Re: abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode search)

Michal Januszewski
2010-11-19T22:28:33Z

On Fri, Nov 19, 2010 at 02:07:21PM -0800, Andrew Morton wrote:
> On Thu, 18 Nov 2010 07:40:19 +0100
> Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> > > 'c' will end up with a value of 0xffffffff instead of the expected 0x1.
> >
> > This happens on 64-bit only, right?

Absolutely, I should have mentioned it in the patch description.

> How does this look?
>
> [..]
>
> Reported-by: Michal Januszewski <michalj@gmail.com>
> Cc: Rolf Eike Beer <eike-kernel@sf-tec.de
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
> include/linux/kernel.h | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff -puN include/linux/kernel.h~include-linux-kernelh-abs-fix-handling-of-32-bit-unsigneds-on-64-bit include/linux/kernel.h
> --- a/include/linux/kernel.h~include-linux-kernelh-abs-fix-handling-of-32-bit-unsigneds-on-64-bit
> +++ a/include/linux/kernel.h
> @@ -143,9 +143,16 @@ extern int _cond_resched(void);
>
> #define might_sleep_if(cond) do { if (cond) might_sleep(); } while (0)
>
> -#define abs(x) ({ \
> - long __x = (x); \
> - (__x < 0) ? -__x : __x; \
> +#define abs(x) ({ \
> + long ret; \
> + if (sizeof(x) == sizeof(long)) { \
> + long __x = (x); \
> + ret = (__x < 0) ? -__x : __x; \
> + } else { \
> + int __x = (x); \
> + ret = (__x < 0) ? -__x : __x; \
> + } \
> + ret; \
> })
>
> #define abs64(x) ({ \
> _

Looks good to me. I posted essentially the same thing some 3 months ago
(http://marc.info/?l=linux-kernel&m=128033094822201&w=2) but it then
failed to get any traction. At any rate, I like your version better as
it seems more readable.

Michal