linux-kernel - Re: [PATCH v3] linux/kernel.h: Fix DIV_ROUND

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Date:	Thu, 30 Aug 2012 17:35:31 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-kernel@...r.kernel.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 v3] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support
 negative operands

On Thu, 30 Aug 2012 17:10:47 -0700 Guenter Roeck <linux@...ck-us.net> wrote:

> DIV_ROUND_CLOSEST returns a bad result for dividends with different sign:
> 	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).
> 
> ...
>
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -84,8 +84,11 @@
>  )
>  #define DIV_ROUND_CLOSEST(x, divisor)(			\
>  {							\
> -	typeof(divisor) __divisor = divisor;		\
> -	(((x) + ((__divisor) / 2)) / (__divisor));	\
> +	typeof(x) __x = x;				\
> +	typeof(divisor) __d = divisor;			\
> +	((__x) < 0) == ((__d) < 0) ?			\
> +		(((__x) + ((__d) / 2)) / (__d)) :	\
> +		(((__x) - ((__d) / 2)) / (__d));	\
>  }							\
>  )

Your v2 had that sneaky little "(typeof(x))-1 >= 0" trick in it, so
half the code gets elided at compile time if `x' (why isn't this called
"dividend") has an unsigned type.

Would retaining that be of any benefit?  We do want to avoid doing the
compare-and-branch in as many cases as possible.

Also, this would be a great opportunity to document the macro's beahviour
(I do go on).  That would be a useful thing to do, given that we're now
handling the four +/+, +/-, -/+, -/- cases and the behaviour for each
case isn't terribly obvious.
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