libstdc++: Simplify std::_Destroy using 'if constexpr'

This is another place where we can use 'if constexpr' to replace
dispatching to a specialized class template, improving compile times and
avoiding a function call.

libstdc++-v3/ChangeLog:

	* include/bits/stl_construct.h (_Destroy(FwdIter, FwdIter)): Use
	'if constexpr' instead of dispatching to a member function of a
	class template.
	(_Destroy_n(FwdIter, Size)): Likewise.
	(_Destroy_aux, _Destroy_n_aux): Only define for C++98.

Reviewed-by: Patrick Palka <ppalka@redhat.com>

diff --git a/libstdc++-v3/include/bits/stl_construct.h b/libstdc++-v3/include/bits/stl_construct.h
index 9d6111396e1ce60577b5631e98a893bd49744cf7..6889a9bfa0e6e4b3387f7404f99db552d10c229b 100644 (file)
--- a/libstdc++-v3/include/bits/stl_construct.h
+++ b/libstdc++-v3/include/bits/stl_construct.h
@@ -166,6 +166,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif
     }
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // for if-constexpr
+
+#if __cplusplus < 201103L
   template<bool>
     struct _Destroy_aux
     {
@@ -185,6 +189,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         static void
         __destroy(_ForwardIterator, _ForwardIterator) { }
     };
+#endif
 
   /**
    * Destroy a range of objects.  If the value_type of the object has
@@ -201,15 +206,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // A deleted destructor is trivial, this ensures we reject such types:
       static_assert(is_destructible<_Value_type>::value,
                    "value type is destructible");
-#endif
+      if constexpr (!__has_trivial_destructor(_Value_type))
+       for (; __first != __last; ++__first)
+         std::_Destroy(std::__addressof(*__first));
 #if __cpp_constexpr_dynamic_alloc // >= C++20
-      if (std::__is_constant_evaluated())
-       return std::_Destroy_aux<false>::__destroy(__first, __last);
+      else if (std::__is_constant_evaluated())
+       for (; __first != __last; ++__first)
+         std::destroy_at(std::__addressof(*__first));
 #endif
+#else
       std::_Destroy_aux<__has_trivial_destructor(_Value_type)>::
        __destroy(__first, __last);
+#endif
     }
 
+#if __cplusplus < 201103L
   template<bool>
     struct _Destroy_n_aux
     {
@@ -234,6 +245,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          return __first;
        }
     };
+#endif
 
   /**
    * Destroy a range of objects.  If the value_type of the object has
@@ -250,14 +262,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // A deleted destructor is trivial, this ensures we reject such types:
       static_assert(is_destructible<_Value_type>::value,
                    "value type is destructible");
-#endif
+      if constexpr (!__has_trivial_destructor(_Value_type))
+       for (; __count > 0; (void)++__first, --__count)
+         std::_Destroy(std::__addressof(*__first));
 #if __cpp_constexpr_dynamic_alloc // >= C++20
-      if (std::__is_constant_evaluated())
-       return std::_Destroy_n_aux<false>::__destroy_n(__first, __count);
+      else if (std::__is_constant_evaluated())
+       for (; __count > 0; (void)++__first, --__count)
+         std::destroy_at(std::__addressof(*__first));
 #endif
+      else
+       std::advance(__first, __count);
+      return __first;
+#else
       return std::_Destroy_n_aux<__has_trivial_destructor(_Value_type)>::
        __destroy_n(__first, __count);
+#endif
     }
+#pragma GCC diagnostic pop
 
 #if __glibcxx_raw_memory_algorithms // >= C++17
   template <typename _ForwardIterator>