AFAIK the prefetcher doesn't help much if it's mainly busy with prefetching padding bytes of the 'inactive' unions (and I think even a smart prefetcher would at

AFAIK the prefetcher doesn't help much if it's mainly busy with prefetching padding bytes of the 'inactive' unions (and I think even a smart prefetcher would at least always load whole cache lines which could be packed with active data instead). Of course that's also a bandwidth problem, you're wasting bandwidth loading data into the cache which then is ignored.