Synthesize start and stop "events" corresponding to absence of user events

[nfrisby]

I'm using this tool to profile some non-trivial pure code. It's very helpful, thank you.

This issue is a feature request. The feature is for ghc-events-analyze to synthesize events corresponding to START no-genuine-user-events-are-open and STOP no-genuine-user-events-are-open. Informally, my goal is for the synthesized no-genuine-user-events-are-open event to be the complement of all other user events.

This feature would help me determine if I've placed my traceEvent calls carefully and thoroughly enough for my profiling data to be useful.

(In my local branch, I've hacked up something similar by post-processing the quantized user events, but it's severely inaccurate due to double-counting when user events overlap in time within a bucket. However, it already does make it obvious on the graph when none of my genuine user events are active in a given bucket.)

[edsko]

If you implement this, feel free to submit a PR :)

[jberryman]

OT: @nfrisby have you written about your experience using ghc-events-analyze anywhere or have any best practices to share? (I ask partly because I don't totally follow your feature request here; is the idea that you might have missed a "STOP" event somewhere? I'd like to be aware of gotchas as I start to use this tool more heavily)