Micro-optimization: Make ArgKind a regular class instead of enum

[JukkaL]

Mypyc doesn't generate very efficient code for enums, so switch to a regular class. We can later revert the change if/when we can improve enum support in mypyc.

Operations related to ArgKind were pretty prominent in the op trace log (#19457).

By itself this improves performance by ~1.7%, based on perf_compare.py, which is significant:

master                    4.168s (0.0%) | stdev 0.037s
HEAD                      4.098s (-1.7%) | stdev 0.028s

This is a part of a set of micro-optimizations that improve self check performance by ~5.5%.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

django-stubs (https://github.com/typeddjango/django-stubs)
+ django-stubs/contrib/sites/models.pyi:16: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
+ https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
+ Please report a bug at https://github.com/python/mypy/issues
+ version: 1.18.0+dev.3194db2e144e768800ecb9650111103a3f6e9bdf
+ django-stubs/contrib/sites/models.pyi:16: : note: use --pdb to drop into pdb
+ Traceback (most recent call last):
+   File "", line 8, in <module>
+     sys.exit(console_entry())
+   File "/__main__.py", line 15, in console_entry
+     main()
+   File "/main.py", line 127, in main
+     res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
+   File "/main.py", line 211, in run_build
+     res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
+   File "/build.py", line 191, in build
+     result = _build(
+   File "/build.py", line 267, in _build
+     graph = dispatch(sources, manager, stdout)
+   File "/build.py", line 2939, in dispatch
+     process_graph(graph, manager)
+   File "/build.py", line 3337, in process_graph
+     process_stale_scc(graph, scc, manager)
+   File "/build.py", line 3438, in process_stale_scc
+     graph[id].type_check_first_pass()
+   File "/build.py", line 2311, in type_check_first_pass
+     self.type_checker().check_first_pass()
+   File "/checker.py", line 483, in check_first_pass
+     self.accept(d)
+   File "/checker.py", line 590, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 1259, in accept
+     return visitor.visit_class_def(self)
+   File "/checker.py", line 2560, in visit_class_def
+     self.accept(defn.defs)
+   File "/checker.py", line 590, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 1340, in accept
+     return visitor.visit_block(self)
+   File "/checker.py", line 3021, in visit_block
+     self.accept(s)
+   File "/checker.py", line 590, in accept
+     stmt.accept(self)
+   File "/nodes.py", line 1427, in accept
+     return visitor.visit_assignment_stmt(self)
+   File "/checker.py", line 3070, in visit_assignment_stmt
+     self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
+   File "/checker.py", line 3282, in check_assignment
+     rvalue_type = self.expr_checker.accept(rvalue, type_context=type_context)
+   File "/checkexpr.py", line 5982, in accept
+     typ = node.accept(self)
+   File "/nodes.py", line 2080, in accept
+     return visitor.visit_call_expr(self)
+            ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
+   File "/checkexpr.py", line 520, in visit_call_expr
+     return self.visit_call_expr_inner(e, allow_none_return=allow_none_return)
+            ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   File "/checkexpr.py", line 654, in visit_call_expr_inner
+     ret_type = self.check_call_expr_with_callee_type(
+         callee_type, e, fullname, object_type, member
+     )
+   File "/checkexpr.py", line 1504, in check_call_expr_with_callee_type
+     ret_type, callee_type = self.check_call(
+                             ~~~~~~~~~~~~~~~^
+         callee_type,
+         ^^^^^^^^^^^^
+     ...<6 lines>...
+         object_type=object_type,
+         ^^^^^^^^^^^^^^^^^^^^^^^^
+     )
+     ^
+   File "/checkexpr.py", line 1606, in check_call
+     return self.check_callable_call(
+            ~~~~~~~~~~~~~~~~~~~~~~~~^
+         callee,
+         ^^^^^^^
+     ...<6 lines>...
+         object_type,
+         ^^^^^^^^^^^^
+     )
+     ^
+   File "/checkexpr.py", line 1851, in check_callable_call
+     new_ret_type = self.apply_function_plugin(
+         callee,
+     ...<7 lines>...
+         context,
+     )
+   File "/checkexpr.py", line 1278, in apply_function_plugin
+     return callback(
+         FunctionContext(
+     ...<8 lines>...
+         )
+     )
+   File "_django_plugin/transformers/fields.py", line 255, in transform_into_proper_return_type
+     return set_descriptor_types_for_field_callback(ctx, django_context)
+   File "_django_plugin/transformers/fields.py", line 136, in set_descriptor_types_for_field_callback
+     return set_descriptor_types_for_field(ctx)
+   File "_django_plugin/transformers/fields.py", line 145, in set_descriptor_types_for_field
+     null_expr = helpers.get_call_argument_by_name(ctx, "null")
+   File "_django_plugin/lib/helpers.py", line 206, in get_call_argument_by_name
+     if kind == ArgKind.ARG_NAMED and argname == name:
+                ^^^^^^^^^^^^^^^^^
+ AttributeError: type object 'ArgKind' has no attribute 'ARG_NAMED'

[sterliakov]

How difficult would be the relevant changes to enum handling in mypyc? I can try to work on that. This PR incurs a huge maintainability and readability penalty IMO, but probably 1.7% is worth that:(

[JukkaL]

Yeah this is pretty unfortunate. Since this change seems to break some plugins, it may be better to improve the enum support in mypyc instead and forget about this. I can create a mypyc issue detailing the requirements needed to make this better.

[ilevkivskyi]

@sterliakov I think we can optimize enums in general in mypyc (e.g. use some static pointers for enum values, use pointer identity comparison for ==, and inline enum references when compiling, i.e. SomeEnum.some_val will become _CPy_SomeEnum_some_val). Most likely the perf benefit is from not having to go through attribute access logic, but TBH I didn't really think much about this.

That said I agree having optimized enums in mypyc is IMO worth it.

[JukkaL]

@sterliakov @ilevkivskyi I created a mypyc issue about this: mypyc/mypyc#1121

Calling methods of enum classes is the biggest bottleneck that I can see. Accessing enum values is already optimized.