aio-libs / multidict / 36340231354

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Keep the hash table resize out of line (#1613)

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## What do these changes do?

Moves the out-of-line boundary of the hash table resize onto the heavy
bodies. `_md_resize()` is `NOINLINE`, the GIL build's in-place shrink
moves into a `NOINLINE` `_md_shrink_impl()`, and `_md_shrink()` becomes
an inlined trampoline (FT reuses `_md_resize()` at the current size, as
before). `_md_resize_for_insert()` and `_md_resize_for_upd()` differed
only in `marks`, so they are merged into one `NOINLINE`
`_md_resize_for_add()`.

The GIL build used to carry a second, inlined copy of `_md_resize()`
inside `_md_resize_for_insert()`; now each build has a single copy. The
dispatcher stays out of line on purpose: inlining it made
`md_add_with_hash_steal_refs()` save two more registers on every call
(+3 Ir per add on GIL, `ctor_items` +1.8%), and a `COLD` dispatcher
behind `UNLIKELY` cost about 17 Ir on every resize from the empty table
(`copy_small` +0.9%).

C extension only; the pure-Python backend has no equivalent.

## Are there changes in behavior for the user?

No.

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

No; it is an inlining change inside `hashtable.h` with one fewer
function.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [ ] Unit tests for the changes exist: N/A, no behaviour change;
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 Opus 5.5 (Claude Code); reviewed by @asvetlov.

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

Callgrind (`benchmarks/callgrind_driver.py --include-multidict-only`,
`PYTHONHASHSEED=0`), CPython 3.14.7 and 3.14.7t, ag... (continued)

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