[Bug libstdc++/96088] Range insertion into unordered_map is less effective than a loop with insertion

["glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96088

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> ---

(In reply to Jonathan Wakely from comment #2)
> Or use unordered_map<string, int, hash<string_view>, equal_to<>> which
> should perform better.

Good idea.

> We haven't implemented http://wg21.link/p0919r3 and http://wg21.link/p1690r1
> yet, I wonder if those would help, especially if we make the internal
> helpers available pre-C++20. That could allow the range insertion to use the
> heteregenous lookup, to avoid creating temporaries. I'm not sure if that
> would be conforming though. Heterogeneous lookup is observably different,
> and not conforming in C++17.

Restricting it to a few standard types like string should not be observable.

> Adding hash<string>::operator()(string_view) is an interesting idea for the
> standard though.

Indeed. If we want to, I think it is possible to add some overloads for when
the argument is exactly const char* or string_view, which should remain
conforming and provide a significant part of the benefits.