Method recip_est1_pn

engine/dlib/src/basis/encoder/cppspmd_math.h:356–372

}

CPPSPMD_FORCE_INLINE vfloat spmd_kernel::recip_est1_pn(const vfloat& t)
{
	//const int mag = 0x7EF312AC; // 2 NR iters, 3 is  0x7EEEEBB3
	const int mag = 0x7EF311C3;
	const float fMinThresh = .0000125f;

	vfloat s = sign(t);
	vfloat q = abs(t);

	vfloat l = spmd_ternaryf(q >= fMinThresh, q, cast_vint_to_vfloat(vint(mag)));

	vint x_l = vint(mag) - cast_vfloat_to_vint(l);

	vfloat rcp_l = cast_vint_to_vfloat(x_l);

	return rcp_l * vfnma(rcp_l, q, 2.0f) * s;
}

// https://basesandframes.files.wordpress.com/2020/04/even_faster_math_functions_green_2020.pdf
// https://github.com/hcs0/Hackers-Delight/blob/master/rsqrt.c.txt