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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 19 Feb 2020 10:52:16 +0100
Message-ID: <CAK8P3a0SfCwP04CJPThCuDmngGhwtejN8Px_UQpSwi=s_ww=bw@mail.gmail.com>
Subject: Re: Surprising code generated for vdso_read_begin()
To:     Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Segher Boessenkool <segher@kernel.crashing.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Andy Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
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On Wed, Feb 19, 2020 at 9:45 AM Christophe Leroy
<christophe.leroy@c-s.fr> wrote:
> Le 16/02/2020 à 19:10, Arnd Bergmann a écrit :
> > On Sat, Jan 11, 2020 at 12:33 PM Segher Boessenkool
> > <segher@kernel.crashing.org> wrote:
> >>
> >> On Fri, Jan 10, 2020 at 07:45:44AM +0100, Christophe Leroy wrote:
> >>> Le 09/01/2020 à 21:07, Segher Boessenkool a écrit :
> >>>> It looks like the compiler did loop peeling.  What GCC version is this?
> >>>> Please try current trunk (to become GCC 10), or at least GCC 9?
> >>>
> >>> It is with GCC 5.5
> >>>
> >>> https://mirrors.edge.kernel.org/pub/tools/crosstool/ doesn't have more
> >>> recent than 8.1
> >>
> >> Arnd, can you update the tools?  We are at 8.3 and 9.2 now :-)  Or is
> >> this hard and/or painful to do?
> >
> > To follow up on this older thread, I have now uploaded 6.5, 7.5, 8.3 and 9.2
> > binaries, as well as a recent 10.0 snapshot.
> >
>
> Thanks Arnd,
>
> I have built the VDSO with 9.2, I get less performant result than with
> 8.2 (same performance as with 5.5).
>
> After a quick look, I see:
> - Irrelevant NOPs to align loops and stuff, allthough -mpcu=860 should
> avoid that.
> - A stack frame is set for saving r31 in __c_kernel_clock_gettime. GCC
> 8.1 don't need that, all VDSO functions are frameless with 8.1

If you think it should be fixed in gcc, maybe try to reproduce it in
https://godbolt.org/ and open a gcc bug against that.

Also, please try the gcc-10 snapshot, which has the highest chance
of getting fixes if it shows the same issue (or worse).

     Arnd