GNU bug report logs - #78162
[PATCH] hash-table-contains-p: Avoid creating a symbol on every call

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From: Daniel Mendler <mail <at> daniel-mendler.de>
To: Pip Cet <pipcet <at> protonmail.com>
Cc: 78162 <at> debbugs.gnu.org, Stefan Kangas <stefankangas <at> gmail.com>
Subject: bug#78162: [PATCH] hash-table-contains-p: Avoid creating a symbol on every call
Date: Wed, 30 Apr 2025 16:00:40 +0200

Pip Cet <pipcet <at> protonmail.com> writes:

> "Daniel Mendler via \"Bug reports for GNU Emacs, the Swiss army knife of text editors\"" <bug-gnu-emacs <at> gnu.org> writes:
>
>> Tags: patch
>>
>> For performance reasons avoid creating a symbol on every call to
>> `hash-table-contains-p'.  See also https://github.com/emacs-compat/compat/issues/71.
>
> If the performance of hash-table-contains-p is an issue, we should
> reimplement it in C, which can use magic values which aren't accessible
> from Lisp.

For a proper hash table I expect this function to be efficient and not
allocate. A magic value becomes a problem if it is actually used in
Lisp. Is this likely? This should not happen by accident, only if
`hash-table--missing` is used intentionally. Note that the actual symbol
which is used for the check is uninterned.

> Pip

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