Re: [PATCH] linux/kernel.h: Fix warning seen with W=1 due to changein DIV_ROUND_CLOSEST

[Jean Delvare]

On Tue, 18 Sep 2012 21:00:09 -0700, Guenter Roeck wrote:
> 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.
> 
> Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> ---
>  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));	\
>  }							\

Very nice. Let's push this upstream ASAP.

Acked-by: Jean Delvare <khali@xxxxxxxxxxxx>

-- 
Jean Delvare
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