Linus Torvalds wrote:
>
> On Tue, 30 Jun 2009, Yinghai Lu wrote:
> ?
>> +#define round_up(x, y) ({ __typeof__(x) __mask = (y)-1; \
>> + ((x)+__mask) & ~__mask; })
>> +#define round_down(x, y) ({ __typeof__(x) __mask = (y)-1; (x) & ~__mask; })
>
> Yes, except we might as well simplify it. Do it without the statement
> expressions, using just a single 'y'. Like this:
>
> #define __round_mask(x,y) ((__typeof__(x))((y)-1))
> #define round_up(x,y) (((x) | __round_mask(x,y))+1)
> #define round_down(x,y) ((x) & ~__round_mask(x,y))
>
> (Yeah, it uses 'x' twice, but the second one is for 'typeof', which
> doesn't actually cause the value to be evaluated, so it's ok).
>
> Now those 'round_xyz()' operations will always return a value of a type
> that is the same as the type of 'x' except it's gone through the normal C
> integer promotion rules (ie if 'x' is a smaller type than 'int', then it
> will be promoted to 'int').
>
> Not very well tested, but it _looks_ correct, and uses Peter's trick, and
> willlet the compiler notice that
>
> round_up(x,y)-1
>
> is the same thing as
>
> x | (y-1)
>
> which it _could_ have perhaps figured out before, but now it's way more
> obvious.
how about x = 0, y = 0x100?
YH
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