linux-kernel - Re: [lm-sensors] [PATCH 1/2] Create a DIV_ROUND_CLOSEST macro to do division with rounding

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Date:	Tue, 11 Nov 2008 15:05:02 -0800 (PST)
From:	Trent Piepho <tpiepho@...escale.com>
To:	"Darrick J. Wong" <djwong@...ibm.com>
cc:	Jean Delvare <khali@...ux-fr.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	lm-sensors <lm-sensors@...sensors.org>
Subject: Re: [lm-sensors] [PATCH 1/2] Create a DIV_ROUND_CLOSEST macro to do
 division with	rounding

On Mon, 10 Nov 2008, Darrick J. Wong wrote:
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index fba141d..fb02266 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -48,6 +48,12 @@ extern const char linux_proc_banner[];
> #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
> #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
> #define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y))
> +#define DIV_ROUND_CLOSEST(x, divisor)(			\
> +{							\
> +	typeof(divisor) __divisor = divisor;		\
> +	(((x) + ((__divisor) / 2)) / (__divisor));	\
> +}							\
> +)

Maybe you can do away with the statement-expression extension?  I've seen
cases where it cases gcc to generate worse code.  It seems like it
shouldn't, but it does.  I know DIV_ROUND_CLOSEST (maybe DIV_ROUND_NEAR?)
uses divisor twice, but all the also divide macros do that too, so why does
this one need to be different?

Note that if divisor is a signed variable, divisor/2 generates worse code
than divisor>>1.
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