Thank you for the discussion!

A few gaps won't matter, and that to me speaks of a split between major and minor GC making sense. However, I'm not really sold on that meaning a free list making sense. For one, if I split the heap vectors into single value parts, then holding free slot data in any of them will become somewhat complicated. Hence at least for the foreseeable future I'm 100% in on the compacted heap vectors idea :) Time will tell if the aggressive compacting makes sense or not.

Our JavaScript Values are the full 8 bytes (yes, this is large and it pains me, but it does give us all integers on stack, most doubles on stack, and up to 7 bytes of string data on stack), so a field that can point to any kind of object stores a byte tag and a u32 index. I might pack this down to 1+3 bytes or so, at the cost of supporting smaller maximum number of objects in the engine. JS Value itself would still probably remain 8 bytes because of the stack data benefits.

There is indeed dynamic dispatching through match statements, though it generally happens at the specification method level. Eg. A specification method to get a property from an object will match on the tag and then dispatch to a concrete method with the index as parameter. The indexes are typed as well, so from this point on we statically know we're dealing with eg. an Array.

So there is dynamic dispatch yes, but we try to eliminate it at the earliest opportunity. We probably still have more of that than a traditional engine would have though: A traditional engine will keep the tag on the heap and there is some dynamic dispatch done based on that, but at least your data lookup isn't based on dispatch.