Performance of tables slower than built in?

[Ola Fosheim Grøstad]

This appears to get roughly the same results as sin(x):


__gshared double[512+1] QuarterSinTab;

void init(){
     const auto res = QuarterSinTab.length-1;
     for(int i = 0; i < res; i++)
         QuarterSinTab[i] = sin(PI*(0.5*i/cast(double)res));	
	QuarterSinTab[$-1] = sin(PI*0.5);
}

auto fastQuarterLookup(double x){
     const ulong mantissa = cast(ulong)( (x - floor(x)) * 
(cast(double)(1UL<<54)) );
     const double sign = cast(double)(1 - 
cast(int)((mantissa>>52)&2));
     const ulong phase = 
(mantissa^((1UL<<53)-((mantissa>>52)&1)))&((1UL<<53) -1);
     const uint quarterphase = (phase>>43)&511;
     const double frac = 
cast(double)(phase&((1UL<<43)-1))*cast(double)(1.0/(1UL<<43));
     return sign*((1.0-frac)*QuarterSinTab[quarterphase] + 
frac*QuarterSinTab[quarterphase+1]);
}


Ola.