libstdc++: Extend memcmp optimization in std::lexicographical_compare

[Jonathan Wakely]

On 05/06/20 22:24 +0200, François Dumont via Libstdc++ wrote:
>diff --git a/libstdc++-v3/include/debug/safe_iterator.tcc b/libstdc++-v3/include/debug/safe_iterator.tcc
>index 888ac803ae5..ca4e2d52d1d 100644
>--- a/libstdc++-v3/include/debug/safe_iterator.tcc
>+++ b/libstdc++-v3/include/debug/safe_iterator.tcc
>@@ -470,6 +470,80 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       return __equal_aux1(__first1, __last1, __first2);
>     }
> 
>+  template<typename _Ite1, typename _Seq1, typename _Cat1,
>+	   typename _II2>
>+    int
>+    __lexicographical_compare_aux(
>+	const ::__gnu_debug::_Safe_iterator<_Ite1, _Seq1, _Cat1>& __first1,
>+	const ::__gnu_debug::_Safe_iterator<_Ite1, _Seq1, _Cat1>& __last1,
>+	_II2 __first2, _II2 __last2)
>+    {
>+      typename ::__gnu_debug::_Distance_traits<_Ite1>::__type __dist1;
>+      __glibcxx_check_valid_range2(__first1, __last1, __dist1);
>+      __glibcxx_check_valid_range(__first2, __last2);
>+
>+      if (__dist1.second > ::__gnu_debug::__dp_equality)
>+	return std::__lexicographical_compare_aux(__first1.base(),
>+						  __last1.base(),
>+						  __first2, __last2);
>+      return std::__lexicographical_compare_aux1(__first1, __last1,
>+						 __first2, __last2);

What's the rationale for the choice of whether to call aux or aux1
here?

It seems to be that if we know [first1, last1) is a valid range, we
use aux with the unsafe iterators (which means we do overload
resolution again for all the overloads that include _Safe_iterator,
but we know we don't have safe iterators now). Otherwise, if we don't
know it's a valid range, we call aux1 with the safe iterators.

Why don't we use aux1 in both cases?