That would remove the need for touch#, but the point of Entry is that it does not contain references to values that the GC has to follow. If we would add a reference to the region, then the GC will have to look into the Entry, which is what we want to avoid. :)

I thought the cost of GC following a pointer into a compact region is basically constant time, or is that incorrect?

I don't have all the context you have in your app, but you are saying that during a GC the O(n) action of looking at each live Entry and finding it has a pointer into the compact region is too slow? I find that incredible.