Code shared from the Rust Playground

playground.rs

use std::arch::x86_64::*;
use std::mem::transmute;

#[target_feature(enable = "avx2")]
#[target_feature(enable = "avx")] 
pub unsafe fn alpha_mask(a: __m256i, b: __m256i) -> __m256i {
    let mask = _mm256_set_epi8(
        0, 0, 0, -1,
        0, 0, 0, -1,
        0, 0, 0, -1,
        0, 0, 0, -1,
        0, 0, 0, -1,
        0, 0, 0, -1,
        0, 0, 0, -1,
        0, 0, 0, -1,
    );
    _mm256_blendv_epi8(a, b, mask)
}

fn main() {
    unsafe {
        let a = transmute::<[u8; 32], _>([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31]);
        let b = transmute::<[u8; 32], _>([32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63]);
        let c = transmute::<_, [u8; 32]>(alpha_mask(a, b));
        println!("{:?}", c);
    }
}