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

A JIT compiler is usually the last part of a VM's pipeline and generates machine code from the VM's intermediary language.

It improves speed by optimizing the generated code for the environment it runs on (CPU specific instructions, cache size,...).

Traditional compilers also optimize the generated machine code but they have to do it without being aware of the specific resources that will be available at runtime.