Where Just-in-time compilation comes into picture ?
->Here is simple diagram that I have prepared as per my understanding:
JVM implementation is doen using C. JVM development rules are left to developers (developers of Oracle JVM)
->Here is simple diagram that I have prepared as per my understanding:
JVM implementation is doen using C. JVM development rules are left to developers (developers of Oracle JVM)
Just-in-time (JIT) compiler, optimizes & compiles a piece of bytecode into machine code, and that part is subsequently executed.
The pieces of code to be compiled using JIT are decided based on the statistic(profiling information) of run-time received from interpreter. These are called HotSpots at run-time.
JIT process is started as thread in background. It generates code tailored for the target architecture
There are two types of compilers in JIT:
C1 i.e. client compiler: Low processing time but less optimized solution -> Applied for small pieces of code
C2 i.e.server compiler: Higher processing time but more optimized solution - > Applied for big pieces of code
The VM uses the interpreter to execute the code (without optimizations) immediately after starting and profiles it, to decide what optimizations to apply. It keeps track of the number of invocations for each method, and if it exceeds the threshold of the c1 compiler, it makes the method eligible for optimizations by the c1 compiler, so it’s queues the method for compilation. Similarly for the c2, if the number of invocations reaches a certain threshold, eventually it will be compiled by it.
Optimisations like following are done:
1. Inlining methods
2. Synchronisation lock coarsening
3. Dead Code elimination
Replacing variables on stack i.e. On stack Replacement (OSR), which is difficult task, done by JIT in final optimisation steps.
Conclusion:
My conclusion is not to waste time on local optimizations. In most cases, we can’t beat the JIT. A sensible (yet counter-intuitive) optimization is to split long methods into several methods that can be optimized individually by the JIT.
You’d rather focus on global optimizations. In most cases, they have much more impact.
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