gh-127545: Replace _Py_ALIGN_AS(V) by _Py_ALIGNED_DEF(N, T) #135209
+123
−106
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
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`.
|
🤖 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. |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
_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 the C5274 warning.This PR does that for
PyASCIIObject.state.