On Wed, 21 Mar 2007 20:15:37 +0100
Willy Tarreau <[EMAIL PROTECTED]> wrote:
> Hi Stephen,
>
> On Wed, Mar 21, 2007 at 11:54:19AM -0700, Stephen Hemminger wrote:
> > On Tue, 13 Mar 2007 21:50:20 +0100
> > Willy Tarreau <[EMAIL PROTECTED]> wrote:
>
> [...] ( cut my boring part )
>
> > > Here are the results classed by speed :
> > >
> > > /* Sample output on a Pentium-M 600 MHz :
> > >
> > > Function clocks mean(us) max(us) std(us) Avg err size
> > > ncubic_tab0 79 0.66 7.20 1.04 0.613% 160
> > > ncubic_0div 84 0.70 7.64 1.57 4.521% 192
> > > ncubic_1div 178 1.48 16.27 1.81 0.443% 336
> > > ncubic_tab1 179 1.49 16.34 1.85 0.195% 320
> > > ncubic_ndiv3 263 2.18 24.04 3.59 0.250% 512
> > > ncubic_2div 270 2.24 24.70 2.77 0.187% 512
> > > ncubic32_1 359 2.98 32.81 3.59 0.238% 544
> > > ncubic_3div 361 2.99 33.08 3.79 0.170% 656
> > > ncubic32 364 3.02 33.29 3.51 0.247% 544
> > > ncubic 529 4.39 48.39 4.92 0.247% 720
> > > hcbrt 539 4.47 49.25 5.98 1.580% 96
> > > ocubic 732 4.93 61.83 7.22 0.274% 320
> > > acbrt 842 6.98 76.73 8.55 0.275% 192
> > > bictcp 1032 6.95 86.30 9.04 0.172% 768
> > >
>
> [...]
>
> > The following version of div64_64 is faster because do_div already
> > optimized for the 32 bit case..
>
/* 64bit divisor, dividend and result. dynamic precision */
static uint64_t div64_64(uint64_t dividend, uint64_t divisor)
{
uint32_t high, d;
high = divisor >> 32;
if (high) {
unsigned int shift = fls(high);
d = divisor >> shift;
dividend >>= shift;
} else
d = divisor;
do_div(dividend, d);
return dividend;
}
> Cool, this is interesting because I first wanted to optimize it but did
> not find how to start with this. You seem to get very good results. BTW,
> you did not append your changes.
>
> However, one thing I do not understand is why your avg error is about 1/3
> below the original one. Was there a precision bug in the original div_64_64
> or did you extend the values used in the test ?
>
> Or perhaps you used -fast-math to build and the original cbrt() is less
> precise in this case ?
No, but I did use -mtune=pentiumm on the ULV
--
Stephen Hemminger <[EMAIL PROTECTED]>
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