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Re: RFC: x86: Emit rsqrt with only -ffast-math

On 11/05/2009 02:57 PM, Michael Matz wrote: 
>
>     float sqrt(float r) {
>       float q = rsqrt(r);   // approx. 1.0 / sqrt (r);
>       float y = q * r;      // approx sqrt (r)
>       float e = y * y - r;  // y == sqrt (r + e), with e small
>       return y - e / (2.0 * y); // Newton Raphson correction
>     }

That itself has a division in it, and hence wouldn't be better than just
using the normal sqrt (no recriprocal) instruction.

Yes, that would be

return y - e * q * 0.5;

Paolo

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