> > you need to use relative pointers and such

> I don't think this qualifies as a performance overhead, though, beyond the odd isub.

It ends up as one. The reason is less the additional instruction(s), but that you actually need to ferry arround additional data. In common scenarios you'll end up with a number of mappings shared between processes, so you can't just assume a single base address per-process. Instead you've to associate the specific mapping with relative pointers, and that does add to overhead. Both programming wise and runtime efficiency wise.