Function safe_arch::addsub_m256d

#[must_use]pub fn addsub_m256d(a: m256d, b: m256d) -> m256d

This is supported with target feature avx only.

Alternately, from the top, add f64 then sub f64.

let a = m256d::from_array([10.0, 20.0, 30.0, 40.0]);
let b = m256d::from_array([100.0, 200.0, 300.0, 400.0]);
let c = addsub_m256d(a, b).to_array();
assert_eq!(c, [-90.0, 220.0, -270.0, 440.0]);