NWScript JIT engine: Wrap-up (for now)

Yesterday, I provided a brief performance overview of the MSIL JIT backend versus my implementation of an interpretive VM for various workloads.

Today, I’ll mostly pontificate on conclusions from the JIT project. It has certainly been an interesting foray into .NET, program analysis, and code generation; the JIT engine is actually my first non-trivial .NET project. I have to admit that .NET turned out to not be as bad as I thought that it would be (as much as I thought I wouldn’t have said that); that being said, I don’t see myself abandoning C++ anytime soon.

Looking back, I do think that it was worth going with MSIL (.NET) as the first JIT backend. Even though I was picking up .NET Reflection for the first time, aside from some initial frustrations with referencing /clr mixed types from emitted code, things turned out relatively smooth. I suspect that writing the JIT against another backend, such as LLVM, would have likely taken much more time invested to reach a fully functional state, especially with full support for cleaning up lingering state if the script program aborted at any point in time.

Justin is working on a LLVM JIT backend for the JIT system, though, so we’ll have to see how it turns out. I do suspect that it’s probably the case that LLVM may offer slightly better performance in the end, due to more flexibility in cutting out otherwise extraneous bits in the JIT’d native code that .NET insists on (such as the P/Invoke wrapper code, thin as it may be).

That being said, the .NET JIT didn’t take an inordinate amount of time to write, and it fully supports turning IL into optimized x86, amd64, and ia64 code (Andrew Rogers’s 8-year-old Itanium workstation migrated to my office at work, and I tried it the JIT engine out on ia64 on the weekend using it — the JIT system did actually function correctly, without any additional development work necessary, which makes me happy). There was virtually no architecture-specific code that I had to write to make that happen, which in many respects says something impressive about using MSIL as a code generation backend.

MSIL was easy to work with as a target language for the JIT system, and the fact that the JIT optimizes the output freed me from many of the complexities that would be involved had I attempted to target x86 or amd64 machine code directly. While there’s still some (thin) overhead introduced by P/Invoke stubs and the like in the actual machine code emitted by the .NET JIT, the code quality is enough that it performs quite well at the end of the day.

Oh, and if you’re curious, you can check out an example NWScript assembly and its associated IL. Note that this is the 64-bit version of the assembly, as you can see from the action service handler call stubs. For fun, I’ve heard that you can even turn it into C# using Reflector (though without scopes defined, it will probably be a bit of a pain to wade through).

All in all, the JIT engine was a fun vacation project to work on. Next steps might be to work on patching the JIT backend into the stock NWN2 server (currently it operates in my ground-up server implementation), but that’s a topic for another day.


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