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Comment by schemescape | Hacker News Reader

I thought this was going to be about a toy emulator/hobby project, but it's apparently about an emulator used in Time Travel Debugging!

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Ha, I was just reading some of the discussion and thinking it sounded quite similar to the JIT we use in our own (Linuxy) Time Travel Debugger.

It's similarly am x86-on-x86 JIT / emulator but (and I'm sure WinDbg's TTD is similar) most of what you want to do there is just code copying for any instructions that don't need special instrumentation.

And you want to run entirely in the cache of JITted code so you're close to native speed, rather than exit to C code and make decisions.

Funny enough, we tried going down the road of doing a JIT, but for our use cases pure emulation was often fast enough and it wasn't worth the extra complexity to do JIT. Part of that is probably due to the architecture of how TTD works, but part of it is how efficient we were able to get with the emulation mode. Later, a different emulator was created for running x86 code on ARM64, and that one uses an extremely efficient JIT. (I didn't get to work on the x86-on-arm64 emulator though. Would have been fun)

Working on the JIT in TTD was a lot of fun though, and it's a shame it didn't make sense to keep it around.

Very impressive you got such good performance out of straight emulation! And WinDbg TTD runs with truly parallel threaded execution within one process, which we do not (yet).

I'm surprised a JIT wasn't worth it but, from what I'm aware of, I can see a few reasons why the trade offs are different.

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