gh-127545: Specify minimum PyGC_Head alignment to fix build failure #127546
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Misc/NEWS.d/next/Build/2024-12-04-10-00-35.gh-issue-127545.t0THjE.rst
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I think the patch can made more understandable, by making use of the existing macro definitions. I will update the branch. |
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I just noticed that @markshannon refactored the same headers 2 months ago. So I have now updated the fix to resolve those merge conflicts. Please review. This bug is still present in 3.14.0beta1. |
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We're applying this patch in Debian's python3.14 to get it to build on m68k. |
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I see that one of Debian's 3.14.0b2 packages (the one --with-pydebug) fails with an assertion failure that hasn't been addressed in my patch. I'm presently testing a patch to deal with that too, by aligning struct _object. |
As documented in InternalDocs/garbage_collector.md, the garbage collector stores flags in the least significant two bits of the _gc_prev pointer in struct PyGC_Head. Consequently, this pointer is only capable of storing a location that's aligned to a 4-byte boundary. This alignment requirement is documented but it's not actually encoded. The code only works when python happens to run on a platform that has a sufficiently large minimum alignment for the structs in question. The same problem arises with PyObject pointers because the least significant bits get used for PyStackRef tags. Since we know that 2 bits are needed, we also know the minimum alignment that's needed. Let's make that explicit, so the compiler can then make those bits available. This patch fixes a segfault in _bootstrap_python. In 3.14.0 beta 2 this fixes the "Assertion `!PyStackRef_IsTaggedInt(ref)' failed" when built with --config-pydebug. Also, making the requirements explicit improves clarity. This bug was previously investigated by Adrian Glaubitz here: https://lists.debian.org/debian-68k/2024/11/msg00020.html https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1087600 Although Adrian's patch isn't really correct (because natural alignment is not needed), he deserves full credit for finding the root cause.
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I pushed this branch a second time after rebasing on 'main' instead of v3.14.0b2... but I'm still not sure which branch I should be using -- 3.14 or main? |
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Would you opening a new PR? Feel free to tag me on it and I'll apply the necessary labels. |
Sorry for the mess I made when I rebased on 3.14. |
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It's best to avoid rebasing, just merge https://devguide.python.org/getting-started/pull-request-lifecycle/#quick-guide |
#127545