aio-libs / multidict / 38538425053

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Force inlining of the atomic and free-threading helpers (#1796)

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## What do these changes do?

Marks every helper in `atomic_helpers.h` and `freethreading.h`
`ALWAYS_INLINE`, a macro added back to `compiler.h`.

On the GIL build the extension is byte-identical to master: GCC already
inlined all of these. On the FT build none of them had an out-of-line
copy either, but forcing them changes how GCC spends its inlining budget
elsewhere, and `clear()`, `copy()`, `delitem` and key iteration get
cheaper (callgrind numbers below).

Turning the same helpers into macros gives identical inlining on both
builds and the same callgrind numbers, but loses type checking and
evaluates `Py_NewRef()` arguments twice in `publish_value()` and
`replace_compact_key()`, so functions stay.

The pure-Python backend has no equivalent and is unaffected.

## Are there changes in behavior for the user?

No.

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

No.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [ ] Unit tests for the changes exist: N/A, 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`

Drafted with Claude Code (Claude Opus 5.5); reviewed by @asvetlov.

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

Callgrind (`benchmarks/callgrind_driver.py --include-multidict-only`),
3.14.7t, master 407f543f vs this branch, rows that moved by 0.5% or
more;
median 0.00%. GIL not measured: the disassembly is identical.

| op | impl | before Ir/op | after Ir/op | delta |
|---|---|---|---|---|
| clear | MultiDict | 6577.0 | 6176.0 | -6.10% |
| copy | MultiDict | 12952.1 | 12551.1 | -3.10% |
| ... (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