What does a JIT compiler do?

Question

I was just watching the Google IO videos and they talked about the JIT compiler that they included in the android. They showed a demo of performance improvements thanks to t

Answer 1

It's a just-in-time compiler, half-way between an interpreter and a compiler (i.e. it compiles, but only just before the code is executed).

This enables optimisations to the compilation using dynamic information only known at runtime (as compilers usually run statically, and so only have access to compile-time information). They're a lot harder to write, but can give great performance improvements.

For more information, as always, see Wikipedia:

In computing, just-in-time compilation (JIT), also known as dynamic translation, is a technique for improving the runtime performance of a computer program. Traditionally, computer programs had two modes of runtime operation, either interpreted or static (ahead-of-time) compilation. Interpreted code was translated from a high-level language to a machine code continuously during every execution, whereas static compilation translated code into machine code before execution, and only required this translation once.

JIT compilers represented a hybrid approach, with translation occurring continuously, as with interpreters, but with caching of translated code to minimize performance degradation. It also offers other advantages over statically compiled code at development time, such as handling of late-bound data types and the ability to enforce security guarantees.