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 15:42:09 -0800 (PST)
From:	Trent Piepho <tpiepho@...escale.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	djwong@...ibm.com, Jean Delvare <khali@...ux-fr.org>,
	linux-kernel@...r.kernel.org, lm-sensors@...sensors.org
Subject: Re: [lm-sensors] [PATCH 1/2] Create a DIV_ROUND_CLOSEST macro to do
 division with rounding

On Tue, 11 Nov 2008, Andrew Morton wrote:
> On Tue, 11 Nov 2008 15:05:02 -0800 (PST)
> Trent Piepho <tpiepho@...escale.com> wrote:
>> On Mon, 10 Nov 2008, Darrick J. Wong wrote:
>>> #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?
>
> The others need fixing too.

Is it worth generating worse code for these simple macros?

>> Note that if divisor is a signed variable, divisor/2 generates worse code
>> than divisor>>1.
>
> yup.  I wonder why the compiler doesn't do that for itself - is there a
> case where it will generate a different result?

main()
{
     int x = -5;
     printf("%d %d\n", x>>1, x/2);
}
$ a.out
-3 -2
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