Messages in this thread | | | Subject | Re: VolanoMark regression with 2.6.27-rc1 | From | Peter Zijlstra <> | Date | Wed, 20 Aug 2008 22:56:38 +0200 |
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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.
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