Fast sin function - IEEE754 only - double precision FPU mode only - (sizeof(double)/2) == sizeof(int) only - unreadable version :D

gistfile1.c

inline double fast_sin(register double x) {
    register union {
        double d;
        int i;
    } y;
    
    register int k;
    register double z;
    
    y.d = x;
    y.d *= 0.31830988618379067153776752674502872;
    y.d += 6755399441055744.0;
    k = y.i;
    z = k;
    z *= 3.14159265358979323846264338327950288;
    x -= z;
    z = x;
    z *= x;
    y.d = 0.00735246819687011731341356165096815;
    y.d *= z;
    y.d -= 0.16528911397014738207016302002888890;
    y.d *= z;
    y.d += 0.99969198629596757779830113868360584;
    x *= y.d;
    k &= 1;
    k *= 2;
    z = k;
    z *= x;
    x -= z;
    
    return x;
}