Why is this C++ code execution so slow compared to java?

Question

I recently wrote a computation-intensive algorithm in Java, and then translated it to C++. To my surprise the C++ executed considerably slower. I have now written a much shorter

Answer 1

The only time I could get the C++ program to outperform Java was when using profiling information. This shows that there's something in the runtime information (that Java gets by default) that allows for faster execution.

There's not much going on in your program apart from a non-trivial if statement. That is, without analysing the entire program, it's hard to predict which branch is most likely. This leads me to believe that this is a branch misprediction issue. Modern CPUs do instruction pipelining which allows for higher CPU throughput. However, this requires a prediction of what the next instructions to execute are. If the guess is wrong, the instruction pipeline must be cleared out, and the correct instructions loaded in (which takes time).

At compile time, the compiler doesn't have enough information to predict which branch is most likely. CPUs do a bit of branch prediction as well, but this is generally along the lines of loops loop and ifs if (rather than else).

Java, however, has the advantage of being able to use information at runtime as well as compile time. This allows Java to identify the middle branch as the one that occurs most frequently and so have this branch predicted for the pipeline.