Some Comments On Code Optimization

I often code for fun, building tiny toys to amuse myself. Be it simple board games that play themselves (tic-tac-toe and checkers), or some simulations (I have some Windows 7 gadgets laying around), or… well, you get the idea.

In the simulations, for which I developed a taste thanks to a former coworker, I tend to sit down and consider every little calculation I’m going to be performing. I usually run them in an HTML canvas, which both makes them suitable for use as Windows 7 desktop gadgets and lets me have good fun in a healthy environment: It’s easier to e-mail one html file than an exe, and the tools to profile browser javascript code are good to be familiar with.

In any case, one of the things I learned while doing this is that the way in which you run the loops has a tiny impact on performance. It adds up when you try to calculate an immense amount of vectors applied to an immense ammount of particles in a dynamic array… and as we’re running on top of a VM on top of a browser on top of the OS, every little bit counts. Whenever possible, I write the loops in the form

while(n–>0){
// do stuff
}

Why? Javascript implementations, just like the Java VM and the CPython runtime and x86 assembly have special codes to represent a handful of numbers which usually include 0, -1, and 1 through 5 at least. Not a big deal – unless you’re avoiding cache misses. As it’s a cheap optimization to make (doesn’t hurt readability), whenever it’s logical I use it.

This kind of optimization is nice and clean fun, but not necessarily impactful. The best way to optimize code is:

  1. Write it in a readable way. Forget about optimizations.
  2. Measure the hotspots, that is: measure where time is being spent
  3. Optimize that

Of course, before ever writing a line of code, some thought must be given to what you’re going to write. The fastest code is the one which does the least, at any given level. In the example above, the optimization is at the bytecode or assembly level; when you choose the wrong data structure or the wrong algorithm, you affect every level. This is the value of that “algorithms and data structures” class or book which you didn’t pay too much mind to at school.

I’m going to put forward two examples of choice, one of data structure and one of algorithm.

An Example of Data Structure Choice

If you need to organize a list of existing data, you may choose to use a list – after all, the amount of data to be organized could change in time. But different implementations of list have different characteristics. In Java, for example, you have LinkedList and ArrayList implementations of a List (among others). Any implementation of List can be used as an argument for of those you could invoke Collections.sort(List <T> list). This implementations, of course, mirror what you could write by hand if you wanted to. Indeed, I’d urge you to write an ArrayList implementation.

In any case: Which one would you use?

If you chose the ArrayList, pat yourself in the back. To organize data, you need to constantly compare elements, and the fastest way to do that is with a contiguous, indexed block of memory. The difference in performance can be observed in a desktop organizing random integers.

An Example of Algorithm

I’ll share something very real: An e-mail from GNU grep author Mike Haertel in a FreeBSD mailing list in 2010, in which he dissects the reason GNU grep is faster than BSD grep. Long story short: GNU grep executes fewer operations to achieve the same goal. Read it through, I could hardly improve upon Mr. Haertel’s exposition.

Conclusion

Toy around, and have fun with your micro-optimizations all you like. But be careful with the algorithm and data structure you choose.

Sometimes you’re calling some library or external service only to discard the result if some condition isn’t met. This kind of mistake is especially abundant in inherited codebases which accrue functionality over time.

Most important: if code is being slow, use a profiler. Optimize the hot spots, leave the rest alone. How good can this be? Read the answer here. Have fun :^)

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