[libcxx-commits] [libcxx] [libc++] Optimize lexicographical_compare (PR #65279)

[Louis Dionne via libcxx-commits]

================
@@ -11,39 +11,101 @@
 
 #include <__algorithm/comp.h>
 #include <__algorithm/comp_ref_type.h>
+#include <__algorithm/min.h>
+#include <__algorithm/mismatch.h>
+#include <__algorithm/simd_utils.h>
+#include <__algorithm/unwrap_iter.h>
 #include <__config>
+#include <__functional/identity.h>
 #include <__iterator/iterator_traits.h>
+#include <__string/constexpr_c_functions.h>
+#include <__type_traits/desugars_to.h>
+#include <__type_traits/invoke.h>
+#include <__type_traits/is_equality_comparable.h>
+#include <__type_traits/is_integral.h>
+#include <__type_traits/is_trivially_lexicographically_comparable.h>
+#include <__type_traits/is_volatile.h>
+#include <cwchar>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Compare, class _InputIterator1, class _InputIterator2>
+template <class _Iter1, class _Sent1, class _Iter2, class _Sent2, class _Proj1, class _Proj2, class _Comp>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool __lexicographical_compare(
-    _InputIterator1 __first1,
-    _InputIterator1 __last1,
-    _InputIterator2 __first2,
-    _InputIterator2 __last2,
-    _Compare __comp) {
-  for (; __first2 != __last2; ++__first1, (void)++__first2) {
-    if (__first1 == __last1 || __comp(*__first1, *__first2))
+    _Iter1 __first1, _Sent1 __last1, _Iter2 __first2, _Sent2 __last2, _Comp& __comp, _Proj1& __proj1, _Proj2& __proj2) {
+  while (__first2 != __last2) {
+    if (__first1 == __last1 ||
+        std::__invoke(__comp, std::__invoke(__proj1, *__first1), std::__invoke(__proj2, *__first2)))
       return true;
-    if (__comp(*__first2, *__first1))
+    if (std::__invoke(__comp, std::__invoke(__proj2, *__first2), std::__invoke(__proj1, *__first1)))
       return false;
+    ++__first1;
+    ++__first2;
   }
   return false;
 }
 
+#ifndef _LIBCPP_CXX03_LANG
+
+template <class _Tp,
+          class _Proj1,
+          class _Proj2,
+          class _Comp,
+          __enable_if_t<__desugars_to_v<__totally_ordered_less_tag, _Comp, _Tp, _Tp> && !is_volatile<_Tp>::value &&
+                            __libcpp_is_trivially_equality_comparable<_Tp, _Tp>::value &&
+                            __is_identity<_Proj1>::value && __is_identity<_Proj2>::value,
+                        int> = 0>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
+__lexicographical_compare(_Tp* __first1, _Tp* __last1, _Tp* __first2, _Tp* __last2, _Comp&, _Proj1&, _Proj2&) {
+  if constexpr (__is_trivially_lexicographically_comparable_v<_Tp, _Tp>) {
+    auto __res =
+        std::__constexpr_memcmp(__first1, __first2, __element_count(std::min(__last1 - __first1, __last2 - __first2)));
+    if (__res == 0)
+      return __last1 - __first1 < __last2 - __first2;
+    return __res < 0;
+  }
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+  else if constexpr (is_same<__remove_cv_t<_Tp>, wchar_t>::value) {
+    auto __res = std::__constexpr_wmemcmp(__first1, __first2, std::min(__last1 - __first1, __last2 - __first2));
+    if (__res == 0)
+      return __last1 - __first1 < __last2 - __first2;
+    return __res < 0;
+  }
+#endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
+  else {
+    auto __res = std::mismatch(__first1, __last1, __first2, __last2);
+    if (__res.second == __last2)
+      return false;
+    if (__res.first == __last1)
+      return true;
+    return *__res.first < *__res.second;
+  }
+}
+
+#endif // _LIBCPP_CXX03_LANG
+
 template <class _InputIterator1, class _InputIterator2, class _Compare>
 _LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool lexicographical_compare(
     _InputIterator1 __first1,
     _InputIterator1 __last1,
     _InputIterator2 __first2,
     _InputIterator2 __last2,
     _Compare __comp) {
-  return std::__lexicographical_compare<__comp_ref_type<_Compare> >(__first1, __last1, __first2, __last2, __comp);
----------------
ldionne wrote:

https://github.com/llvm/llvm-project/issues/101500

Don't have to fix in this PR, since I think we want to address this more generally. But it's worth noting that we are technically regressing coverage in this PR.

https://github.com/llvm/llvm-project/pull/65279