Messages in this thread | | | From | YingChi Long <> | Subject | RE: [PATCH v2] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN | Date | Wed, 28 Sep 2022 00:44:11 +0800 |
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> Interesting - what justification do they give? > Linux kernel requires that the compiler add no unnecessary padding > so that structure definitions are well defined.
Yes, that's a clarification given in 2019.
> So using a type definition inside offsetof() won't give a > useful value - but it still isn't really UB.
WG14 may worry about commas and the scope of new definitions. So they provide new words into the standard and said:
> If the specified type defines a new type or if the specified member is a > bit-field, the behavior is undefined.
https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
I've provided this link in the patch.
> Has that ever worked? > Given: > struct foo { > int a; > char b; > char c; > };
TYPE_ALIGN(struct foo) evaluates to 4 in the previous approach (based on offsetof). _Align(struct foo) evaluates to the same value.
See https://godbolt.org/z/sqebhEnsq
> I think CHECK_MEMBER_AT_END_OF_TYPE(struct foo, b) is true.
Hmm, both the previous version and after this patch the macro gives me false. (See the godbolt link).
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