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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 8 May 2020 13:49:44 +0200
Message-ID: <CAK8P3a0dJ0vNnktcoWFiPKB4NJbeyf7nvwWf0YLyeUyxT5pvQg@mail.gmail.com>
Subject: Re: [PATCH] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for
 gcc >= 10
To:     Oleksandr Natalenko <oleksandr@redhat.com>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.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 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

It's a relatively short list, so someone familiar with the options could
perhaps look into whether we want to change the default for all
of them, or if it makes sense to be more selective.

Personally, I'm more interested in improving compile speed of the kernel
and eventually supporting -Og or some variant of it for my own build
testing, but of course I also want to make sure that the other optimization
levels do not produce warnings, and -Og leads to more problems than
-O3 at the moment.

       Arnd