Benchmark results show a hint of a speedup, but in the noise. No slowdown, though.

Some things I observed. Correct me if I'm wrong:

1. This actually splits out the tier 2 interpreter from the tier 1 interpreter. Which is likely a significant saving for our tier 2 debug build time and tier 2 interpreter. Nice!
2. The shim/trampoline is only compiled once lazily and reused across the whole runtime. Then reused across all JIT functions. We currently do two jumps (one into the shim/trampoline, one into the exec->jit_code). You need to lock this with a mutex in case multiple threads try to compile the shim at the same time.

If my understanding above is correct, then this is alright to me.

That is all correct. I'll add a comment explaining it.

Nice, sorry I wasn't able to review in time, but it looks mostly good (love the unification of the two JIT execution engines).

Two notes:

• I prefer the old name "shim" throughout to the new name "trampoline". Trampoline already has another meaning in the JIT compiler, and we renamed this to "shim" a while back to make the distinction clearer.
• I don't love that we leak the shim code when the runtime is finalized. We should free it in Py_Finalize or whatever.