Messages in this thread |  | | Date | Thu, 22 Aug 2002 10:21:57 +0000 | From | Gabriel Paubert <> | Subject | Re: [PATCH]: fix 32bits integer overflow in loops_per_jiffy calculation |
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Yoann Vandoorselaere wrote: > Hi, > > The "low_part * mult" multiplication of the old function may overflow a > 32bits integer... > > This patch both fix the overflow issue (tested with frequencies up to > 20Ghz), and make the result of the function lose less precision. > > Please apply, > > > > ------------------------------------------------------------------------ > > --- linux-benh/kernel/cpufreq.c 2002-08-21 17:27:52.000000000 +0200 > +++ linux-yoann/kernel/cpufreq.c 2002-08-22 11:27:09.000000000 +0200 > @@ -78,14 +78,16 @@ static unsigned int cpufreq_ > */ > static unsigned long scale(unsigned long old, u_int div, u_int mult) > { > - unsigned long low_part, high_part; > - > - high_part = old / div; > - low_part = (old % div) / 100; > - high_part *= mult; > - low_part = low_part * mult / div; > - > - return high_part + low_part * 100; > + unsigned long val, carry = 0; > + > + mult /= 100; > + div /= 100;
if(abs(div)<100) div=0;
> + val = old / div * mult;
Now happily divide by zero.
> + > + carry = old % div;
Again.
> + carry = carry * mult / div;
Again.
> + > + return val + carry; > } >
And I can't see how it can be more precise, you divide the numerator and denominator of the fraction by 100 and then proceed forgetting everything about the rest. Basically this looses about 7 bits of precision.
Now altogether I believe that such a function pertains to a per arch optimized routine.
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