
0x7FDE623822FC16E6 : a magic constant for double float reciprocal - yan
http://www.pvk.ca/Blog/LowLevel/software-reciprocal.html
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cperciva
If you want to compute lots of reciprocals, don't mind some rounding errors,
and have reasonable-sized values, you can compute them with one reciprocal and
a few multiplications, e.g.,

    
    
      tmp = 1 / (a * b)
      oneovera = tmp * b
      oneoverb = tmp * a

