[gcc r13-10323] libstdc++: Simplify std::_Destroy using 'if constexpr'

[Jonathan Wakely]

https://gcc.gnu.org/g:8f61b86eac661c623979440ab026d75c9c0c7b3c

commit r13-10323-g8f61b86eac661c623979440ab026d75c9c0c7b3c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Nov 28 12:32:59 2024 +0000

    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>
    (cherry picked from commit 1467409beb27a693e96994899f30e4db015b5c2c)

Diff:
---
 libstdc++-v3/include/bits/stl_construct.h | 33 +++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_construct.h b/libstdc++-v3/include/bits/stl_construct.h
index 3403c3a4aeed..6ce0cdfa2869 100644
--- a/libstdc++-v3/include/bits/stl_construct.h
+++ b/libstdc++-v3/include/bits/stl_construct.h
@@ -152,6 +152,10 @@ _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
     {
@@ -171,6 +175,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         static void
         __destroy(_ForwardIterator, _ForwardIterator) { }
     };
+#endif
 
   /**
    * Destroy a range of objects.  If the value_type of the object has
@@ -187,15 +192,21 @@ _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
     {
@@ -220,6 +231,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  return __first;
 	}
     };
+#endif
 
   /**
    * Destroy a range of objects.  If the value_type of the object has
@@ -236,14 +248,23 @@ _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 __cplusplus >= 201703L
   template <typename _ForwardIterator>