GeezerVison is showing github.com — GH-148047: Check early whether tail-calling is possible for MSVC builds on Windows by chris-eibl · Pull Request #148036 · python/cpython
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chris-eibl
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Apr 3, 2026
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Rather than failing late when compiling e.g. a debug configuration
build.bat -c debug --tail-call-interp
with hundreds of
error C4737: Unable to perform required tail call. Performance may be degraded.
lets fail early with an explicit error message for configurations that are not supported by MSVC.
This is a follow-up on #140513 / #140548.
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chris-eibl
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Apr 3, 2026
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TSan https://github.com/python/cpython/actions/runs/23948631142/job/69850491565?pr=148036 failure for sure unreleated: this is a pure Windows change and would result in a build error on onsupported (Windows) hosts when using --tail-call-interp. Note that there are no such hosts in CI nor in buildbots, and if there were, they would have already failed (but with tons of Unable to perform required tail call messages).
I've checked manually that I get all the expected errors for unsupported configurations.
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Please open a separate issue for this, rather than reusing the same closed issue. Thanks!
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chris-eibl
changed the title
GH-139922: Check early whether tail calling is possible
GH-148047: Check early whether tail calling is possible
Apr 3, 2026
chris-eibl
changed the title
GH-148047: Check early whether tail calling is possible
GH-148047: Check early whether tail-calling is possible for MSVC builds on Windows
Apr 3, 2026
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Overall looks good, thank you for this!
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A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.
Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.
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⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️
Hi! The buildbot x86 Debian Installed with X 3.x (no tier) has failed when building commit cbd81d5.
What do you need to do:
- Don't panic.
- Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
- Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1244/builds/7983) and take a look at the build logs.
- Check if the failure is related to this commit (cbd81d5) or if it is a false positive.
- If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.
You can take a look at the buildbot page here:
https://buildbot.python.org/#/builders/1244/builds/7983
Summary of the results of the build (if available):
==
Click to see traceback logs
Traceback (most recent call last):
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/contextlib.py"�[0m, line �[35m85�[0m, in �[35minner�[0m
return func(*args, **kwds)
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m649�[0m, in �[35mtest_interrupt�[0m
exitcode = self._kill_process(multiprocessing.Process.interrupt)
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/contextlib.py"�[0m, line �[35m85�[0m, in �[35minner�[0m
return func(*args, **kwds)
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m630�[0m, in �[35m_kill_process�[0m
self.assertEqual(�[31mjoin�[0m�[1;31m()�[0m, None)
�[31m~~~~�[0m�[1;31m^^�[0m
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m303�[0m, in �[35m__call__�[0m
return �[31mself.func�[0m�[1;31m(*args, **kwds)�[0m
�[31m~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^�[0m
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/process.py"�[0m, line �[35m156�[0m, in �[35mjoin�[0m
res = self._popen.wait(timeout)
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/popen_fork.py"�[0m, line �[35m44�[0m, in �[35mwait�[0m
return �[31mself.poll�[0m�[1;31m(os.WNOHANG if timeout == 0.0 else 0)�[0m
�[31m~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^�[0m
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/popen_fork.py"�[0m, line �[35m28�[0m, in �[35mpoll�[0m
pid, sts = �[31mos.waitpid�[0m�[1;31m(self.pid, flag)�[0m
�[31m~~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^�[0m
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m626�[0m, in �[35mhandler�[0m
raise RuntimeError('join took too long: %s' % p)
�[1;35mRuntimeError�[0m: �[35mjoin took too long: <Process name='Process-158' pid=28453 parent=26696 started daemon>�[0m
Traceback (most recent call last):
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/contextlib.py"�[0m, line �[35m85�[0m, in �[35minner�[0m
return func(*args, **kwds)
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m649�[0m, in �[35mtest_interrupt�[0m
exitcode = self._kill_process(multiprocessing.Process.interrupt)
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/contextlib.py"�[0m, line �[35m85�[0m, in �[35minner�[0m
return func(*args, **kwds)
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m630�[0m, in �[35m_kill_process�[0m
self.assertEqual(�[31mjoin�[0m�[1;31m()�[0m, None)
�[31m~~~~�[0m�[1;31m^^�[0m
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m303�[0m, in �[35m__call__�[0m
return �[31mself.func�[0m�[1;31m(*args, **kwds)�[0m
�[31m~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^�[0m
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/process.py"�[0m, line �[35m156�[0m, in �[35mjoin�[0m
res = self._popen.wait(timeout)
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/popen_fork.py"�[0m, line �[35m44�[0m, in �[35mwait�[0m
return �[31mself.poll�[0m�[1;31m(os.WNOHANG if timeout == 0.0 else 0)�[0m
�[31m~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^�[0m
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/popen_fork.py"�[0m, line �[35m28�[0m, in �[35mpoll�[0m
pid, sts = �[31mos.waitpid�[0m�[1;31m(self.pid, flag)�[0m
�[31m~~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^�[0m
File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m626�[0m, in �[35mhandler�[0m
raise RuntimeError('join took too long: %s' % p)
�[1;35mRuntimeError�[0m: �[35mjoin took too long: <Process name='Process-1' pid=2089 parent=2087 started daemon>�[0m
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<Error Text="MSVC supports tail-calling only for x64."
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Condition="$(Platform) != 'x64'" />
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<Error Text="Platform toolset >= v145 is required for tail-calling."
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Condition="$(PlatformToolset.Replace('v', '0')) < '145'" />
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We can't do this check - PlatformToolset is an arbitrary string, you can't assume that it's comparable to a number in any meaningful way.
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<Error Text="Platform toolset >= v145 is required for tail-calling."
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Condition="$(PlatformToolset.Replace('v', '0')) < '145'" />
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<Error Text="MSVC requires optimization to be enabled for tail-calling."
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Condition="$(Configuration) == 'Debug'" />
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Why would we do this? It makes it annoyingly hard to set tail calling enabled in my environment and then choose between debug/release builds.
A warning is fine, if you really want, but a debug build is inherently without optimisations, so there's no reason to remind users that that's what they're getting.
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<Target Name="_CheckTailCalling" BeforeTargets="PrepareForBuild" Condition="'$(UseTailCallInterp)' == 'true' and $(PlatformToolset) != 'ClangCL'">
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<Error Text="MSVC supports tail-calling only for x64."
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Condition="$(Platform) != 'x64'" />
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Not convinced this needs to be an error - if/when MSVC gets support, they may want to test it on CPython themselves before shipping, and we're just going to block them.
As with Debug, make it a warning if you really want, but ultimately it's up to the user to decide whether they're enabling a feature or not. We shouldn't prevent users from doing what they want to do.
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Really sorry @chris-eibl, but I'm going to revert this change. I think users can just let the compiler fail to build if it's not available.
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Really sorry @chris-eibl, but I'm going to revert this change. I think users can just let the compiler fail to build if it's not available.
No worries, totally fine with me. That's why we have discussions and reviews.
Sorry for the churn.
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No problem, nothing to apologise for!
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ljfp
pushed a commit
to ljfp/cpython
that referenced
this pull request
Apr 25, 2026
…C builds on Windows (python#148036)
Rather than failing late when compiling e.g. a debug configuration
```
build.bat -c debug --tail-call-interp
```
with hundreds of
```
error C4737: Unable to perform required tail call. Performance may be degraded.
```
-- fail early with an explicit error message for configurations that are not supported by MSVC.
This is a follow-up on python#140513 / python#140548
ljfp
pushed a commit
to ljfp/cpython
that referenced
this pull request
Apr 25, 2026
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