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I am converting some C++ code to Java and I was wondering what I can do about the inlined functions. Can I assume that functions will be inlined by the VM (as an when necessary) and just not worry about this? How do I profile to observe this behaviour? Suppose there is a main outer function, and I throw a for loop around it and cause a million invocations. Should I expect to see an improvements as the VM inlines more and more?
Yes Java does inline method calls. The inlining is performed by the JIT compiler, so you won’t see it by examining the bytecode files.
Whether inlining actually occurs for a given method call will depend on the size of the method body, and whether the call is inlineable. (If a method call involves dispatching … after the JVM has a bunch of global optimization designed to remove unnecessary dispatching … then it cannot be inlined.)
The same applies to your example with your outer main function. It depends on how big the method body is. On the other hand, if the method takes a significant time to execute, then the relative importance of the optimization decreases correspondingly.
My advice is to not worry about things like this at this stage. Just write the code clearly and simply, and let the JIT compiler deal with the problem of optimizing. When your application is working, you can profile it and see if there are any “hot spots” in the code that are worthwhile optimizing by hand.
It may be observable and it may not, depending on the amount time spent in making the calls relative to executing the method bodies. (Profiling often relies on sampling the program counter. The reported times may be inaccurate if the number of samples for a given region of code is too small … and for other reasons.)
It also depends on the JVM you are using. Not all JVMs will re-optimize code that they have previously optimized.
Finally, there is a way to get the JVM to dump the native code output by the JIT compiler. That will give you a definitive answer as to what has been inlined … if you are prepared to read the machine instructions.