aio-libs / multidict / 35971413209

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Always inline the hash-table probe initializer (#1554)

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## What do these changes do?

`htkeysiter_init()` sets five fields of the struct that walks a hash
chain, and every lookup, insertion and deletion in the extension starts
by calling it. GCC was emitting it out of line, so each of those paths
opened with a call. Marking it `ALWAYS_INLINE` removes the call.

Instructions per operation on a GIL build of CPython 3.13, measured with
`benchmarks/callgrind_driver.py` against `3336132`:

| operation | `MultiDict` | `CIMultiDict` |
| --- | ---: | ---: |
| `for k, v in d.items()` | -8.8% | -8.7% |
| `d.setdefault(key, v)`, new key | -5.0% | -2.5% |
| `d.get(key)`, miss | -4.5% | -2.2% |
| `d.pop(key)` | -4.1% | -2.2% |
| `d.update(other)` | -2.9% | -2.3% |
| `d[key] = v`, existing key | -2.7% | -2.2% |
| `d.getall(key)` | -1.7% | -1.2% |
| `d.popitem()` | -1.7% | -0.6% |

Nothing measured slower on either class. Free-threaded builds already
inlined it: `tools/codegen_diff.py` reports the emitted code byte for
byte identical there, so the change is a no-op on that build.

## Are there changes in behavior for the user?

No, other than the speed. The function's body is untouched.

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

No. It is one attribute, next to the existing `ALWAYS_INLINE` on
`md_calc_identity_hash()` and for the same reason.

## Related issue number

None.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist — N/A, no behaviour changes; the
existing suite covers every path this touches
- [x] Documentation reflects the changes — N/A
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt` — N/A, already listed
- [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`

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