From: Paul Eggert <eggert@cs.ucla.edu>
To: Eli Zaretskii <eliz@gnu.org>, martin rudalics <rudalics@gmx.at>
Cc: rgm@gnu.org, emacs-devel@gnu.org
Subject: Re: emacs-26 9e59de9: Use GCALIGNED properly for GCC
Date: Fri, 10 Nov 2017 00:26:19 -0800 [thread overview]
Message-ID: <4ee60712-789e-f160-52a9-4e24a9733a11@cs.ucla.edu> (raw)
In-Reply-To: <83zi7upnfo.fsf@gnu.org>
Eli Zaretskii wrote:
> Paul, maybe we should simply go back a few notches and not mark
> main_thread as GCALIGNED, except on that single platform which needed
> it (i.e. via #ifdef)? Then we could return all the other symbols to
> their previous syntax.
>
> Or maybe we should move GCALIGNED to yet another position, like at the
> beginning or the end of the whole construct. Like this:
>
> GCALIGNED struct foo FOO;
> or
> struct foo FOO GCALIGNED;
>
> Not sure what else can we do, but this seems to be an exceptionally
> fragile feature, at least with GCC 7 that many people are using.
I've investigated this further, and neither solution is likely to work. It is
merely the luck of the draw that we have seen the bug only on that platform so
far, and a similar bug is likely to occur on other platforms. Neither GCALIGNED
syntax works in general (see GCC bug 82914) and we've seen examples of failures
with either syntax.
I have thought of a solution that fixes the problem by dropping use of GCALIGNED
and instead using classic C unions along with 'char alignas (8)'. The idea is to
gcalign a 'struct foo' this way:
union gcaligned_foo { struct foo s; char alignas (8) gcaligned; };
Something like this should work on all platforms that Emacs ports to. I plan to
work on a first cut tomorrow.
next prev parent reply other threads:[~2017-11-10 8:26 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20171109031206.7056.28312@vcs0.savannah.gnu.org>
[not found] ` <20171109031208.D2CAF2033E@vcs0.savannah.gnu.org>
2017-11-09 23:31 ` emacs-26 9e59de9: Use GCALIGNED properly for GCC Glenn Morris
2017-11-10 7:10 ` martin rudalics
2017-11-10 8:06 ` Eli Zaretskii
2017-11-10 8:26 ` Paul Eggert [this message]
2017-11-10 9:57 ` Eli Zaretskii
2017-11-10 16:23 ` Stefan Monnier
2017-11-10 17:58 ` Paul Eggert
2017-11-10 18:02 ` Stefan Monnier
2017-11-10 18:11 ` Philipp Stephani
2017-11-10 19:19 ` Paul Eggert
2017-11-10 20:31 ` Stefan Monnier
2017-11-10 20:45 ` Paul Eggert
2017-11-11 7:08 ` Paul Eggert
2017-11-11 7:57 ` Paul Eggert
2017-11-11 8:34 ` martin rudalics
2017-11-11 8:50 ` Paul Eggert
2017-11-11 9:46 ` martin rudalics
2017-11-11 8:33 ` martin rudalics
2017-11-13 18:19 ` Paul Eggert
2017-11-13 18:45 ` martin rudalics
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