fn classify_arg<'a, Ty, C>(
    cx: &C,
    arg: &mut ArgAbi<'a, Ty>,
    xlen: u64,
    flen: u64,
    is_vararg: bool,
    avail_gprs: &mut u64,
    avail_fprs: &mut u64
)where
    Ty: TyAbiInterface<'a, C> + Copy,