Given that the frontend of the JIT compiler is finished with the bytecode, having turned it into some other form that is easier to process, what happens next? Typically, the code goes through several levels of transformations and optimizations, each level becoming increasingly platform-dependent. The final level of code is native code for a particular platform. The native code is emitted into a code buffer and executed whenever the function it represents is called.

Naturally, it makes sense to keep the JIT compiler portable as far as possible. So, most optimizations are usually done when the intermediate code format is still platform-independent. This makes it easier to port the JIT compiler to different architectures. However, low-level, platform-specific optimizations must naturally be implemented as well to achieve industrial strength performance.

This section describes how the JIT compiler gets from bytecode to native code and the stages involved. We concentrate...