From: Stephen Hemminger <[EMAIL PROTECTED]>
Date: Tue, 6 Mar 2007 10:29:41 -0800
> /* calculate the cubic root of x using Newton-Raphson */
> static uint32_t ncubic(uint64_t a)
> {
> uint64_t x;
>
> /* Initial estimate is based on:
> * cbrt(x) = exp(log(x) / 3)
> */
> x = 1u << (fls64(a)/3);
>
> /* Converges in 3 iterations to > 32 bits */
>
> x = (2 * x + div64_64(a, x*x)) / 3;
> x = (2 * x + div64_64(a, x*x)) / 3;
> x = (2 * x + div64_64(a, x*x)) / 3;
>
> return x;
> }
Indeed that will be the fastest variant for cpus with hw
integer division.
I did a quick sparc64 port, here is what I got:
Function clocks mean(us) max(us) std(us) total error
ocubic 529 0.35 15.16 0.66 545101
ncubic 498 0.33 12.83 0.36 576263
acbrt 427 0.28 11.04 0.33 547562
hcbrt 393 0.26 10.18 0.47 2410
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