aio-libs / multidict / 38357421796

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Mark the exception paths in the C extension as unlikely (#1791)

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## What do these changes do?

Wraps the C extension checks whose taken branch means a Python exception
is set in `UNLIKELY()`, including the `hash == -1` checks after
`unicode_hash()`. Checks where `NULL` or `-1` means "not found" or "take
the fallback path" stay as they are.

Some error checks stay unmarked on purpose. GCC sits at its inlining
budget, and marking them changed which helpers it inlines on the add,
update, iteration and istr paths, even though each mark is harmless on
its own:

- `_multidict.c`: inlines `md_add` into `add()` on FT, not measured yet.
- `bulk_update.h:156`, `:531`, `:535` (the `hash == -1` in
`_md_extend_item()`), `:538`, and the `capsule.h` type check macro: on
FT, `md_add_with_hash_steal_refs`, `_md_update_matched` and
`_md_set_default_locked` move out of line, which made the constructor
7.5% slower and `update()` 10.7% slower.
- `bulk_update.h:181`: FT `CIMultiDict.update()` +2.2%.
- `bulk_update.h:482`: together with the `extend` sites, it takes
`_md_update_key()` out of line in `_multidict_bulk_locked()`, making GIL
`update()` 4% slower.
- `iter.h:311`: together with `:317`, `for k in d` costs one extra
instruction per key.
- `istr.h:48`: CIMultiDict `popitem()` costs 12 extra instructions.

## Are there changes in behavior for the user?

No.

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

No. New error checks can follow the same pattern, but a mark on a hot
path should be measured, since it can shift inlining elsewhere.

## 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; the
existing suite covers these paths
- [ ] 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/` folde... (continued)

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