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

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Date:	Sun, 9 Sep 2012 09:17:59 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
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

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)
> 
Hi Gert,

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.
Can you send me your configuration ?
Also, do you know where I can find the Ubuntu/Debian package for your compiler
version, by any chance ?

Thanks,
Guenter
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