Not all JIT compilation needs to be fast. I think it was hotspot that first does interpretation, then - when interpretation proves too expensive - switches over to a quick jit. And when code gets executed lots of times it runs jit again, this time with deep optimization settings.
What I personally don't understand: Why don't we cache JIT results between runs? That might be a worthwile optimization and even possible in the face of class loading.
It probably would be like running ngen on .net, just WITH performance statistics of a program. (Enabling specialization of calls for types commonly passed or eliminating constant expressions while keeping the generic version of a function around - that's hard in AOT as you need profiling information. I think sun's C/C++ compiler was able to do that for AOT, resulting in large speedups. But maybe it only used it for branch prediction).
Edit: What I forgot to add - I like the way you could always AOT things in .NET with ngen but also use a JIT where possible. Now that Java turned out to be owned by the evil empire and .NET the one by the company committed to open source - imagine reading that 10 years ago - I'm really curious in which way things will develop. And with all the new contenders as well. JVM (and .net) is not dead, but a lot of interesting alternatives are getting traction now.
> I think it was hotspot that first does interpretation, then - when interpretation proves too expensive - switches over to a quick jit. And when code gets executed lots of times it runs jit again, this time with deep optimization settings.
This is configurable, Hotspot can JIT right away when application starts, but then be prepared to wait a bit.
> What I personally don't understand: Why don't we cache JIT results between runs?
They do, just not the OpenJDK that many only care about.
All commercial JDKs support code caches between executions and AOT compilation.
What I personally don't understand: Why don't we cache JIT results between runs? That might be a worthwile optimization and even possible in the face of class loading.
It probably would be like running ngen on .net, just WITH performance statistics of a program. (Enabling specialization of calls for types commonly passed or eliminating constant expressions while keeping the generic version of a function around - that's hard in AOT as you need profiling information. I think sun's C/C++ compiler was able to do that for AOT, resulting in large speedups. But maybe it only used it for branch prediction).
Edit: What I forgot to add - I like the way you could always AOT things in .NET with ngen but also use a JIT where possible. Now that Java turned out to be owned by the evil empire and .NET the one by the company committed to open source - imagine reading that 10 years ago - I'm really curious in which way things will develop. And with all the new contenders as well. JVM (and .net) is not dead, but a lot of interesting alternatives are getting traction now.