pdep in Zig with select implementation that find the nth set bit of an integer

pdep.zig

fn pdep(src: u64, mask: u64) u64 {
    return asm ("pdep %[mask], %[src], %[ret]"
        : [ret] "=r" (-> u64),
        : [src] "r" (src),
          [mask] "r" (mask),
    );
}

fn select(x: u64, k: u6) u6 {
    @setRuntimeSafety(false);
    return @intCast(u6, @ctz(pdep(@as(u64, 1) << k, x)));
}