perf: add __slots__ to SubtypeContext

[ngnpope]

Follow up to afd5a38.

master                    6.544s (0.0%) | stdev 0.056s 
slotted-subtypecontext    6.512s (-0.5%) | stdev 0.060s 

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[sterliakov]

Nice, thanks! This shouldn't harm, but should probably have less impact: one SubtypeContext is reused in nested is_subtype calls, while visitors are created in each of them.

[JukkaL]

Adding __slots__ isn't expected to have any real performance impact after compilation, since mypyc essentially applies the rough equivalent of the __slots__ optimization automatically to compiled (native) classes. So any performance change here is likely noise -- random fluctuations are often about 0.5%, when using the perf_compare.py tool, unfortunately.

A potential workaround to noise is to add a few different optimizations in a branch and measure their overall impact. If they add up to close to 1.0% performance improvement, we can be reasonably sure that they are beneficial in aggregate.

[ngnpope]

Ok. Good to know! 🙂

[ngnpope]

Just out of interest, I did another run to see how this and Stanislav's previous one stack over the commit before:

cb3bddd4                  6.356s (0.0%) | stdev 0.042s 
afd5a382                  6.354s (-0.0%) | stdev 0.039s 
slotted-subtypecontext    6.338s (-0.3%) | stdev 0.030s 

So neither of them are significant. I'll leave this up to you as to whether you want to merge or close it - it won't do any harm, but isn't going to make much of a dent as you say.

[sterliakov]

add a few different optimizations in a branch

I thought about that, but finally ruled this out: mixing multiple updates together means some of them can be masked regressions, with total improvement being caused by other changes. Maybe we should write a benchmark suite for subtype checks instead? Something that doesn't take tens of minutes to run.