linux-kernel - RE: [PATCH v2] x86/fpu: use _Alignof to avoid UB in TYPE

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Date:   Wed, 28 Sep 2022 00:44:11 +0800
From:   YingChi Long <me@...lyc.cn>
To:     david.laight@...lab.com
Cc:     bp@...en8.de, chang.seok.bae@...el.com,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        linux-kernel@...r.kernel.org, me@...lyc.cn, mingo@...hat.com,
        ndesaulniers@...gle.com, pbonzini@...hat.com, tglx@...utronix.de,
        x86@...nel.org
Subject: RE: [PATCH v2] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN

> 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).