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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 27 May 2020 14:50:20 +0200
Message-ID: <CAK8P3a3i0kPf8dRg7Ko-33hsb+LkP=P05uz2tGvg5B43O-hFvg@mail.gmail.com>
Subject: Re: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar_typeof
 compilation time
To:     Sedat Dilek <sedat.dilek@gmail.com>
Cc:     Marco Elver <elver@google.com>, Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>
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On Wed, May 27, 2020 at 2:35 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> On Wed, May 27, 2020 at 2:31 PM Arnd Bergmann <arnd@arndb.de> wrote:
> > On Wed, May 27, 2020 at 1:36 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> > > On Wed, May 27, 2020 at 1:27 PM Arnd Bergmann <arnd@arndb.de> wrote:
> > > > On Wed, May 27, 2020 at 12:33 PM Marco Elver <elver@google.com> wrote:
> > > >
> > > > This gives us back 80% of the performance drop on clang, and 50%
> > > > of the drop I saw with gcc, compared to current mainline.
> > > >
> > > > Tested-by: Arnd Bergmann <arnd@arndb.de>
> > > >
> > >
> > > Hi Arnd,
> > >
> > > with "mainline" you mean Linux-next aka Linux v5.8 - not v5.7?
> >
> > I meant v5.7.
> >
> > > I have not seen __unqual_scalar_typeof(x) in compiler_types.h in Linux v5.7.
> > >
> > > Is there a speedup benefit also for Linux v5.7?
> > > Which patches do I need?
> >
> > v5.7-rc is the baseline and is the fastest I currently see. On certain files,
> > I saw an intermittent 10x slowdown that was already fixed earlier, now
> > linux-next
> > is more like 2x slowdown for me and 1.2x with this patch on top, so we're
> > almost back to the speed of linux-5.7.
> >
>
> Which clang version did you use - and have you set KCSAN kconfigs -
> AFAICS this needs clang-11?

I'm currently using clang-11, but I see the same problem with older
versions, and both with and without KCSAN enabled. I think the issue
is mostly the deep nesting of macros that leads to code bloat.

        Arnd