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…sometimes, at least. When compared to Java under OpenJDK.

I was performing some experiments with various ways of implementing persistent random-access sequences. At one point, after implementing some of them in Java, I had the brilliant idea to try and implement some of them in C# as well and see how performance compares.

Unfortunately, this comparison was not very favorable towards C#: it consistently performed around 4-6× slower, four to six times, than the same data structure in Java. That was certainly… not what I expected. I was expecting the performance to be very close, if not C#-favored. After all, the Common Language Runtime supports value types, whereas the Java Virtual Machine at the moment does not, and some of the data structures I tested can benefit from reduced indirection.

Profiling revealed that the hottest functions were those that created new objects. While this does make sense, as persistent data structures using path copying do create many objects, spending north of 80% wall clock time on just allocating new objects, instead of on actual logic, is certainly a lot.

The next step I took to understand why those functions take so much time was, naturally, looking at the generated code. The differences were quite stark, and explained the performance gap almost immediately, so why don't we take a look?