Style nit:

Typically, we unindent #if/#else/#endif one level for easier readability. I find this a bit easier to parse:

I know that's traditional, but honestly I'm not too happy with that convention and I find it less readable that what Irit proposes here. Also, she tried outdenting it in an early regmachine branch and the generated code was indentded weirdly.

I think this change isn't needed unless you also want to disable superinstructions. That seems like something separate, and less useful, than what you're proposing here.

IIRC it had something to do with making the interpreter easier to debug when nothing works. The more you can rule out the less code you have left to debug.

I wonder if the changes in ceval.c would be cleaner if we just changed the "ADAPTIVE_COUNTER" macro definitions when ENABLE_SPECIALIZATION is zero (instead of sprinkling these #ifs everywhere). Might be about the same level of complexity, though.

Possibly. But if we want to exclude this code from compilation we would need to rewrite ADAPTIVE_COUNTER_IS_ZERO so that it's not in an if condition.

I think this change isn't needed unless you also want to disable superinstructions. That seems like something separate, and less useful, than what you're proposing here.

We could have a separate flat ENABLE_SUPERINSTRUCTIONS for that. We could have toggles for different kinds of optimisations, with a global one to turn them all off for debugging (the individual ones can be used to assess the performance gain of the parts, and to better document what is what):

ENABLE_BYTECODE_OPTIMIZATIONS = True
ENABLE_SPECIALIZATION = ENABLE_BYTECODE_OPTIMIZATIONS and True
ENABLE_SUPERINSTRUCTIONS = ENABLE_BYTECODE_OPTIMIZATIONS and True
ENABLE_PEEPHOLER = ENABLE_BYTECODE_OPTIMIZATIONS and True
...

This is a feature for extreme debugging. I'd keep it as simple as possible.

I'll merge then. We can iterate on this.