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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/96088] Range insertion into unordered_map is less
 effective than a loop with insertion
Date: Thu, 09 Jul 2020 08:31:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96088

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Fran=C3=A7ois Dumont from comment #1)
> I'll check if we can be smarter here. A nice improvement would be to chan=
ge
> std::hash<std::string> operator signature to:
>=20
>       size_t
>       operator()(const string_view& __str) const noexcept
>=20
> but that's a Standard modification.

Or use unordered_map<string, int, hash<string_view>, equal_to<>> which shou=
ld
perform better.

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 helpe=
rs
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 wo=
uld
be conforming though. Heterogeneous lookup is observably different, and not
conforming in C++17.

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