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

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Message-ID: <20101119222823.GA28797@quadria>
Date:	Fri, 19 Nov 2010 23:28:23 +0100
From:	Michal Januszewski <michalj@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Rolf Eike Beer <eike-kernel@...tec.de>,
	linux-kernel@...r.kernel.org,
	Linux Fbdev development list <linux-fbdev@...r.kernel.org>
Subject: Re: abs() vs. abs64() (was: Re: [PATCH] fbdev: fix nearest mode
 search)

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@...ux-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@...il.com>
> Cc: Rolf Eike Beer <eike-kernel@...tec.de
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Signed-off-by: Andrew Morton <akpm@...ux-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
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