The Python interpreter core loop sounds like the perfect problem for AlphaEvolve. Or it's open source equivalent OpenEvolve if DeepMind doesn't want to speed up Python for the competition.

> This has caused many issues for compilers in the past, too many to list in fact. I have a EuroPython 2025 talk about this.

Looks like it refers to this:

https://youtu.be/pUj32SF94Zw

(wish it's a link in the article)

TLDR: The tail-calling interpreter is slightly faster than computed goto.

> I used to believe the the tailcalling interpreters get their speedup from better register use. While I still believe that now, I suspect that is not the main reason for speedups in CPython.

> My main guess now is that tail calling resets compiler heuristics to sane levels, so that compilers can do their jobs.

> Let me show an example, at the time of writing, CPython 3.15’s interpreter loop is around 12k lines of C code. That’s 12k lines in a single function for the switch-case and computed goto interpreter.

> […] In short, this overly large function breaks a lot of compiler heuristics.

> One of the most beneficial optimisations is inlining. In the past, we’ve found that compilers sometimes straight up refuse to inline even the simplest of functions in that 12k loc eval loop.

Really nice results on MSVC. The idea that tail calls effectively reset compiler heuristics and unblock inlining is pretty convincing. One thing that worries me though is the reliance on undocumented MSVC behavior — if this becomes widely shipped, CPython could end up depending on optimizer guarantees that aren’t actually stable. Curious how you’re thinking about long-term maintainability and the impact on debugging/profiling.

if the author of this blog reads this: can we can an RSS, please?

This seems like very low hanging fruit. How is the core loop not already hyper optimized?

I'd have expected it to be hand rolled assembly for the major ISAs, with a C backup for less common ones.

How much energy has been wasted worldwide because of a relatively unoptimized interpreter?

The money shot (wish this were included in the blog post):

  #   if defined(_MSC_VER) && !defined(__clang__)
  #      define Py_MUSTTAIL [[msvc::musttail]]
  #      define Py_PRESERVE_NONE_CC __preserve_none
  #   else
  #       define Py_MUSTTAIL __attribute__((musttail))
  #       define Py_PRESERVE_NONE_CC __attribute__((preserve_none))
  #   endif

https://github.com/python/cpython/pull/143068/files#diff-45b...

Apparently(?) this also needs to be attached to the function declarator and does not work as a function specifier: `static void *__preserve_none slowpath();` and not `__preserve_none static void *slowpath();` (unlike GCC attribute syntax, which tends to be fairly gung-ho about this sort of thing, sometimes with confusing results).

Yay to getting undocumented MSVC features disclosed if Microsoft thinks you’re important enough :/