Skip useless hash table probe when deleting from split dicts

[hetaozdh]

Feature or enhancement

Summary

delitem_common() currently calls lookdict_index() before checking whether the dictionary is combined or split.

For split dictionaries, the returned hash-table slot is not used. Therefore, the probe performed by lookdict_index() is redundant for split-table deletions.

I propose moving lookdict_index() into the combined-table branch avoiding a full probe sequence for every deletion from a split dictionary.

Benchmark

Measured with a free-threaded build, -Og, without PGO. The benchmark deletes keys from materialized instance dictionaries; medians over 12 alternating runs:

                    before   after    change
del d[k]             71.2     69.2 ns/op   -2.8%
d.pop(k)              98.9     96.1 ns/op   -2.8%
colliding hashes      78.0     74.4 ns/op   -4.6%

Combined-table deletions are unchanged.

Clang also does not sink the lookdict_index() call into the branch at -O3 -DNDEBUG, so the improvement is not specific to the local -Og build.

Validation

The following test modules pass:

• test_dict
• test_dictviews
• test_dictcomps

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-158816: Skip useless hash table probe when deleting from split dicts #158819