How to speed up floating-point to integer number conversion? [duplicate]

Question

We're doing a great deal of floating-point to integer number conversions in our project. Basically, something like this

for(int i = 0; i < HUGE_NUMBER; i++)
     int_array[i] = float_array[i];

The default C function which performs the conversion turns out to be quite time consuming.

Is there any work around (maybe a hand tuned function) which can speed up the process a little bit? We don't care much about a precision.

Answer 1

Most of the other answers here just try to eliminate loop overhead.

Only deft_code's answer gets to the heart of what is likely the real problem -- that converting floating point to integers is shockingly expensive on an x86 processor. deft_code's solution is correct, though he gives no citation or explanation.

Here is the source of the trick, with some explanation and also versions specific to whether you want to round up, down, or toward zero: Know your FPU

Sorry to provide a link, but really anything written here, short of reproducing that excellent article, is not going to make things clear.

Answer 2

inline int float2int( double d )
{
   union Cast
   {
      double d;
      long l;
    };
   volatile Cast c;
   c.d = d + 6755399441055744.0;
   return c.l;
}

// this is the same thing but it's
// not always optimizer safe
inline int float2int( double d )
{
   d += 6755399441055744.0;
   return reinterpret_cast<int&>(d);
}

for(int i = 0; i < HUGE_NUMBER; i++)
     int_array[i] = float2int(float_array[i]);

The double parameter is not a mistake! There is way to do this trick with floats directly but it gets ugly trying to cover all the corner cases. In its current form this function will round the float the nearest whole number if you want truncation instead use 6755399441055743.5 (0.5 less).