c++ - code optimising for if-else function -

i want write partition function. output values obtained comparing values of input dx , dy. below code run slow. suggestion speedup code?

i search web, seems using lookup table or sse instructions possible. little idea in implementing them.

thank much.

inline int fastatan2(      int dy,     int dx ) {         if(dy<0) { dx *= -1; dy *= -1; }        if( dx > 0 ){          if( dy <  dx ) return (dy <= ((  17560*dx)>>16))? 0:1;         else           return (dy <= (( 244583*dx)>>16))? 2:3;      }else{          if( dy < -dx ) return (dy <= (( -17560*dx)>>16))? 0:5;         else           return (dy <= ((-244583*dx)>>16))? 4:3;       }   }  

here answer. first remove branches , implement sse. haven't check speed yet.

const int t1    =  17560; const int t2    = 244583; const int shift = 16;  int = 2*((dx ^ dy) >= 0)-1;  //check (dy,dx) opposite sign   dy    = abs(dy);  dx    = abs(dx);   int b1 = (dy < dx); int b2 = (dy > ((t1*dx) >> shift)); int b3 = (dx > ((t1*dy) >> shift));  const int table1[]={3,2,3,2,0,0,1,1}; int ori = table1[b3 + b2*2 + b1*4]; //implement bit shuffle (_mm_shuffle_epi8)   const int table2[]={0, 1, 2, 3, 4, 5,  0, 1, 2, 3, 4, 5}; ori = table2[a*ori + 6];   //implement bit shuffle (_mm_shuffle_epi8)  return ori; 

the code above fast quantization of angle shown below. there differences @ (dx==0, dy==0) , boundary cases (abs(dx)==abs(dy), @ angle 45, 135 degrees).

int exact_atan2(     int dx,     int dy ){     float angle = atan2((float)dy, (float)dx) / 3.14159265359f * 180.f;     float o = (angle + 360.f) / (180.f / 6) ;      int ori = int(o+0.5)%6;        return ori; }