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Date:   Tue, 12 May 2020 10:44:29 +0200 (CEST)
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>,
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        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
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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)
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