fsqrt

• x86'da karekok alan fpu instruction... (bkz: fdiv) (bkz: fmul)
• önemli edit:
  "doh. this method (newton-rhapson) has been used for years, and the theory behind it is hundreds of years old. it's a damn shame if people begin to call it carmack's sqrt now, just because carmack has a guru's status and a cool sounding name.
  
  - mikko kauppila (whose name doesn't sound cool"
  
  --
  
  intel işlemcilerde çalıştırıldığında john carmack'in q_sqrt'undan dört kat daha yavaş olan instruction. diğer işlemcilerde pek hız farkı olmayabilir.
  
  quake 3'ün kodundan aşırdığım fonksiyonu da yazayım da tam olsun:
  
  float q_rsqrt( float number )
  {
  long i;
  float x2, y;
  const float threehalfs = 1.5f;
  
  x2 = number * 0.5f;
  y = number;
  i = * ( long * ) &y; // evil floating point bit level hacking
  i = 0x5f3759df - ( i >> 1 ); // what the fuck?
  y = * ( float * ) &i;
  y = y * ( threehalfs - ( x2 * y * y ) ); // 1st iteration
  // y = y * ( threehalfs - ( x2 * y * y ) ); // 2nd iteration, this can be removed
  
  #ifndef q3_vm
  #ifdef __linux__
  assert( !isnan(y) ); // bk010122 - fpe?
  #endif
  #endif
  return y;
  }
  
  aylar yıllar sonra gelen edit: http://www.beyond3d.com/content/articles/8/
  
  -----original message-----
  from: john carmack
  sent: 26 april 2004 19:51
  subject: re: origin of fast approximated inverse square root
  
  at 06:38 pm 4/26/2004 +0100, you wrote:
  
  >hi john,
  >
  >there's a discussion on beyond3d.com's forums about who the author of
  >the following is:
  >
  >float invsqrt (float x){
  > float xhalf = 0.5f*x;
  > int i = *(int*)&x;
  > i = 0x5f3759df - (i>>1);
  > x = *(float*)&i;
  > x = x*(1.5f - xhalf*x*x);
  > return x;
  >}
  >
  >is that something we can attribute to you? analysis shows it to be
  >extremely clever in its method and supposedly from the q3 source.
  >most people say it's your work, a few say it's michael abrash's. do
  >you know who's responsible, possibly with a history of sorts?
  
  not me, and i don't think it is michael. terje matheson perhaps?
  
  john carmack