From: "Jason A. Donenfeld" <Jason@zx2c4.com> To: Linus Torvalds <email@example.com> Cc: Linux Kernel Mailing List <firstname.lastname@example.org>, Linux Kbuild mailing list <email@example.com>, "the arch/x86 maintainers" <firstname.lastname@example.org>, stable <email@example.com>, "H.J. Lu" <firstname.lastname@example.org>, Peter Zijlstra <email@example.com>, Jakub Jelinek <firstname.lastname@example.org>, Oleksandr Natalenko <email@example.com>, Arnd Bergmann <firstname.lastname@example.org>, Andrew Morton <email@example.com>, David Laight <David.Laight@aculab.com>, Masahiro Yamada <firstname.lastname@example.org> Subject: Re: [PATCH v2] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for gcc >= 10 Date: Mon, 11 May 2020 18:43:48 -0600 [thread overview] Message-ID: <CAHmME9pXnYurszyvX8FAYHbMysknpegUSF1g2wZPdBybxD-xZQ@mail.gmail.com> (raw) In-Reply-To: <CAHk-=wi87j=wj0ijkYZ3WoPVkZ9Fq1U2bLnQ66nk425B5kW0Cw@mail.gmail.com> On Mon, May 11, 2020 at 6:05 PM Linus Torvalds <email@example.com> wrote: > There's a reason -O3 isn't even offered as an option. > > 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. Alright. It might be possible to produce some benchmarks, and then isolate the precise inlining parameter that makes the difference, and include that for gcc-10. But you made a compelling argument in that old gcc bug report about not going down the finicky rabbit hole of gcc inlining switches that seem to change meaning between releases, which is persuasive. The other possibility would be if -O3 actually isn't bad like it used to be and the codegen is markedly better, alongside some numbers to back it up. I'm not presently making that argument and don't have those numbers, but perhaps others who were interested in this patch for other reasons do have strong arguments there and want to chime in. Otherwise, no problem dropping this.
next prev parent reply other threads:[~2020-05-12 0:44 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 2020-05-12 0:43 ` Jason A. Donenfeld [this message] 2020-05-12 8:44 ` Richard Biener 2020-05-13 11:27 ` [PATCH] " Artem S. Tashkinov
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