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There is also an emulator layer. It's used as a fallback when the much faster ahead-of-time binary translator can't work. The emulator is generally not a good experience -- if you have to use it, you really want to go native asap.

Examples where it is required is any kind of self-modifying code, such as JITs and the like. Since IDEA is a java application that ships with it's own JRE, the aot translator can only translate the JRE parts and when the execution first jumps to freshly written memory the system bails to the emulator. Since the emulator is an order of magnitude (probably more...) slower than native, and the entire IDE runs on JIT, this is very bad.

It includes a full emulator, there's no other way to run code generated at runtime which it can do.

You're right.