How can adding code to a loop make it faster?

Question

I have a simple function with an inner loop - it scales the input value, looks up an output value in a lookup table, and copies it to the destination. (ftol_ambient is a tri

Answer 1

Are you just testing this inner loop, or are you testing your undisclosed outer loop as well? If so, look at these three lines:

if (((int *)SRGBCeiling)[iSRGB] <= *((int *)pSource))  
    ++iSRGB;
*pDestination = (unsigned char) iSRGB;

Now, it looks like *pDestination is the counter for the outer loop. So by sometimes doing an extra increment of the iSRGB value you get to skip some of the iterations in the outer loop, thereby reducing the total amount of work the code needs to do.