {
const APInt &TrueVal = TrueV->getAsAPIntVal();
const APInt &FalseVal = FalseV->getAsAPIntVal();

// Prefer these over Zicond to avoid materializing an immediate:
// (select (x < 0), y, z) -> x >> (XLEN - 1) & (y - z) + z
// (select (x > -1), z, y) -> x >> (XLEN - 1) & (y - z) + z
if (CondV.getOpcode() == ISD::SETCC &&
CondV.getOperand(0).getValueType() == VT && CondV.hasOneUse()) {
ISD::CondCode CCVal = cast<CondCodeSDNode>(CondV.getOperand(2))->get();
if ((CCVal == ISD::SETLT && isNullConstant(CondV.getOperand(1))) ||
(CCVal == ISD::SETGT && isAllOnesConstant(CondV.getOperand(1)))) {
int64_t TrueImm = TrueVal.getSExtValue();
int64_t FalseImm = FalseVal.getSExtValue();
if (CCVal == ISD::SETGT)
std::swap(TrueImm, FalseImm);
if (isInt<12>(TrueImm) && isInt<12>(FalseImm) &&
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@dtcxzyw dtcxzyw Feb 4, 2025

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Do we really need isInt<12>(FalseImm)?

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@topperc topperc Feb 4, 2025

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It's needed to make guarantee the ISD::ADD becomes an ADDI. The one we probably don't need is isInt<12>(TrueImm). It currently protects the TrueImm - FalseImm from signed overflow, but we could change it to an unsigned subtract.

I copied the checks from how it was written in the RISCVISD::SELECT_CC combine.

isInt<12>(TrueImm - FalseImm)) {
SDValue SRA =
DAG.getNode(ISD::SRA, DL, VT, CondV.getOperand(0),
DAG.getConstant(Subtarget.getXLen() - 1, DL, VT));
SDValue AND =
DAG.getNode(ISD::AND, DL, VT, SRA,
DAG.getSignedConstant(TrueImm - FalseImm, DL, VT));
return DAG.getNode(ISD::ADD, DL, VT, AND,
DAG.getSignedConstant(FalseImm, DL, VT));
}
}
}

const int TrueValCost = RISCVMatInt::getIntMatCost(
TrueVal, Subtarget.getXLen(), Subtarget, /*CompressionCost=*/true);
const int FalseValCost = RISCVMatInt::getIntMatCost(
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