Add pass to instrument memory.grow instructions

[loganek]

I've implemented this functionality as a new pass, however, it could also be an extension of the existing instrument-memory pass. The main reason for that is because I'd like to be able to use this instrumentation in production code, not just for debugging - the instrument-memory pass adds lot more instrumentation which would impact the performance of my code.

Alternatively, I could extend the instrument-memory pass and add a parameter to select instructions that should be instrumented - I'm open for feedback; if that's preferred approach, I'd be happy to update the PR.

[kripken]

Those use cases make sense to me. How about this?

1. Make the existing InstrumentMemory pass also instrument grows - it should do everything basically.
2. Add a new pass that just instruments grows, which is what you mainly want.
3. Can maybe put them both in the same file to save a little code.

[loganek]

Thanks @kripken for feedback. I don't mind updating the InstrumentMemory to include the memory.grow tracking too. Having said that, I wonder if it makes sense to keep the InstrumentMemoryGrow pass at all, and instead just add some an optional enabled-instructions parameter to the InstrumentMemory pass that allows for choosing which instructions to pick? That'd be a bit more generic and we'd avoid duplicating the same functionality for different passes.

I don't insist on any of the options though, and I'd be happy to leave the decision to you.

[kripken]

@loganek A pass argument is a nice idea. So by default it instruments everything, but you can pick an explicit subset? That sounds good to me.

[loganek]

Yes, that's exactly what I meant. I'll update the PR soon.

[kripken]

Please add a comment explaining that only i32.load and memory.grow should be instrumented below.

[kripken]

I don't see the comment - where is it?

The first function should have a comment "we instrument grow and loads because we were asked to by the flags. The second, "we do not instrument store since we were not asked to by the flags".

[kripken]

I don't see the comment - where is it?

The first function should have a comment "we instrument grow and loads because we were asked to by the flags. The second, "we do not instrument store since we were not asked to by the flags".