aio-libs / multidict / 37211102432

86%

push

github

Use the key alone as the hole marker (#1674)

<!-- Thank you for your contribution! -->

## What do these changes do?

`entry_is_hole()` tested the key in a compact table and the identity in
an anystr one. Every path that deletes an anystr entry clears both
fields under the critical section, and every insert sets both, so for
any caller holding the lock the key alone tells a hole. The helper now
reads `entry->key` in every kind, drops its `kind` argument and is
`ALWAYS_INLINE`. Lock-free readers never used it; they check the
identity slot through `try_get_ref()` and are unchanged.

The old kind-dependent test made GCC split the iterator and `popitem()`
scans into one loop per kind with a constant entry size; reading only
the key undid that and cost about 2 instructions per `__next__`. Those
two scans now branch on the kind by hand. `tools/check_inlining.py`
gained rules for `_iter_scan`, `_md_last_live` and `entry_is_hole`.

`_md_check_consistency()` now also asserts the reverse direction, that a
hole has a NULL key, so a path that clears one field without the other
trips debug builds.

The pure-Python implementation has no equivalent helper and is
unaffected.

## Are there changes in behavior for the user?

No.

## Is it a substantial burden for the maintainers to support this?

No; it removes a per-kind branch from the walk loops.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist (N/A for new tests; covered by
the existing suite and the debug consistency check)
- [x] Documentation reflects the changes (N/A)
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt` (N/A)
- [x] Add a new news fragment into the `CHANGES/` folder
- [x] `make doc-spelling` passes and any new technical words are added
to `docs/spelling_wordlist.txt`

Callgrind instructions per operation (`benchmarks/callgrind_driver.py
--include-multidict-only`, release builds). "Key only" is ... (continued)

792 of 1584 branches covered (50.0%)

Branch coverage included in aggregate %.

9315 of 10167 relevant lines covered (91.62%)

1.83 hits per line