Re: [Quesa-develop] Re: Quesa - Optimization ideas.

[Edward K. C. <ek...@lg...>]

On Feb 10, 2004, at 16:44, Seth Willits wrote:

> What about __fsqrt? Brian used it in all of his code. I'm assuming it 
> was because it was/is faster than sqrt.

I think __fsqrt is only implemented on the G5 (PPC970), am I right?  
Also, I am not sure it is supported by all development environments 
yet.  Does anyone know if CodeWarrior 9 has it?  I heard it has yet to 
be tuned for the G5, which is why I am not in a hurry to upgrade from 
8.3.

It would be great to use __fsqrt, but presumably you would then need 
two version of the Quesa library: one for the G5 and one for pre-G5s.  
For the latter, __frsqrte has been around since the PPC603.  FWIW, I 
wrote a function like this and it speeded up my code quite a bit:

float mysqrtf(float x) {
	float y;
	if(x == 0.0f) y = 0.0f;
	else {
		const float k = 1.5f;
		const float halfx = 0.5f * x;
		y = __frsqrte(x);
		y *= k - halfx * y * y;
		y *= k - halfx * y * y;  // add 2 more iterations for double
		y *= x;
	}
	return y;
}

If you need a whole batch of single-precision square roots, I would 
recommend vec_rsqrte as an alternative to __frsqrte.  Not only does 
altivec crunch four at a time, but vec_rsqrte gives you 20 bits of 
precision (compared to only 5 for __frsqrte), meaning you can generally 
get away with only one iterative refinement.

-Ted