Re: [BUG 2.6.31-rc1] HIGHMEM64G causes hang in PCI init on 32-bit x86

Linus Torvalds
2009-06-30T22:12:22Z

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.
Linus