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

[Jonathan Wakely]

On 08/06/20 21:07 +0100, Jonathan Wakely wrote:
>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?

Oh, is it because aux will still use __niter_base and so turn
__normal_iterator arguments into pointers?