[Bug libstdc++/110879] [14 Regression] Unnecessary reread from memory in a loop with std::vector

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110879

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Ah I think that's probably expected. In _M_realloc_insert (and now
_M_realloc_append) we have:

#if __cplusplus >= 201103L
        if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
          {
            // Relocation cannot throw.
            __new_finish = _S_relocate(__old_start, __position.base(),
                                       __new_start, _M_get_Tp_allocator());
            ++__new_finish;
            __new_finish = _S_relocate(__position.base(), __old_finish,
                                       __new_finish, _M_get_Tp_allocator());
          }
        else
#endif

and then an alternative path used for non-trivial types and for C++98. That
alternative path does more work and probably can't be optimized as well, so the
reads from _M_end_of_storage aren't optimized out.

I think we can just use { target c++11 } for the test.