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[DAG] visitTRUNCATE - early out from computeKnownBits/ComputeNumSignBits failures. NFC. #154111

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Merged
merged 2 commits into from
Aug 18, 2025
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[DAG] visitTRUNCATE - early out from computeKnownBits/ComputeNumSignBits failures. NFC. #154111

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a8da17e
[DAG] visitTRUNCATE - early out from computeKnownBits/ComputeNumSignB…
RKSimon Aug 18, 2025
3fb4dae
Early out from the RHS first - more likely to be constant
RKSimon Aug 18, 2025
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15 changes: 6 additions & 9 deletions llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -16332,25 +16332,22 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
// (trunc (abdu/abds a, b)) -> (abdu/abds (trunc a), (trunc b))
if ((!LegalOperations || N0.hasOneUse()) &&
TLI.isOperationLegal(N0.getOpcode(), VT)) {
EVT SrcVT = N0.getValueType();
EVT TruncVT = VT;
unsigned SrcBits = SrcVT.getScalarSizeInBits();
unsigned TruncBits = TruncVT.getScalarSizeInBits();
unsigned NeededBits = SrcBits - TruncBits;

SDValue A = N0.getOperand(0);
SDValue B = N0.getOperand(1);
bool CanFold = false;

if (N0.getOpcode() == ISD::ABDU) {
KnownBits KnownA = DAG.computeKnownBits(A);
KnownBits KnownB = DAG.computeKnownBits(B);
CanFold = KnownA.countMinLeadingZeros() >= NeededBits &&
KnownB.countMinLeadingZeros() >= NeededBits;
APInt UpperBits = APInt::getBitsSetFrom(SrcBits, TruncBits);
CanFold = DAG.MaskedValueIsZero(B, UpperBits) &&
DAG.MaskedValueIsZero(A, UpperBits);
} else {
unsigned SignBitsA = DAG.ComputeNumSignBits(A);
unsigned SignBitsB = DAG.ComputeNumSignBits(B);
CanFold = SignBitsA > NeededBits && SignBitsB > NeededBits;
unsigned NeededBits = SrcBits - TruncBits;
CanFold = DAG.ComputeNumSignBits(B) > NeededBits &&
DAG.ComputeNumSignBits(A) > NeededBits;
}

if (CanFold) {
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