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

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Message-Id: <1219266243.8651.86.camel@twins>
Date:	Wed, 20 Aug 2008 23:04:03 +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 13:58 -0700, Ray Lee wrote:
> On Wed, Aug 20, 2008 at 1:30 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > 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.
> 
> I think you'd want to cast it with a (signed) instead? as in:
> 
> #include <stdio.h>
> 
> #define avg(x, y) ({            \
>        typeof(x) _x = (x);      \
>        typeof(y) _y = (y);      \
>        (void) (&_x == &_y);     \
>        _x + (signed)(_y - _x)/2; })

signed is short for signed int, which is too short for say long or long
long input.

Anyway, see my previous mail in which I explained that I got the cast
order wrong.

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