On Tue, 11 Nov 2008, Andrew Morton wrote:
> On Tue, 11 Nov 2008 15:05:02 -0800 (PST)
> Trent Piepho <tpiepho at freescale.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
