From: Linus Torvalds <torvalds@linux-foundation.org>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: 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: Mon, 11 May 2020 17:09:20 -0700 [thread overview]
Message-ID: <CAHk-=wioWmE+Xy+RfVpc3q9EMh4NhqPVvWZp5=GqtoU6nZfxcA@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wi87j=wj0ijkYZ3WoPVkZ9Fq1U2bLnQ66nk425B5kW0Cw@mail.gmail.com>
On Mon, May 11, 2020 at 5:04 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> 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).
In case people care, the bugzilla case I mentioned is this one:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49194
with example code on why it's actively wrong to inline.
Obviously, in the kernel, we can fix the obvious cases with "noinline"
and "always_inline", but those take care of the outliers. Having a
compiler that does reasonably well by default is a good thing, and
that very much includes *not* inlining mindlessly.
Linus
next prev parent reply other threads:[~2020-05-12 0:09 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 [this message]
2020-05-12 0:43 ` Jason A. Donenfeld
2020-05-12 8:44 ` Richard Biener
2020-05-13 11:27 ` [PATCH] " Artem S. Tashkinov
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