gh-127545: Specify minimum PyGC_Head alignment to fix build failure #127546
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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool. If this change has little impact on Python users, wait for a maintainer to apply the |
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 works when python happens to run on a platform that has a large minimum alignment, but fails otherwise. 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 sure those 2 bits are available. This patch fixes a segfault in _bootstrap_python. It also clarifies the code by making the actual requirement explicit. This bug was 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.
Misc/NEWS.d/next/Build/2024-12-04-10-00-35.gh-issue-127545.t0THjE.rst
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…han a plain number. Move the definition accordingly.
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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#127545