GH-148047: Check early whether tail-calling is possible for MSVC builds on Windows by chris-eibl · Pull Request #148036 · python/cpython

chris-eibl · 2026-04-03T13:54:22Z

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.

Issue: Check early whether tail-calling is possible for MSVC builds on Windows #148047

chris-eibl · 2026-04-03T14:21:44Z

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.

Fidget-Spinner · 2026-04-03T16:28:30Z

Please open a separate issue for this, rather than reusing the same closed issue. Thanks!

itamaro

Overall looks good, thank you for this!

PCbuild/pythoncore.vcxproj

Misc/NEWS.d/next/Build/2026-04-03-20-09-46.gh-issue-148047.HE6iGK.rst

bedevere-app · 2026-04-04T15:57:00Z

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.

bedevere-bot · 2026-04-12T05:56:18Z

⚠️⚠️⚠️ 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

zooba · 2026-04-14T14:48:10Z

PCbuild/pythoncore.vcxproj

+    <Error Text="MSVC supports tail-calling only for x64."
+           Condition="$(Platform) != 'x64'" />
+    <Error Text="Platform toolset >= v145 is required for tail-calling."
+           Condition="$(PlatformToolset.Replace('v', '0')) &lt; '145'" />


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.

zooba · 2026-04-14T14:49:26Z

PCbuild/pythoncore.vcxproj

+    <Error Text="Platform toolset >= v145 is required for tail-calling."
+           Condition="$(PlatformToolset.Replace('v', '0')) &lt; '145'" />
+    <Error Text="MSVC requires optimization to be enabled for tail-calling."
+           Condition="$(Configuration) == 'Debug'" />


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.

zooba · 2026-04-14T14:52:52Z

PCbuild/pythoncore.vcxproj

+
+  <Target Name="_CheckTailCalling" BeforeTargets="PrepareForBuild" Condition="'$(UseTailCallInterp)' == 'true' and $(PlatformToolset) != 'ClangCL'">
+    <Error Text="MSVC supports tail-calling only for x64."
+           Condition="$(Platform) != 'x64'" />


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.

Fidget-Spinner · 2026-04-14T15:16:22Z

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.

… for MSVC builds on Windows (python#148036)" This reverts commit cbd81d5.

Fidget-Spinner · 2026-04-14T15:19:18Z

Revert up at #148558

chris-eibl · 2026-04-14T15:39:54Z

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.

Fidget-Spinner · 2026-04-14T15:41:53Z

No problem, nothing to apologise for!

…sible for MSVC builds on Windows (#148036)" (#148558) This reverts commit cbd81d5.

chris-eibl added 2 commits March 27, 2026 08:31

check tail-calling support in pythoncore.vcxproj

15b0bf1

Merge branch 'main' into CheckTailCalling

92a236d

chris-eibl requested a review from a team as a code owner April 3, 2026 13:54

bedevere-app bot added the awaiting review label Apr 3, 2026

bedevere-app bot mentioned this pull request Apr 3, 2026

Tail calling interpreter for MSVC #139922

Closed

chris-eibl requested review from Fidget-Spinner and itamaro April 3, 2026 13:56

chris-eibl added the skip news label Apr 3, 2026

chris-eibl mentioned this pull request Apr 3, 2026

GH-139922: add interpreter types to InternalDocs/interpreter.md #148035

Merged

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

bedevere-app bot mentioned this pull request Apr 3, 2026

Check early whether tail-calling is possible for MSVC builds on Windows #148047

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chris-eibl added OS-windows build The build process and cross-build and removed skip news labels Apr 3, 2026

chris-eibl added 2 commits April 3, 2026 20:08

Merge remote-tracking branch 'upstream/main' into CheckTailCalling

3a28aeb

blurb it

8768351

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

itamaro requested changes Apr 4, 2026

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update news entry

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itamaro merged commit cbd81d5 into python:main Apr 12, 2026
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chris-eibl deleted the CheckTailCalling branch April 12, 2026 05:58

zooba reviewed Apr 14, 2026

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Revert "pythonGH-148047: Check early whether tail-calling is possible…

0550f09

… for MSVC builds on Windows (python#148036)" This reverts commit cbd81d5.

Fidget-Spinner added a commit that referenced this pull request Apr 14, 2026

gh-148047: Revert "GH-148047: Check early whether tail-calling is pos…

11da7d4

…sible for MSVC builds on Windows (#148036)" (#148558) This reverts commit cbd81d5.

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