How can I count operations in C++?

Question

How can I count operations in C++? I'd like to analyze code in a better way than just timing it since the time is often rounded to 0 millisec.

Answer 1

If you are timing code, it's worth running it a lot of times in a loop to avoid the effect of the timer resolution. So you might run the thing you're timing 10,000 times and measure the amount of time it takes to run all the iterations. It will probably only take a few seconds to run and you'll get better timing data.

Answer 2

Using "the number of operations" is a bad idea when thinking about performance. It doesn't take into account the differences between the best case/worst case cycle counts for each operation, the costs of cache misses, pipeline misses, potential (automatic) parallelisation etc.

As Greg says, usually it's a better idea for a microbenchmark to just run the same code enough times to get a decent span of time.

Even better is to run your whole application with a realistic workload and measure the metrics you're really interested in, but that's a different matter...

What is certainly useful is to work out the complexity of your code - know when a method is going to be O(1), O(log n), O(n) etc. That typically doesn't involve knowing the details of what the individual instructions in C++ do - although you do need to know the complexity of anything you call. (Joel's story of Shlemiel the Painter and strlen being the most obvious example.)