Here's a first, partial pass. I've tried to focus on things that confused me. I haven't fully reviewed optimizer.c yet, nor the _COLD_EXIT uop definition, so I'll focus on those when I find some time.

Maybe it would be useful to add TODO comments indicating things you're planning to tackle in the near future (subsequent PRs)? Like the counters.

Didn't look too closely at the cases generator or optimizer (I'm assuming there are other eyes on that).

• I appreciate the writeup!
• I like that the interpreter/JIT interfaces are a lot more unified on this iteration. I know you're not crazy about conditional macro stuff but I think it makes things a lot easier to follow in this case. Thanks.
• Suggestions to remove bit of tricky code duplication in template.c.
• A couple of questions about setting tstate->previous_executor.
• A few other random questions/suggestions I had while reading through.
• One thing I find sort of tricky to keep track of is the nuances of the first couple of instructions in the trace (when/where _START_EXECUTOR is added, when/where pointers to optimizers/executors are smuggled in as operands, etc). This might be worth cleaning up now or in the future, or at least writing up. Not sure.