[Bug libstdc++/114149] New: lexicographical_compare should use memcmp for C++20 contiguous iterators as well as pointers

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114149

            Bug ID: 114149
           Summary: lexicographical_compare should use memcmp for C++20
                    contiguous iterators as well as pointers
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

__lexicographical_compare_aux1 has:

      const bool __simple =
        (__is_memcmp_ordered_with<_ValueType1, _ValueType2>::__value
         && __is_pointer<_II1>::__value
         && __is_pointer<_II2>::__value
#if __cplusplus > 201703L && __glibcxx_concepts
         // For C++20 iterator_traits<volatile T*>::value_type is non-volatile
         // so __is_byte<T> could be true, but we can't use memcmp with
         // volatile data.
         && !is_volatile_v<remove_reference_t<iter_reference_t<_II1>>>
         && !is_volatile_v<remove_reference_t<iter_reference_t<_II2>>>
#endif

We should use memcmp for contiguous iterators, not only pointers.

There's a similar condition in ranges::lexicographical_compare.