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

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Message-ID: <aa329b6fa5764e03ad6891142c2311fc@AcuMS.aculab.com>
Date:   Thu, 6 Oct 2022 08:12:04 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Nick Desaulniers' <ndesaulniers@...gle.com>,
        YingChi Long <me@...lyc.cn>
CC:     "bp@...en8.de" <bp@...en8.de>,
        "chang.seok.bae@...el.com" <chang.seok.bae@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>
Subject: RE: [PATCH v2] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN

From: Nick Desaulniers
> Sent: 05 October 2022 19:58
...
> https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3054.pdf
> Section 6.2.8 "Alignment of objects" refers to "fundamental alignment"
> and "extended alignment."
> 
> I wonder if it would be precise to say that "_Alignof evaluates to the
> fundamental alignment while __alignof__ evaluates to the extended
> alignment (which is implementation defined, typically by the machine
> specific ABI)." Though even that seems imprecise since it sounds like
> a fundamental alignment could be less than or equal to what alignof
> evaluates to.

Except that neither of those terms makes any sense to most
people.
Something like "__alignof__() is the preferred alignment and
_Alignof() the minimal alignment. For 'long long' on x86 these
are 8 and 4 respectively."

	David

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