[issue38613] Optimize some set operations in dictkeys object

[Inada Naoki]

New submission from Inada Naoki <songofacandy at gmail.com>:

-, |, and ^ of dictkeys are implemented as:

PyObject *result = PySet_New(self);
// Call set.difference_update, set.update, set.symmetric_difference_update with other.

PySet_New(iterable) has optimized step for iterable is dict.
But since iterable is dictkeys, PyIter_Next() is called for all elements in the dict.

We can pass dict instead of dictkey object to PySet_New.

  $ ./python -m pyperf timeit -o patched.json -s 'k = dict.fromkeys("abcdefghijklmnopqrstuvwxyz").keys(); s={1,2,3}' -- 'k | {1,2,3}'
  $ ./python -m pyperf compare_to master.json patched.json
  Mean +- std dev: [master] 778 ns +- 17 ns -> [patched] 550 ns +- 24 ns: 1.42x faster (-29%)

----------
components: Interpreter Core
messages: 355536
nosy: inada.naoki
priority: normal
severity: normal
status: open
title: Optimize some set operations in dictkeys object
type: performance
versions: Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38613>
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