gh-121192: Add fast path to `deepcopy()` for empty list/tuple/dict/set

[lgeiger]

deepcopy() can be surprisingly slow when called with empty containers like lists, tuples, dicts, sets or frozensets.

This adds a fast path for this case similar to #114266 which speeds up these cases by about 4x - 21x while having a small impact on the default path.

Here are some benchmarks comparing this PR against main. Let me know if there are more benchmarks that I can run to verify that there a no performance regressions for the common case:

import pyperf
runner = pyperf.Runner()

setup = """
import copy

a = {"list": [1, 2 ,3 ,4], "t": (1, 2, 3), "str": "hello", "subdict": {"a": True}}

t = ()
fs = frozenset()
l = []
s = set()
d = {}
deep = [[], (), {}, set(), frozenset()]
"""

runner.timeit(name="deepcopy dict", stmt=f"b = copy.deepcopy(a)", setup=setup)
runner.timeit(name="deepcopy empty tuple", stmt=f"b = copy.deepcopy(t)", setup=setup)
runner.timeit(name="deepcopy empty frozenset", stmt=f"b = copy.deepcopy(fs)", setup=setup)
runner.timeit(name="deepcopy empty list", stmt=f"b = copy.deepcopy(l)", setup=setup)
runner.timeit(name="deepcopy empty set", stmt=f"b = copy.deepcopy(s)", setup=setup)
runner.timeit(name="deepcopy empty dict", stmt=f"b = copy.deepcopy(d)", setup=setup)
runner.timeit(name="deepcopy multiple empty containers", stmt=f"b = copy.deepcopy(deep)", setup=setup)

deepcopy empty tuple: Mean +- std dev: [baseline] 287 ns +- 4 ns -> [opt-empty] 49.8 ns +- 1.7 ns: 5.77x faster
deepcopy empty frozenset: Mean +- std dev: [baseline] 1.46 us +- 0.02 us -> [opt-empty] 67.1 ns +- 1.8 ns: 21.70x faster
deepcopy empty list: Mean +- std dev: [baseline] 327 ns +- 6 ns -> [opt-empty] 67.1 ns +- 8.2 ns: 4.88x faster
deepcopy empty set: Mean +- std dev: [baseline] 1.44 us +- 0.02 us -> [opt-empty] 67.9 ns +- 3.8 ns: 21.26x faster
deepcopy empty dict: Mean +- std dev: [baseline] 329 ns +- 4 ns -> [opt-empty] 68.1 ns +- 3.0 ns: 4.83x faster

Benchmark hidden because not significant (2): deepcopy dict, deepcopy multiple empty containers

Geometric mean: 4.85x faster

• Issue: gh-121192