[vorbis-dev] Another good optimization (for PPC only, though)

[Segher Boessenkool]

> // Vanilla PPC code, but since PPC has a reciprocal square root estimate instruction,
> // runs *much* faster than calling sqrt().  We'll use two Newton-Raphson
> // refinement steps to get bunch more precision in the 1/sqrt() value for very little cost.
> // We'll then invert using the PPC reciprocal estimate instruction and also refine that
> // (although we should only need a single refinement step there).

Nonononono! If you got y = 1/sqrt(x), then "y *= x;" will give you y = sqrt(x),
easier, cheaper & better :-)

>     if (x == 0.0)

There are separate +0.0 and -0.0, so you better watch out.

>     /* Second refinement step */
>     y0 = y1;
>     y1 = y0 + half*y0*(one - B*y0*y0);

Why the copying?

Ciao,

Segher

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