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What happens to the machine code/instructions that are generated by the JIT compiler when a program is terminated or restarted. Does the JIT compilation occur again? If so, why would it use this approach?
To keep the scope simple, I would like to know specifically about .NET but for the benefit of others, answers about other implementations are welcome
Yes, in general.
For one thing, it avoids questions about where the JITted code should be stored, how long for etc. For another, it avoids the code becoming invalid due to someone moving their hard drive to a machine with a different CPU, for example. I suspect it’s felt that JIT compilation is a relatively small part of the total time taken for most programs.
Now you can “pre-JIT” an assembly using NGen – but there are some optimizations I believe it can’t make, and again the generated code becomes invalid if you change CPU etc.
Also note that the .NET framework itself will run a background service when it’s first installed, to JIT compile the framework libraries (which will probably form the bulk of the code run for most apps). Presumably it has some smarts to recompile if necessary… I’ve never looked into it in too much detail.