gh-127545: Replace _Py_ALIGN_AS(V) by _Py_ALIGNED_DEF(N, T) #135209
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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.
This is a common façade for the various `_Alignas` alternatives, which behave in interesting ways. The standard `alignas` errors if it would decrease the alignment; to prevent that it can be used again with the defined variable's type. The standard `alignas` can't be used on a `struct` definition, the workaround is to use it on one of the `struct`'s members. MSVC `declspec(aligned)` only takes integer literals. MSVC `declspec(aligned)` had a bug when applied to a combined struct+member definition. The workaround is to separate the struct definition. Do that for `PyASCIIObject.state`.
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🤖 New build scheduled with the buildbot fleet by @encukou for commit 512fe58 🤖 Results will be shown at: https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F135209%2Fmerge If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again. |
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@fthain, would you be able to test this on a m68k? [edit] Buildbot failures look unrelated. |
This was referenced Jun 7, 2025
encukou
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Jun 7, 2025
encukou
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Jun 7, 2025
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_Py_ALIGNED_DEF(N, T)might be a better way to make the various_Alignasalternatives,which behave in interesting ways, increase (rather than set) a variable's alignment.
The standard
alignas/_Alignaserror out if they would decrease the alignment. To prevent that, they can be used again with the defined variable's type.The standard
alignas/_Alignascan't be used on astructdefinition, the workaround is to use it on one of thestruct's members.MSVC
declspec(aligned)only takes integer literals.MSVC
declspec(aligned)had a bug when applied to a combined struct+member definition; a workaround is to separate the struct definition. This avoids a potentially ABI-breaking bug in older MSVC versions (which newer versions warn about, using code C5274).This PR does that for
PyASCIIObject.state.