From: Jonathan Wakely <jwakely@redhat.com>
Date: Tue, 16 Sep 2025 11:00:19 +0000 (+0100)
Subject: libstdc++: Optimize determination of std::tuple_cat return type
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=5d774ec80bcb633a393790154c9969218b7c782f;p=thirdparty%2Fgcc.git

libstdc++: Optimize determination of std::tuple_cat return type

The std::tuple_cat function has to determine a std::tuple return type
from zero or more tuple-like arguments. This uses the __make_tuple class
template to transform a tuple-like type into a std::tuple, and the
__combine_tuples class template to combine zero or more std::tuple types
into a single std::tuple type.

This change optimizes the __make_tuple class template to use an
_Index_tuple and pack expansion instead of recursive instantiation, and
optimizes __combine_tuples to use fewer levels of recursion.

For ranges::adjacent_view's __detail::__repeated_tuple helper we can
just use the __make_tuple class template directly, instead of doing
overload resolution on std::tuple_cat to get its return type.

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::__repeated_tuple): Use
	__make_tuple helper alias directly, instead of doing overload
	resolution on std::tuple_cat.
	* include/std/tuple (__make_tuple_impl): Remove.
	(__do_make_tuple): Replace recursion with _Index_tuple and pack
	expansion.
	(__make_tuple): Adjust to new __do_make_tuple definition.
	(__combine_tuples<tuple<T1s...>, tuple<T2s...>, Rem...>): Replace
	with a partial specialization for exactly two tuples and a
	partial specialization for three or more tuples.

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

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 1c76dcc85ac..fd290ea362c 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -5383,7 +5383,7 @@ namespace views::__adaptor
   {
     // Yields tuple<_Tp, ..., _Tp> with _Nm elements.
     template<typename _Tp, size_t _Nm>
-      using __repeated_tuple = decltype(std::tuple_cat(std::declval<array<_Tp, _Nm>>()));
+      using __repeated_tuple = typename __make_tuple<array<_Tp, _Nm>>::__type;
 
     // For a functor F that is callable with N arguments, the expression
     // declval<__unarize<F, N>>(x) is equivalent to declval<F>(x, ..., x).
diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 2e6499eab22..0ca616f1b4f 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -2681,54 +2681,53 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return tuple<_Elements&&...>(std::forward<_Elements>(__args)...); }
 
   /// @cond undocumented
-  template<size_t, typename, typename, size_t>
-    struct __make_tuple_impl;
-
-  template<size_t _Idx, typename _Tuple, typename... _Tp, size_t _Nm>
-    struct __make_tuple_impl<_Idx, tuple<_Tp...>, _Tuple, _Nm>
-    : __make_tuple_impl<_Idx + 1,
-			tuple<_Tp..., __tuple_element_t<_Idx, _Tuple>>,
-			_Tuple, _Nm>
-    { };
+  template<typename _Tuple, typename _Idx_tuple>
+    struct __do_make_tuple;
 
-  template<size_t _Nm, typename _Tuple, typename... _Tp>
-    struct __make_tuple_impl<_Nm, tuple<_Tp...>, _Tuple, _Nm>
+  template<typename _Tuple, size_t... _Idx>
+    struct __do_make_tuple<_Tuple, _Index_tuple<_Idx...>>
     {
-      typedef tuple<_Tp...> __type;
+      using __type = tuple<__tuple_element_t<_Idx, _Tuple>...>;
     };
 
-  template<typename _Tuple>
-    struct __do_make_tuple
-    : __make_tuple_impl<0, tuple<>, _Tuple, tuple_size<_Tuple>::value>
-    { };
-
   // Returns the std::tuple equivalent of a tuple-like type.
-  template<typename _Tuple>
+  template<typename _Tuple,
+	   typename _Tup = __remove_cvref_t<_Tuple>,
+	   typename _Indices = _Build_index_tuple<tuple_size<_Tup>::value>>
     struct __make_tuple
-    : public __do_make_tuple<__remove_cvref_t<_Tuple>>
+    : __do_make_tuple<_Tup, typename _Indices::__type>
     { };
 
-  // Combines several std::tuple's into a single one.
+  // Combines several std::tuple types into a single one.
   template<typename...>
     struct __combine_tuples;
 
   template<>
     struct __combine_tuples<>
     {
-      typedef tuple<> __type;
+      using __type = tuple<>;
     };
 
   template<typename... _Ts>
     struct __combine_tuples<tuple<_Ts...>>
     {
-      typedef tuple<_Ts...> __type;
+      using __type = tuple<_Ts...>;
+    };
+
+  template<typename... _T1s, typename... _T2s>
+    struct __combine_tuples<tuple<_T1s...>, tuple<_T2s...>>
+    {
+      using __type = tuple<_T1s..., _T2s...>;
     };
 
-  template<typename... _T1s, typename... _T2s, typename... _Rem>
-    struct __combine_tuples<tuple<_T1s...>, tuple<_T2s...>, _Rem...>
+  template<typename... _T1s, typename... _T2s, typename... _T3s,
+	   typename... _Rem>
+    struct __combine_tuples<tuple<_T1s...>, tuple<_T2s...>, tuple<_T3s...>,
+			    _Rem...>
     {
-      typedef typename __combine_tuples<tuple<_T1s..., _T2s...>,
-					_Rem...>::__type __type;
+      using _First = tuple<_T1s..., _T2s..., _T3s...>;
+      using _Second = typename __combine_tuples<_Rem...>::__type;
+      using __type = typename __combine_tuples<_First, _Second>::__type;
     };
 
   // Computes the result type of tuple_cat given a set of tuple-like types.