[mypyc] feat: call native __len__ when defined

[BobTheBuidler]

When a native class defines __len__, we don't need to use python's dispatch for a vtable lookup. Instead, we can call the C function directly.

[BobTheBuidler]

I think this line should be f.__len__() instead of C.__len__(f) because neither C nor C.__len__ nor F is final

But maybe its fine because F isn't decorated with mypyc_attr(allow_interpreted_subclasses=True)? Seeking feedback on this point

[JukkaL]

Mypyc performs static analysis on class hierarchies, and if you aren't doing separate compilation (most tests don't use it), it can identify all possible subclasses and thus determine that F is effectively final, since there are no (can be no) subclasses.

[JukkaL]

Have you measured that this improves performance? Method calls should already be optimized and call the native __len__ method if it's defined. The goal is that gen_method_call generates the fastest possible method call, so any further optimizations should likely go there instead of every place where we generate method calls.

[JukkaL]

Mypyc performs static analysis on class hierarchies, and if you aren't doing separate compilation (most tests don't use it), it can identify all possible subclasses and thus determine that F is effectively final, since there are no (can be no) subclasses.

[BobTheBuidler]

In a perfect world, should gen_method_call output the IR on the left or on the right?

I'll try to reimplement the logic from this PR in there and see what happens

[JukkaL]

I think in this case they should do the same thing, but the original IR is more consistent with the intent of the code, since we aren't calling a static method, so the it would be better.

[BobTheBuidler]

Okay, I misunderstood what the IR was representing and no longer think this PR is necessary. Closing so we can focus on the others.