In the past they've talked about the speed and memory improvements they've gotten from using MultiArrayList in the compiler, storing tags separately from the un

In the past they've talked about the speed and memory improvements they've gotten from using MultiArrayList in the compiler, storing tags separately from the unions themselves. If you have a union with a size of 16 bytes and you add a tag to that which is 1 byte, a lot of space is wasted due to padding. If you keep the tags in a separate array, both arrays are individually densely packed. Less memory wasted due to padding == less memory use overall and better utilization of cache. But in terms of the implementation, this means working with untagged unions, because the tags are maintained externally.