[FRIAM] Q_rsqrt() vs 1/sqrt()

[uǝlƃ ↙↙↙]

https://en.wikipedia.org/wiki/Fast_inverse_square_root

So, why is Q_rsqrt() *slower* than 1/sqrt()?

1/sqrt() took 0.294771 s
Q_rsqrt() took 0.51579 s

-- 
↙↙↙ uǝlƃ

#include <stdio.h>
#include <math.h>
#include <time.h>
float Q_rsqrt( float number, int iterates) {
  long i=0L;
  float x2,y;
  const float threehalfs = 1.5F;
  x2 = number * 0.5F;
  y = number;
  i = * (long *) &y;
  i = 0x5f3759df - (i>>1);
  y = * (float *) &i;
  for (int j=0;j<iterates;j++)
    y = y * (threehalfs - (x2 * y * y));
  return y;
}

int main() {
  float result;
  float x;
  clock_t begin = clock();
  for (x=1 ; x<2e6; x+=0.1) result = 1/sqrt(x);
  clock_t end = clock();
  printf("1/sqrt() took %lg s\n",(double)(end-begin)/CLOCKS_PER_SEC);

  begin = clock();
  for (x=1 ; x<2e6; x+=0.1) result = Q_rsqrt(x,1);
  end = clock();

  printf("Q_rsqrt() took %lg s\n",(double)(end-begin)/CLOCKS_PER_SEC);
  //printf("(%f)^-½ = %f ≃ %f\n", x, 1/sqrt(x), Q_rsqrt(x,1));
  return 0;
}

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