linux-kernel - Re: [PATCH v4] linux/kernel.h: Fix DIV_ROUND

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Message-ID: <CAMuHMdUirbD8MjyZJfuHv6ht0iK5u2WJ3m0WanXsFQFWgcoPJw@mail.gmail.com>
Date:	Sun, 9 Sep 2012 22:38:41 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jean Delvare <khali@...ux-fr.org>, lm-sensors@...sensors.org
Subject: Re: [PATCH v4] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support
 negative dividends

Hi Günter,

On Sun, Sep 9, 2012 at 6:17 PM, Guenter Roeck <linux@...ck-us.net> wrote:
> On Sun, Sep 09, 2012 at 04:57:34PM +0200, Geert Uytterhoeven wrote:
>> On Fri, Aug 31, 2012 at 5:02 PM, Guenter Roeck <linux@...ck-us.net> wrote:
>> > DIV_ROUND_CLOSEST returns a bad result for negative dividends:
>> >         DIV_ROUND_CLOSEST(-2, 2) = 0
>> >
>> > Most of the time this does not matter. However, in the hardware monitoring
>> > subsystem, DIV_ROUND_CLOSEST is sometimes used on integers which can be
>> > negative (such as temperatures).
>>
>> This seems to cause 3 new warnings in my m68k builds of v3.6-rc5:
>>
>> drivers/hwmon/mcp3021.c:76: warning: comparison is always true due to
>> limited range of data type
>> drivers/ide/ide-cd.c:1158: warning: comparison is always true due to
>> limited range of data type
>> drivers/ide/ide-cd.c:1159: warning: comparison is always true due to
>> limited range of data type
>>
>> $ m68k-linux-gnu-gcc -v
>> Using built-in specs.
>> Target: m68k-linux-gnu
>> Configured with: ../src/configure -v --enable-languages=c,c++
>> --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib
>> --without-included-gettext --enable-threads=posix --enable-nls
>> --with-gxx-include-dir=/usr/m68k-linux-gnu/include/c++/4.1.2
>> --program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu
>> --enable-libstdcxx-debug --disable-libssp --disable-werror
>> --enable-checking=release --program-prefix=m68k-linux-gnu-
>> --includedir=/usr/m68k-linux-gnu/include --build=x86_64-linux-gnu
>> --host=x86_64-linux-gnu --target=m68k-linux-gnu
>> Thread model: posix
>> gcc version 4.1.2 20061115 (prerelease) (Ubuntu 4.1.1-21)
>>
> The "always true" condition is on purpose to cause the compiler to optimize the
> expression away. I don't see the warnings myself, on none of the targets I am
> building (x86, ppc, arm, mips), but then I am using much more recent compiler
> versions. Odd is that you only see the warnings on "u16" and not elsewhere.
>
> I don't immediately see the warnings with a more recent m68k compiler (4.6.3),
> nor with any other target (x86, ppc, arm, mips) in my nightly builds.

I also don't see it in any of the linux-next builds of v3.6-rc5. So
probably it's just
a gcc 4.1.2 issue...

> Can you send me your configuration ?

The ide-cd one shows up in any config that enables ide-cd, incl. defconfig and
allmodconfig.
The mcp3021 one I only see in allmodconfig.

> Also, do you know where I can find the Ubuntu/Debian package for your compiler
> version, by any chance ?

It's not from an official binary package. I built the cross-compiler
from the then
current gcc 4.1.1-21 sources in Ubuntu.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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