[localzet]

Summary

Copy live entries directly into a compact keys table when cloning a sparse combined dictionary, rather than inserting each surviving key/value pair through the generic merge path.

• Preserve insertion order, key/value identities, and cached hashes without invoking user-defined __hash__ methods again.
• Use the compact-copy path for dict.copy() and copying into empty dictionaries (including frozendict construction).
• Keep the existing path for very sparse tables (fewer than approximately 1/64 live entries), where scanning tombstones is less beneficial.
• Preserve dictionary watcher notifications when an empty destination clones a source.

Tests

The patch was tested in the contributor fork with:

• Debug builds: test_dict, test_capi.test_dict, and test_capi.test_watchers (249 tests).
• Debug free-threaded build: the same test modules (249 tests).
• Reference-leak checks: -R 3:3 on these modules.
• AddressSanitizer build: the same test modules (detect_leaks=0, matching CPython's ASan CI configuration).
• Additional randomized sparse-dictionary stress tests, including hash collisions and concurrent copies, in a separate research branch.

Verification: https://github.com/localzet/cpython/actions/runs/42510556632

Performance

Linux x86-64, GCC PGO + LTO; median of repeated source.copy() measurements against the unmodified baseline (3c92a06e). Times are ns per operation.

Keys (live / original)	Key type	Baseline	Patch	Speedup
30,000 / 100,000	str	891,644	235,320	3.79x
30,000 / 100,000	int	592,779	206,562	2.87x
3,000 / 100,000	str	118,462	46,237	2.56x
3,000 / 100,000	int	111,283	76,323	1.46x
1,500 / 100,000	int	87,039	89,021	0.98x

These are results from one CI benchmark run, not a statistically controlled multi-machine study. Extremely sparse integer-key cases are approximately unchanged; some cases show a small measured slowdown.

Benchmark: https://github.com/localzet/cpython/actions/runs/42497448717

AI assistance was used for source exploration, patch development, and test/benchmark automation. The patch and its behavior were examined and tested against the configurations listed above.

• Issue: Optimising sparse dict copy #161524