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Subject: Re: Surprising code generated for vdso_read_begin()
To:     Arnd Bergmann <arnd@arndb.de>,
        Segher Boessenkool <segher@kernel.crashing.org>
Cc:     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>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>
References: <cover.1577111363.git.christophe.leroy@c-s.fr>
 <bd4557a7-9715-59aa-5d8e-488c5e516a98@c-s.fr>
 <20200109200733.GS3191@gate.crashing.org>
 <77a8bf25-6615-6c0a-56d4-eae7aa8a8f09@c-s.fr>
 <20200111113328.GX3191@gate.crashing.org>
 <CAK8P3a11wX1zJ+TAacDTkYsrzvfdVmNrcB6OC23aFvCxF57opQ@mail.gmail.com>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <305fcee5-2e1b-ea4d-9a2a-a0e8034d40a8@c-s.fr>
Date:   Wed, 19 Feb 2020 09:45:45 +0100
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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

Christophe