GH-125837: Rework the instructions for loading constants and returning values.

[markshannon]

Adds:

• RETURN_VALUE_FUNC
• RETURN_VALUE_GEN
• INSTRUMENTED_RETURN_VALUE_FUNC
• INSTRUMENTED_RETURN_VALUE_GEN
• LOAD_CONST_IMMORTAL

Removes:

• RETURN_VALUE
• RETURN_CONST
• INSTRUMENTED_RETURN_VALUE
• INSTRUMENTED_RETURN_CONST

• Issue: gh-125837

[markshannon]

Performance impact without the JIT is in the noise. Nominally 0.2% slower

Performance impact with the JIT is also in the noise. Nominally 0.2% faster.

Stats shows that almost 90% of LOAD_CONST are converted to LOAD_CONST_IMMORTAL. There is a 3% increase in EXTENDED_ARG due to LOAD_CONST RETURN_VALUE taking more space than RETURN_CONST.

My hypothesis is that the slowdown caused by extra dispatching is mostly offset by the more efficient LOAD_CONST in tier 1 and more streamlined RETURN_VALUEs. In tier 2 there is no extra overhead for dispatching, which might be why we see a tiny speedup instead of a tiny slowdown.

[bedevere-app[bot]]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[markshannon]

The magic comment is "I have made the requested changes; please review again". I haven't, but I think I've justified my decisions. @iritkatriel

[bedevere-app[bot]]

Thanks for making the requested changes!

@iritkatriel: please review the changes made to this pull request.

[markshannon]

I'm closing this for now. We can revisit splitting up RETURN_VALUE when the full refactoring I suggested above is ready.

https://github.com/python/cpython/pull/125972 splits up LOAD_CONST and removes RETURN_CONST and shows slightly better performance.