Function round_pack 

pub fn round_pack(b: &mut Builder, sign: u32, exp: &Mw, sig: &Mw) -> Mw

Round sig × 2^(exp − 62) to binary64, nearest-even, and pack it with sign. sig has its leading bit at 62 (a nonzero significand), with any lost low bits folded into bit 0 as a sticky bit. The one rounding every operation performs: normal results, gradual underflow to subnormals, and overflow to infinity.