Re: VolanoMark regression with 2.6.27-rc1

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2008-08-20 at 22:30 +0200, Peter Zijlstra wrote:
> On Wed, 2008-08-20 at 11:15 -0700, Ray Lee wrote:
> > On Wed, Aug 20, 2008 at 10:55 AM, Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> > > On Thursday 21 August 2008 03:21, Peter Zijlstra wrote:
> > >> Ok, so one last time (I hope!)..
> > >>
> > >> Everybody happy with this?
> > >
> > >
> > >> Index: linux-2.6/include/linux/kernel.h
> > >> ===================================================================
> > >> --- linux-2.6.orig/include/linux/kernel.h
> > >> +++ linux-2.6/include/linux/kernel.h
> > >> @@ -367,6 +367,12 @@ static inline char *pack_hex_byte(char *
> > >>       (void) (&_max1 == &_max2);              \
> > >>       _max1 > _max2 ? _max1 : _max2; })
> > >>
> > >> +#define avg(x, y) ({                         \
> > >> +     typeof(x) _avg1 = (x);                  \
> > >> +     typeof(y) _avg2 = (y);                  \
> > >> +     (void) (&_avg1 == &_avg2);              \
> > >> +     _avg1 + (_avg2 - _avg1)/2; })
> > >
> > > That's not going to work with unsigned types.
> > 
> > Uhm, I think it works fine, even with unsigned, even where _avg2 is
> > smaller than _avg1. Underflow is a good thing here. And I mocked up a
> > little test harness and it gives the correct answers for a half dozen
> > sets of values I tossed at it
> > 
> > But maybe I'm forgetting an obscure unsigned or signed int type
> > widening rule, so, care to elaborate?
> 
> Nick is right, try:
> 
> int main(int argc, char **argv)
> {
>         unsigned int x = 7, y = 5;
>         printf("%d\n", avg(x,y));
>         return 0;
> }
> 
> It fails because 5-7 = -2, which needs a signed division or sign
> extending right shift.
> 
> we'd need something like:
> 
> #define avg(x, y) ({                            \
>         typeof(x) _avg1 = (x);                  \          
>         typeof(y) _avg2 = (y);                  \
>         (void) (&_avg1 == &_avg2);              \
>         _avg1 + (signed typeof(x))(_avg2 - _avg1)/2; })
> 
> except that typeof() doesn't work that way.
> 
> #define avg(x, y) ({                            \
>         typeof(x) _avg1 = (x);                  \          
>         typeof(y) _avg2 = (y);                  \
>         (void) (&_avg1 == &_avg2);              \
>         _avg1 + (long)(_avg2 - _avg1)/2; })
> 
> works for the above example, but when I make it long long, so as to
> match the longest supported type, it goes boom again - for as of yet
> unknown reasons.

Ok, people pointed out I got my promotion rules mixed up, I casted the
result of the division to signed, instead of ending up with a signed
division.

#define avg(x, y) ({                            \
        typeof(x) _avg1 = (x);                  \
        typeof(y) _avg2 = (y);                  \
        (void) (&_avg1 == &_avg2);              \
        (typeof(x))(_avg1 + ((long long)_avg2 - _avg1)/2); })

seems to work.