[Bug libstdc++/121905] New: std::rotate optimizations for POD types and random access iterators

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121905

            Bug ID: 121905
           Summary: std::rotate optimizations for POD types and random
                    access iterators
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

For the __rotate overload taking random access iterators, check that this
optimizes to memmove for contiguous iterators:

              if (__is_pod(_ValueType) && __k == 1)
                {
                  _ValueType __t = _GLIBCXX_MOVE(*__p);
                  _GLIBCXX_MOVE3(__p + _Distance(1), __p + __n, __p);
                  __p[_Distance(__n - 1)] = _GLIBCXX_MOVE(__t);
                  return __ret;
                }

and this one:

              if (__is_pod(_ValueType) && __k == 1)
                {
                  _RandomAccessIterator __p_n_1 = __p + _Distance(__n - 1);
                  _ValueType __t = _GLIBCXX_MOVE(*__p_n_1);
                  _GLIBCXX_MOVE_BACKWARD3(__p, __p_n_1, __p + __n);
                  *__p = _GLIBCXX_MOVE(__t);
                  return __ret;
                }

Is there any point using std::move here when it's a POD anyway? Removing the
_GLIBCXX_MOVE uses and changing _GLIBCXX_MOVE3 and _GLIBCXX_MOVE_BACKWARD3 to
std::copy and std::copy_backward would make the code easier to read.

We should replace __is_pod with more precise checks (trivially copyable +
trivially assignable?)

And check ranges::rotate too.