JIT compiler vs offline compilers
Are there scenarios where JIT compiler is faster than other compilers like C++?
Do you think in the future JIT compiler will just see minor optimizations, features but follow a similar performance, or will there be breakthroughs that will make it infinitely superior to other compilers?
It looks like the multi core paradigm has some promise but it's not universal magic.
Any insights?
Answer
Yes, there certainly are such scenarios.
- JIT compilation can use runtime profiling to optimize specific cases based on measurement of the characteristics of what the code is actually doing at the moment, and can recompile "hot" code as necessary. That's not theoretical; Java's HotSpot actually does this.
- JITters can optimize for the specific CPU and memory configuration in use on the actual hardware where the program happens to be executing. For example, many .NET applications will run in either 32-bit or 64-bit code, depending upon where they are JITted. On 64 bit hardware they will use more registers, memory, and a better instruction set.
- Virtual method calls inside of a tight loop can be replaced with static calls based on runtime knowledge of the type of the reference.
I think there will be breakthroughs in the future. In particular, I think that the combination of JIT compilation and dynamic typing will be significantly improved. We are already seeing this in the JavaScript space with Chrome's V8 and TraceMonkey. I expect to see other improvements of similar magnitude in the not-too-distant future. This is important because even so-called "statically typed" languages tend to have a number of dynamic features.