linux-kernel - Re: VolanoMark regression with 2.6.27-rc1

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Date:	Wed, 20 Aug 2008 22:30:36 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ray Lee <ray-lk@...rabbit.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>, adobriyan@...il.com,
	Ingo Molnar <mingo@...e.hu>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Aneesh Kumar KV <aneesh.kumar@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ibm.com>,
	Chris Friesen <cfriesen@...tel.com>
Subject: Re: VolanoMark regression with 2.6.27-rc1

On Wed, 2008-08-20 at 11:15 -0700, Ray Lee wrote:
> On Wed, Aug 20, 2008 at 10:55 AM, Nick Piggin <nickpiggin@...oo.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.

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