linux-kernel - Re: [PATCH] vsyscall: use __iter_div_u64

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Message-ID: <CAK8P3a3TfQnQsXzmktedsKaY7Tihue_PbLhLoD1z1jZBMTxWMg@mail.gmail.com>
Date:   Thu, 11 Jul 2019 22:55:16 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Vincenzo Frascino <vincenzo.frascino@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-mips@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Huw Davies <huw@...eweavers.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] vsyscall: use __iter_div_u64_rem()

On Thu, Jul 11, 2019 at 7:14 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.com> wrote:
>
> On Thu, Jul 11, 2019 at 5:28 AM Arnd Bergmann <arnd@...db.de> wrote:
> > clang does not like an inline assembly with a "=q" contraint for
> > a 64-bit output:
>
> Seems like starting in GCC 7, GCC may not like it either:
> https://godbolt.org/z/UyBUfh
> it simply warns then proceeds with code gen.  Another difference may
> come from when GCC vs Clang perform dead code elimination (DCE) vs
> semantic analysis.

Right, I also had the idea to work around it with a set of
__builtin_choos_expr()
instead of the switch()/case but did not complete that patch as the percpu
code is rather complex and this would touch lots of code.

       Arnd