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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

  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] " 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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