[PATCH] libstdc++: Use constexpr-if for std::basic_string::_S_copy_chars

[Jonathan Wakely]

For C++11 and later we can remvoe four overloads of _S_copy_chars and
use constexpr-if in the generic _S_copy_chars. This simplifies overload
resolution for _S_copy_chars, and also means that we optimize for other
iterators such as std::vector<char>::iterator.

We still need all the _S_copy_chars overloads to be part of the explicit
instantiation definition, so make them depend on the macro that is
defined by src/c++11/string-inst.cc for that purpose.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (_S_copy_chars): Replace overloads
	with constexpr-if and extend optimization to all contiguous
	iterators.
	* src/c++11/string-inst.cc: Extend comment.
---

Tested x86_64-linux.

 libstdc++-v3/include/bits/basic_string.h | 31 ++++++++++++++++++------
 libstdc++-v3/src/c++11/string-inst.cc    |  3 ++-
 2 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 886e7e6b19e..067c7915c76 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -468,6 +468,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	  traits_type::assign(__d, __n, __c);
       }
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions"
       // _S_copy_chars is a separate template to permit specialization
       // to optimize for the common case of pointers as iterators.
       template<class _Iterator>
@@ -475,31 +477,44 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
         static void
         _S_copy_chars(_CharT* __p, _Iterator __k1, _Iterator __k2)
         {
+#if __cplusplus >= 201103L
+	  using _IterBase = decltype(std::__niter_base(__k1));
+	  if constexpr (__or_<is_same<_IterBase, _CharT*>,
+			      is_same<_IterBase, const _CharT*>>::value)
+	    _S_copy(__p, std::__niter_base(__k1), __k2 - __k1);
+#if __cpp_lib_concepts
+	  else if constexpr (contiguous_iterator<_Iterator>
+			       && is_same_v<iter_value_t<_Iterator>, _CharT>)
+	    {
+	      const auto __d = __k2 - __k1;
+	      (void) (__k1 + __d); // See P3349R1
+	      _S_copy(__p, std::to_address(__k1), static_cast<size_type>(__d));
+	    }
+#endif
+	  else
+#endif
 	  for (; __k1 != __k2; ++__k1, (void)++__p)
 	    traits_type::assign(*__p, *__k1); // These types are off.
 	}
+#pragma GCC diagnostic pop
 
-      _GLIBCXX20_CONSTEXPR
+#if __cplusplus < 201103L || defined _GLIBCXX_DEFINING_STRING_INSTANTIATIONS
       static void
-      _S_copy_chars(_CharT* __p, iterator __k1, iterator __k2) _GLIBCXX_NOEXCEPT
+      _S_copy_chars(_CharT* __p, iterator __k1, iterator __k2)
       { _S_copy_chars(__p, __k1.base(), __k2.base()); }
 
-      _GLIBCXX20_CONSTEXPR
       static void
       _S_copy_chars(_CharT* __p, const_iterator __k1, const_iterator __k2)
-      _GLIBCXX_NOEXCEPT
       { _S_copy_chars(__p, __k1.base(), __k2.base()); }
 
-      _GLIBCXX20_CONSTEXPR
       static void
-      _S_copy_chars(_CharT* __p, _CharT* __k1, _CharT* __k2) _GLIBCXX_NOEXCEPT
+      _S_copy_chars(_CharT* __p, _CharT* __k1, _CharT* __k2)
       { _S_copy(__p, __k1, __k2 - __k1); }
 
-      _GLIBCXX20_CONSTEXPR
       static void
       _S_copy_chars(_CharT* __p, const _CharT* __k1, const _CharT* __k2)
-      _GLIBCXX_NOEXCEPT
       { _S_copy(__p, __k1, __k2 - __k1); }
+#endif
 
       _GLIBCXX20_CONSTEXPR
       static int
diff --git a/libstdc++-v3/src/c++11/string-inst.cc b/libstdc++-v3/src/c++11/string-inst.cc
index c4864794d40..34df909b31a 100644
--- a/libstdc++-v3/src/c++11/string-inst.cc
+++ b/libstdc++-v3/src/c++11/string-inst.cc
@@ -40,7 +40,8 @@
 // replaced by constrained function templates, so that we instantiate the
 // pre-C++17 definitions.
 // This also causes the instantiation of the non-standard C++0x-era
-// insert(iterator, initializer_list<C>) overload, see PR libstdc++/83328
+// insert(iterator, initializer_list<C>) overload, see PR libstdc++/83328,
+// and overloads of _S_copy_chars for string iterators and pointers.
 #define _GLIBCXX_DEFINING_STRING_INSTANTIATIONS 1
 
 #include <string>
-- 
2.49.0