Code optimization

Question

When i\'m trying to optimize my code, I often run into a dilemma:

I have an expression like this:

int x = 5 + y * y;
int z = sqrt(12) + y * y;


        

Answer 1

You don't need a temporary variable:

int z = y * y;
int x = z + 5
z = z + sqrt(12);

but the only way to be sure if this is (a) faster and (b) truly where you should focus your attention, is to use a profiler and benchmark your entire application.

Answer 2

Trying to optimize your code most often means giving the compiler the permission (through flags) to do its own optimization. Trying to do it yourself will more often then not, either just be a waste of time (no improvement over the compiler) or worse.

In your specific example, I seriously doubt there is anything you can do to change the performance.

Answer 3

One of the older compiler optimisations is "common subexpression elimination" - in this case y * y is such a common subexpression.

It may still make sense to show a reader of the code that the expression only needs calculating once, but any compiler produced in the last ten years will calculate this perfectly fine without repeating the multiplication.

Trying to "beat the compiler on it's own game" is often futile, and certainly needs measuring to ensure you get a better result than the compiler. Adding extra variables MAY cause the compiler to produce worse code, because it gets "confused", so it may not help at all.

And ALWAYS when it comes to performance (or code size) results from varying optimizations, measure, measure again, and measure a third time to make sure you get the results you expect. It's not very easy to predict from looking at code which is faster, and which is slower. But it'd definitely be surprised if y * y is calculated twice even with a low level of optimisation in your compiler.