So, on the JIT branch, we don't actually keep the executor handy when executing instructions (only when entering/exiting traces). I'm guessing we'll need it a lot more in the future once we start doing stitching and such, but I wonder: can we just stick it on the tstate or the frame? It feels a bit painful to dedicate a register to this thing, especially since it'll be hardly used in optimized traces (I think).

Not a big deal though, and can be deferred to a later date. Just something to ponder, or to do if it's easy now.

Idea: put the executor's address in _CHECK_VALIDITY's operand? Then the JIT would require no modifications to support this change.

Ah, never mind, the JIT can burn in the executor. I withdraw my comments. :)