From: Carlos O'Donell <carlos@redhat.com>
To: John David Anglin <dave.anglin@bell.net>,
Jason Merrill <jason@redhat.com>,
Jakub Jelinek <jakub@redhat.com>
Cc: Florian Weimer <fweimer@redhat.com>,
Bernd Edlinger <bernd.edlinger@hotmail.de>,
"gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>,
Jeff Law <law@redhat.com>
Subject: Re: [PATCH] Implement new hook for max_align_t_align
Date: Wed, 12 Oct 2016 19:59:00 -0000 [thread overview]
Message-ID: <5772a64a-9f2b-9134-cf67-6af05e997b25@redhat.com> (raw)
In-Reply-To: <cc06e9db-311e-1ad1-8c3f-c5279e49adff@bell.net>
On 10/12/2016 10:17 AM, John David Anglin wrote:
> On 2016-10-12 9:48 AM, Jason Merrill wrote:
>> On Wed, Oct 12, 2016 at 4:02 AM, Jakub Jelinek <jakub@redhat.com> wrote:
>>> On Wed, Oct 12, 2016 at 09:52:04AM +0200, Florian Weimer wrote:
>>>> dropping the alignment means that the padding before the lock member
>>>> vanishes. Consequently, we have just created a silent ABI change in
>>>> application code, which is a big no-no.
>>> Sure, it would be an ABI change, but how many users would it affect?
>>>
>>>> Since this is PA-RISC, which is essentially dead (neither HPE nor Debian
>>>> ship it anymore), I stand by my suggestion to bump the fundamental alignment
>>> Or just drop support for a dead arch?
>>>
>>>> instead. Sure, it is a bit inefficient, but this will only affect PA-RISC
>>>> users. It does not even cause work for PA-RISC porters. Conversely, if we
>>>> work on this to come up with a different fix, many more people will be
>>>> affected (because they don't get all the nice things we could work on
>>>> instead), and we may need to maintain a special GCC kludge for the
>>>> alternative solution, impacting GCC developers in particular.
>>> But sure, bumping malloc alignment is probably easiest. And people who want
>>> performance have better options than to stay on 32-bit PA-RISC anyway.
>> Or we could do nothing and tell people to ignore the harmless warning.
> The warning is an issue because of -Werror. However, it appears easy to suppress it in the PA
> backend. I have a patch that I'm testing.
>
> We are discussing offline regarding the glibc issue. It easy to bump the alignment of malloc
> but I take Jakub's point and maybe we should break the ABI. Debian unstable churns
> quickly, and I think we would be better off being consistent with the current max_align_t
> and 8-byte aligned malloc.
I am against breaking the ABI.
I would rather see us bump malloc alignment up to 16-bytes.
The last time I changed this alignment it _immediately_ broken libstdc++ boostrap
because it's using exactly the kind of embedded pthread_mutext_t we're talking about
breaking.
So in that case the debian builds in unstable broke right away, and I had to revert
the change. We'd have to BinNMU a bunch of things to get this working again.
Again I think our two options are, in my order of preference:
- Disable the warning via a PA backend change.
- Bump malloc alignment.
I am sensitive to the first change being something that carries with it extra
maintenance burden, so I'm happy to see the second solution chosen if that's what
everyone wants (Florian's suggestion).
--
Cheers,
Carlos.
next prev parent reply other threads:[~2016-10-12 19:59 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-08 17:01 Bernd Edlinger
2016-10-08 17:36 ` John David Anglin
2016-10-09 8:35 ` Bernd Edlinger
2016-10-09 17:52 ` John David Anglin
2016-10-10 18:21 ` John David Anglin
2016-10-11 18:51 ` Jason Merrill
2016-10-11 18:59 ` DJ Delorie
2016-10-11 20:12 ` Jason Merrill
2016-10-11 20:55 ` John David Anglin
2016-10-11 20:57 ` Jakub Jelinek
2016-10-11 21:27 ` Jason Merrill
2016-10-11 20:04 ` John David Anglin
2016-10-12 7:02 ` Carlos O'Donell
2016-10-12 7:25 ` Jakub Jelinek
2016-10-12 7:52 ` Florian Weimer
2016-10-12 8:02 ` Jakub Jelinek
2016-10-12 12:13 ` John David Anglin
2016-10-12 12:33 ` Bernd Schmidt
2016-10-12 12:43 ` Richard Biener
2016-10-12 12:46 ` Bernd Schmidt
2016-10-12 15:51 ` Joseph Myers
2016-10-12 13:48 ` Jason Merrill
2016-10-12 14:17 ` John David Anglin
2016-10-12 19:59 ` Carlos O'Donell [this message]
2016-10-12 16:14 ` Jeff Law
2016-10-12 17:24 ` John David Anglin
2017-02-25 17:46 ` Gerald Pfeifer
2017-02-25 22:13 ` John David Anglin
2017-02-25 22:46 ` Gerald Pfeifer
2017-02-25 23:46 ` John David Anglin
2016-10-12 18:01 ` Florian Weimer
2016-10-12 18:13 ` John David Anglin
-- strict thread matches above, loose matches on Subject: below --
2016-10-08 16:43 Bernd Edlinger
2016-10-08 15:45 John David Anglin
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