aio-libs / multidict / 38289122748

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Fold _md_replace_identity() into md_replace() (#1788)

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## What do these changes do?

Folds `_md_replace_identity()`, the single-caller helper behind `d[key]
= v`, into `md_replace()` in `_multilib/hashtable.h`. A failed hash now
releases the identity and returns early; the identity is still released
last, after the watcher flush. The pure-Python backend has no equivalent
helper and is unaffected.

Callgrind (`benchmarks/callgrind_driver.py --include-multidict-only`,
3.14.7 and 3.14.7t, against master 2933772c): GIL is flat, no row moved
by more than 0.2%. FT `d[key] = v` drops 0.6% to 0.85% (4 to 6
instructions per call) for existing keys, new keys and istr keys, on
both `MultiDict` and `CIMultiDict`.

## Are there changes in behavior for the user?

No.

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

No; it removes a helper.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [ ] Unit tests for the changes exist: N/A, a refactor covered by the
existing suite
- [ ] 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`

<details>
<summary>Agent run details (optional, for reviewers)</summary>

Tests: `pytest -q` passed on 3.11.13 GIL C extension (4274 passed),
3.14.7t C extension (4255 passed), and 3.11.13 pure Python with
`MULTIDICT_NO_EXTENSIONS=1 --no-c-extensions` (1697 passed).
Inlining: `tools/check_inlining.py` ok on 3.14.7 and 3.14.7t.
Docs: `make doc-spelling` passed.

| FT row | impl | master | this PR | delta |
| --- | --- | ---: | ---: | ---: |
| `d[key] = v`, existing key | MultiDict | 470.7 | 466.7 | -0.85% |
| `d[key] = v`, existing key | CIMultiDict | 501.9 | 497.9 | -0.80% |
| `d[key] = v`, new key | MultiDict | ... (continued)

842 of 1684 branches covered (50.0%)

Branch coverage included in aggregate %.

9799 of 10709 relevant lines covered (91.5%)

1.83 hits per line