I'm not really an expert on high performance Lisp, but my impression is that the compilers can optimize things pretty well and that you can have a fairly low le

I'm not really an expert on high performance Lisp, but my impression is that the compilers can optimize things pretty well and that you can have a fairly low level of control over things like allocation patterns. Edit: looks like I misread what you were saying. Yeah, if you want to pack two things next to each other in memory, that might be tricky in the language defined in the ansi standard. Although I think some implementations give you control over this sort of thing.