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Serhiy Storchaka: "Calling _PyStack_AsDict() with non-string or non-unique keywords means that the var-keyword argument was first unpacked to arrays of keyword names and values and then converted back to a dict. Seems something is done non-efficiently." (msg286159 of the issue #29360)
Python code from test_dict:
 dict(**invalid)
Bytecode:
 LOAD_GLOBAL 1 (dict)
 BUILD_TUPLE 0
 LOAD_FAST 1 (invalid)
 CALL_FUNCTION_EX 1
Call stack:
* _PyEval_EvalFrameDefault()
* do_call_core()
* PyObject_Call()
* _Py_RawFastCallDict() -- conversion from dict to stack+kwnames
* type_call()
* call_init()
* _PyStack_AsDict() -- convertsion from stack+kwnames to dict
Oh right, there are two conversions using a temporary FASTCALL format for (keyword) arguments.
This code was not upstream yet, it comes from the pull request of this issue.
Maybe we need two flavors of type_call(): type_call(args: tuple, kwargs: tuple) if tp_fastinit isn't set, type_fastcall(stack, nargs, kwnames) (FASTCALL) if tp_fastinit is set.
But it seems that the logic should be implemented in PyObject_Call() and _PyObject_FastCallDict(), it cannot be implemented in type_call(), it's too late.
For best performances (avoid any kind of conversion and avoid any temporary object), we should implement something like that.