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Python 3.15’s interpreter for Windows x86-64 should hopefully be 15% faster

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Re: Python 3.15’s interpreter for Windows x86-64 should hopefully be 15% faster

#11

Post by kenjin4096 »

mishrapravin441 wrote: post #5

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/…

Thanks for reading! For now, we maintain all 3 of the interpreters in CPython. We don't plan to remove the other interpreters anytime soon, probably never. If MSVC breaks the tail calling interpreter, we'll just go back to building and distributing the switch-case interpreter. Windows binaries will be slower again, but such is life :(.

Also the interpreter loop's dispatch is autogenerated and can be selected via configure flags. So there's almost no additional maintenance overhead. The main burden is the MSVC-specific changes we needed to get this working (amounting to a few hundred lines of code).

> Impact on debugging/profiling

I don't think there should be any, at least for Windows. Though I can't say for certain.

Re: Python 3.15’s interpreter for Windows x86-64 should hopefully be 15% faster

#12

Post by mishrapravin441 »

kenjin4096 wrote: post #11
mishrapravin441 wrote: post #5

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/…

Thanks for reading! For now, we maintain all 3 of the interpreters in CPython. We don't plan to remove the other interpreters anytime soon, probably never. If MSVC breaks the tail calling interpreter, we'll just go back to building and distributing the switch-case interpreter. Windows binaries will be slower again, but such is life :(. Also the interpreter loop's dispatch is autogenerated and can be selected via conf…

That makes sense, thanks for the detailed clarification. Having the switch-case interpreter as a fallback and keeping the dispatch autogenerated definitely reduces the long-term risk.

Re: Python 3.15’s interpreter for Windows x86-64 should hopefully be 15% faster

#13

Post by kccqzy »

redox99 wrote: post #7

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?

Python’s goal is never really to be fast. If that were its goal, it would’ve had a JIT long ago instead of toying with optimizing the interpreter. Guido prioritized code simplicity over speed. A lot of speed improvements including the JIT (PEP 744 – JIT Compilation) came about after he stepped down.

Re: Python 3.15’s interpreter for Windows x86-64 should hopefully be 15% faster

#14

Post by [deleted] »

mananaysiempre wrote: post #10

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…

[deleted]

Re: Python 3.15’s interpreter for Windows x86-64 should hopefully be 15% faster

#15

Post by publicdebates »

mananaysiempre wrote: post #10

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…

Important enough, or benefits them directly? I have no good guesses how improving Python's performance would benefit them, but I would guess that's the real reason.

Re: Python 3.15’s interpreter for Windows x86-64 should hopefully be 15% faster

#16

Post by HPsquared »

publicdebates wrote: post #15
mananaysiempre wrote: post #10

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…

Important enough, or benefits them directly? I have no good guesses how improving Python's performance would benefit them, but I would guess that's the real reason.

I wonder if this is related to Python in Excel. You'll have lots of people running numerical stuff written in Python, running on Microsoft servers.

Re: Python 3.15’s interpreter for Windows x86-64 should hopefully be 15% faster

#17

Post by mkoubaa »

publicdebates wrote: post #15
mananaysiempre wrote: post #10

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…

Important enough, or benefits them directly? I have no good guesses how improving Python's performance would benefit them, but I would guess that's the real reason.

A lot of commercial engineering and scientific software runs on windows.

Re: Python 3.15’s interpreter for Windows x86-64 should hopefully be 15% faster

#18

Post by pxeger1 »

mishrapravin441 wrote: post #5

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/…

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Re: Python 3.15’s interpreter for Windows x86-64 should hopefully be 15% faster

#19

Post by monster_truck »

mananaysiempre wrote: post #10

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…

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