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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Hugh Dickins' <hughd@google.com>,
        Kees Cook <keescook@chromium.org>
CC:     Vlastimil Babka <vbabka@suse.cz>,
        Joel Fernandes <joel@joelfernandes.org>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: RE: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE
Thread-Topic: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE
Thread-Index: AQHY1LmyNvo5HN+EEECxPXiP8YKQXa33zoaAgABIuACAAGaCkA==
Date:   Fri, 30 Sep 2022 21:34:23 +0000
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From: Hugh Dickins
> Sent: 30 September 2022 17:22
> 
> On Fri, 30 Sep 2022, David Laight wrote:
> > > > > Regarding "conflicting" alignment requests: yes, I agree with you,
> > > > > it would have to be a toolchain bug if when asked to align 2 and to
> > > > > align 4, it chose not to align 4.
> >
> > See https://godbolt.org/z/3nGsTaf5e
> > the align() directive takes precedence.
> >
> > Here you only want to ensure the alignment is at least 4.
> 
> Sorry, I don't understand the point you are making,
> nor how to deduce it from the link which you give:
> I'll leave it to those who understand better.

(I've copied Kees Cook - I think it is his patches that are
increasing the function alignment.
He'll need to find the thread history...)

IIRC -malign_functions is being used to put functions onto
16 byte boundaries to get aligned pad space before the function.
Adding __align(4) overrides this and (I think) will remove
the pad space.

I could only see the alignment directives in gcc ouput.
clang didn't seem to generate them.

	David

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