From: Jakub Jelinek <jakub@redhat.com>
To: "Carlos O'Donell" <carlos@redhat.com>
Cc: John David Anglin <dave.anglin@bell.net>,
Jason Merrill <jason@redhat.com>,
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 07:25:00 -0000 [thread overview]
Message-ID: <20161012072540.GJ7282@tucnak.redhat.com> (raw)
In-Reply-To: <88e889ee-dec9-5fef-db19-02f4d91d9156@redhat.com>
On Wed, Oct 12, 2016 at 03:01:51AM -0400, Carlos O'Donell wrote:
> On 10/11/2016 04:04 PM, John David Anglin wrote:
> > On 2016-10-11 2:50 PM, Jason Merrill wrote:
> >> /* Alignment, in bits, a C conformant malloc implementation has to
> >> provide.
> >> The HP-UX malloc implementation provides a default alignment of 8
> >> bytes.
> >> This can be increased with mallopt. The glibc implementation also
> >> provides
> >> 8-byte alignment. Note that this isn't enough for various POSIX
> >> types such
> >> as pthread_mutex_t. However, since we no longer need the 16-byte
> >> alignment
> >> for atomic operations, we ignore the nominal alignment specified
> >> for these
> >> types. The same is true for long double on 64-bit HP-UX. */
> >>
> >> If PA malloc doesn't actually provide 16-byte alignment, this change
> >> seems problematic; it will mean any type that wants 16-byte alignment
> >> will silently get 8-byte alignment instead.
> >
> > I agree the situation is something of a mess. On linux, we could bump the alignment
> > of malloc to 16-bytes. However, Carlos argued that we don't need to and I think doing
> > so would be detrimental to performance.
>
> Correct, we do not need a 16-byte alignment at runtime.
The problem with cheating is that gcc will then assume the structure is
properly aligned and optimize based on that (optimize away alignment checks
etc.).
>
> > The 16-byte alignment was used originally because the ldcw instruction used for atomic
> > operations in linux threads needs 16-byte alignment. However, the nptl pthread
> > implementation now uses a kernel helper for atomic operations. It only needs
> > 4-byte alignment. The largest alignment actually needed is for long double (8 bytes).
> > However, we can't change the 16-byte alignment without affecting the layout of various
> > structures.
>
> Correct, the structure padding needs to continue to be there to match the original ABI.
No, you can just drop the aligned attributes for HPUX 32-bit, basically
introduce a new ABI. If needed, you could add new symbol versions for
pthread_mutex_* etc. (though, if the current code doesn't care about the
alignment, perhaps you could get away without bumping that).
> I think the case where a user specifically requests a larger aligment is still always
> bound to fail if they exceed malloc's aligment. So removing the warning just leaves
If users use posix_memalign/memalign/aligned_alloc or the C++17 aligned new,
they should be fine.
Jakub
next prev parent reply other threads:[~2016-10-12 7:25 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 [this message]
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
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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