linux-kernel - Re: [PATCH] linux/kernel.h: Fix warning seen with W=1 due to change in DIV_ROUND

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Date:	Wed, 19 Sep 2012 05:33:06 -0300
From:	Mauro Carvalho Chehab <mchehab@...hat.com>
To:	Guenter Roeck <linux@...ck-us.net>
CC:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jean Delvare <khali@...ux-fr.org>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	lm-sensors@...sensors.org
Subject: Re: [PATCH] linux/kernel.h: Fix warning seen with W=1 due to change
 in DIV_ROUND_CLOSEST

Em 19-09-2012 01:00, Guenter Roeck escreveu:
> After commit b6d86d3d (Fix DIV_ROUND_CLOSEST to support negative dividends),
> the following warning is seen if the kernel is compiled with W=1 (-Wextra):
> 
> warning: comparison of unsigned expression >= 0 is always true
> 
> The warning is due to the test '((typeof(x))-1) >= 0', which is used to detect
> if the variable type is unsigned. Research on the web suggests that the warning
> disappears if '>' instead of '>=' is used for the comparison.
> 
> Tests after changing the macro along that line show that the warning is gone,
> and that the result is still correct:
> 
> i=-4: DIV_ROUND_CLOSEST(i, 2)=-2
> i=-3: DIV_ROUND_CLOSEST(i, 2)=-2
> i=-2: DIV_ROUND_CLOSEST(i, 2)=-1
> i=-1: DIV_ROUND_CLOSEST(i, 2)=-1
> i=0: DIV_ROUND_CLOSEST(i, 2)=0
> i=1: DIV_ROUND_CLOSEST(i, 2)=1
> i=2: DIV_ROUND_CLOSEST(i, 2)=1
> i=3: DIV_ROUND_CLOSEST(i, 2)=2
> i=4: DIV_ROUND_CLOSEST(i, 2)=2
> 
> Code size is the same as before.

It is great to have this warning disappearing.

Tested-by: Mauro Carvalho Chehab <mchehab@...hat.com>
> 
> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> ---
>  include/linux/kernel.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 594b419..2451f1f 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -91,7 +91,7 @@
>  {							\
>  	typeof(x) __x = x;				\
>  	typeof(divisor) __d = divisor;			\
> -	(((typeof(x))-1) >= 0 || (__x) >= 0) ?		\
> +	(((typeof(x))-1) > 0 || (__x) > 0) ?		\
>  		(((__x) + ((__d) / 2)) / (__d)) :	\
>  		(((__x) - ((__d) / 2)) / (__d));	\
>  }							\
> 

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