SetLeastSignificantBits.cs

public static T SetLeastSignificantBits<T>(this T value) where T : IBinaryInteger<T>, IUnsignedNumber<T> {
    var x = (value | (value >> 1));

    x |= (x >> 2);
    x |= (x >> 4);

    if (BinaryIntegerConstants<T>.Size > T.CreateChecked(value: 8UL)) {
        x |= (x >> 8);
    }

    if (BinaryIntegerConstants<T>.Size > T.CreateChecked(value: 16UL)) {
        x |= (x >> 16);
    }

    if (BinaryIntegerConstants<T>.Size > T.CreateChecked(value: 32UL)) {
        x |= (x >> 32);
    }

    if (BinaryIntegerConstants<T>.Size > T.CreateChecked(value: 64UL)) {
        x |= (x >> 64);
    }

    if (BinaryIntegerConstants<T>.Size > T.CreateChecked(value: 128UL)) {
        var i = int.CreateChecked(value: (BinaryIntegerConstants<T>.Size >> 1));

        do {
            x |= (x >> i);
        } while ((i >>= 1) > 128);
    }

    return x;
}