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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Fri, 8 May 2020 06:07:02 -0600
Message-ID: <CAHmME9p7Hri-vHfwR9GY1gPo91jVx4-hTqsZVRHiioUENRHibA@mail.gmail.com>
Subject: Re: [PATCH] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for
 gcc >= 10
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Oleksandr Natalenko <oleksandr@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
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On Fri, May 8, 2020 at 5:56 AM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Fri, May 8, 2020 at 1:33 PM Oleksandr Natalenko <oleksandr@redhat.com> wrote:
> >
> > On Fri, May 08, 2020 at 05:21:47AM -0600, Jason A. Donenfeld wrote:
> > > > Should we untangle -O3 from depending on ARC first maybe?
> > >
> > > Oh, hah, good point. Yes, I'll do that for a v2, but will wait another
> > > day for feedback first.
> >
> > Just keep in mind that my previous attempt [1] failed because of too
> > many false positive warnings despite -O3 really uncovered a couple of
> > bugs in the codebase.
>
> I think my warning fixes were mostly picked up in the meantime, but
> if there are any remaining, they would be mixed in with random other
> fixes in my testing tree, so it's hard to know for sure.
>
> I also want to hear the feedback from the gcc developers about what
> the general recommendations are between O2 and O3, and how
> they may have changed over time. According to the gcc-10 documentation,
> the difference between -O2 and -O3 is exactly this set of options:
>
> -fgcse-after-reload
> -fipa-cp-clone
> -floop-interchange
> -floop-unroll-and-jam
> -fpeel-loops
> -fpredictive-commoning
> -fsplit-loops
> -fsplit-paths
> -ftree-loop-distribution
> -ftree-loop-vectorize
> -ftree-partial-pre
> -ftree-slp-vectorize
> -funswitch-loops
> -fvect-cost-model
> -fvect-cost-model=dynamic
> -fversion-loops-for-strides

The other significant thing -- and what prompted this patchset -- is
it looks like gcc 10 has lowered the inlining degree for -O2, and put
gcc 9's inlining parameters from -O2 into gcc-10's -O3.