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

[fdumont at gcc dot gnu.org]

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

--- Comment #1 from François Dumont <fdumont at gcc dot gnu.org> ---
The core issue here is that unordered_map key type is std::string while you
insert const char* which explains the temporary.

In f2 you use insert(Pair&&) method so a temporary is generated but then moved
into the container. In f1 we try to check for insertion before generating the
value_type and to do so we use hash<std::string> that generates an additional
temporary.

I'll check if we can be smarter here. A nice improvement would be to change
std::hash<std::string> operator signature to:

      size_t
      operator()(const string_view& __str) const noexcept

but that's a Standard modification.