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

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Date:   Mon, 22 Jul 2019 12:48:08 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Andy Lutomirski <luto@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vsyscall: use __iter_div_u64_rem()

On Mon, Jul 22, 2019 at 12:39 PM Jan Beulich <JBeulich@...e.com> wrote:
> On 22.07.2019 12:10, Thomas Gleixner wrote:
> > On Thu, 11 Jul 2019, Arnd Bergmann wrote:
> "q" was used in that commit exclusively for byte sized operands, simply
> because that _is_ the constraint to use in such cases. Using "r" is
> wrong on 32-bit, as it would include inaccessible byte portions of %sp,
> %bp, %si, and %di. This is how it's described in gcc sources / docs:
>
>   "Any register accessible as @code{@...{r}l}.  In 32-bit mode, @code{a},
>    @code{b}, @code{c}, and @code{d}; in 64-bit mode, any integer register."
>
> What I'm struggling with is why clang would evaluate that asm() in the
> first place when a 64-bit field (perf_ctr_virt_mask) is being accessed.

clang does the optimization and warning checking in a different order,
in this case the argument type checks for the inline assembly is done
before it eliminates the dead code.

        Arnd