From: Richard Biener <rguenther@suse.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
the arch/x86 maintainers <x86@kernel.org>,
stable <stable@vger.kernel.org>, "H.J. Lu" <hjl.tools@gmail.com>,
Peter Zijlstra <peterz@infradead.org>,
Jakub Jelinek <jakub@redhat.com>,
Oleksandr Natalenko <oleksandr@redhat.com>,
Arnd Bergmann <arnd@arndb.de>,
Andrew Morton <akpm@linux-foundation.org>,
David Laight <David.Laight@aculab.com>,
Masahiro Yamada <yamada.masahiro@socionext.com>
Subject: Re: [PATCH v2] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for gcc >= 10
Date: Tue, 12 May 2020 10:44:29 +0200 (CEST) [thread overview]
Message-ID: <nycvar.YFH.7.76.2005121037491.4397@zhemvz.fhfr.qr> (raw)
In-Reply-To: <CAHk-=wi87j=wj0ijkYZ3WoPVkZ9Fq1U2bLnQ66nk425B5kW0Cw@mail.gmail.com>
[-- Attachment #1: Type: text/plain, Size: 2651 bytes --]
On Mon, 11 May 2020, Linus Torvalds wrote:
> On Mon, May 11, 2020 at 2:57 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >
> > GCC 10 appears to have changed -O2 in order to make compilation time
> > faster when using -flto, seemingly at the expense of performance, in
> > particular with regards to how the inliner works. Since -O3 these days
> > shouldn't have the same set of bugs as 10 years ago, this commit
> > defaults new kernel compiles to -O3 when using gcc >= 10.
>
> I'm not convinced this is sensible.
Note the real thing that changed for GCC 10 at -O2 is that -O2
now includes -finline-functions which means GCC considers inlining
of functions not marked with 'inline' at -O2. To counter code-size
growth and tune that back to previous levels the inlining limits
in effect at -O2 have been lowered.
Note this has been done based on analyzing larger C++ code and obviously
not because the kernel would benefit (IIRC kernel folks like 'inline'
to behave as written and thus rather may dislike the change to default to
-finline-functions).
> -O3 historically does bad things with gcc. Including bad things for
> performance. It traditionally makes code larger and often SLOWER.
>
> And I don't mean slower to compile (although that's an issue). I mean
> actually generating slower code.
>
> Things like trying to unroll loops etc makes very little sense in the
> kernel, where we very seldom have high loop counts for pretty much
> anything.
>
> There's a reason -O3 isn't even offered as an option.
And I think that's completely sensible. I would not recommend
to use -O3 for the kernel. Somehow feeding back profile data
might help - though getting such data at all and with enough
coverage is probably hard.
As you said in the followup I wouldn't recommend tweaking GCCs
defaults for the various --param affecting inlining. The behavior
with this is not consistent across releases.
Richard.
> Maybe things have changed, and maybe they've improved. But I'd like to
> see actual numbers for something like this.
>
> Not inlining as aggressively is not necessarily a bad thing. It can
> be, of course. But I've actually also done gcc bugreports about gcc
> inlining too much, and generating _worse_ code as a result (ie
> inlinging things that were behind an "if (unlikely())" test, and
> causing the likely path to grow a stack fram and stack spills as a
> result).
>
> So just "O3 inlines more" is not a valid argument.
>
> Linus
>
--
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
next prev parent reply other threads:[~2020-05-12 8:44 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-07 22:45 [PATCH] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for gcc >= 10 Jason A. Donenfeld
2020-05-08 8:35 ` Peter Zijlstra
2020-05-08 9:02 ` Oleksandr Natalenko
2020-05-08 11:21 ` Jason A. Donenfeld
2020-05-08 11:33 ` Oleksandr Natalenko
2020-05-08 11:49 ` Arnd Bergmann
2020-05-08 12:07 ` Jason A. Donenfeld
2020-05-08 13:04 ` Arnd Bergmann
2020-05-08 15:06 ` Joe Perches
2020-05-08 15:09 ` Arnd Bergmann
2020-05-10 12:47 ` David Laight
2020-05-10 17:45 ` Joe Perches
2020-05-10 18:58 ` David Laight
2020-05-12 1:10 ` Masahiro Yamada
2020-05-11 21:57 ` [PATCH v2] " Jason A. Donenfeld
2020-05-12 0:04 ` Linus Torvalds
2020-05-12 0:09 ` Linus Torvalds
2020-05-12 0:43 ` Jason A. Donenfeld
2020-05-12 8:44 ` Richard Biener [this message]
2020-05-13 11:27 ` [PATCH] " Artem S. Tashkinov
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