fast mode and tracing mode about miri HOT 6 CLOSED

@eddyb pointed out on IRC that this would destroy stack traces.

I think we should have a debug/release mode concept for Miri. (@eddyb called it fast mode and tracking mode.)

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Well, what I mean is that you can run both in succession: if you rarely expect failures, and it's measurably faster not to track debug information, you can re-run on failure to get stack traces, taint analysis and other goodies.
That is, for deterministic use. But one interesting thing is that you can have a third mode: recording only non-determinism (i.e. I/O). Like rr, but specialized to whatever miri will support.

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@eddyb Interesting idea. Determinism allows for some neat features.

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@oli-obk is this still a useful ticket to keep open?

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I think the taint analysis and leak analysis would still be helpful. Leak analysis is pretty easy, we just need to make allocate take a Span and put that Span into the Allocation extra data.

Taint analysis seems harder, since you would have to find a way to store the tag information in integers and other Immediate values.

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With all the perf projects that @saethlin come up with and carried through, and all the reproducability issues we have with rerunning, I think we can just close this and keep focussing on finding other ways to speed up miri.

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