From c3d46672257c23686576c07f490c7d42df44517a Mon Sep 17 00:00:00 2001
From: c8ef <c8ef@outlook.com>
Date: Tue, 28 Oct 2025 00:56:38 +0800
Subject: [PATCH] [libcxx] Optimize rng::generate_n for segmented iterators

---
 libcxx/docs/ReleaseNotes/22.rst                |  5 +++--
 libcxx/include/__algorithm/generate_n.h        | 16 ++++++++++++++--
 libcxx/include/__algorithm/ranges_generate_n.h |  8 ++------
 3 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/libcxx/docs/ReleaseNotes/22.rst b/libcxx/docs/ReleaseNotes/22.rst
index 25d33a9c2eb50..980390c4fe3d7 100644
--- a/libcxx/docs/ReleaseNotes/22.rst
+++ b/libcxx/docs/ReleaseNotes/22.rst
@@ -76,8 +76,9 @@ Improvements and New Features
 - The ``std::{fill, fill_n}`` and ``std::ranges::{fill, fill_n}`` algorithms have been optimized for segmented iterators,
   resulting in a performance improvement of at least 10x for ``std::deque<int>`` iterators and
   ``std::join_view<std::vector<std::vector<int>>>`` iterators.
-- The ``std::generate`` and ``std::generate_n`` algorithms have been optimized for segmented iterators, resulting in a
-  performance improvement for ``std::deque<short>`` and ``std::join_view<vector<vector<short>>>`` iterators.
+- The ``std::{generate, generate_n}`` and ``std::ranges::generate_n`` algorithms have been optimized for segmented
+  iterators, resulting in a performance improvement for ``std::deque<short>`` and
+  ``std::join_view<vector<vector<short>>>`` iterators.
 
 Deprecations and Removals
 -------------------------
diff --git a/libcxx/include/__algorithm/generate_n.h b/libcxx/include/__algorithm/generate_n.h
index e9da133f0570a..23899e49e0b65 100644
--- a/libcxx/include/__algorithm/generate_n.h
+++ b/libcxx/include/__algorithm/generate_n.h
@@ -13,22 +13,34 @@
 #include <__config>
 #include <__functional/identity.h>
 #include <__utility/forward.h>
+#include <__utility/move.h>
 
 #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 _OutputIterator, class _Size, class _Generator>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator
-generate_n(_OutputIterator __first, _Size __orig_n, _Generator __gen) {
+__generate_n(_OutputIterator __first, _Size __orig_n, _Generator& __gen) {
   using __iter_ref = decltype(*__first);
   __identity __proj;
   auto __f = [&](__iter_ref __element) { std::forward<__iter_ref>(__element) = __gen(); };
-  return std::__for_each_n(__first, __orig_n, __f, __proj);
+  return std::__for_each_n(std::move(__first), __orig_n, __f, __proj);
+}
+
+template <class _OutputIterator, class _Size, class _Generator>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator
+generate_n(_OutputIterator __first, _Size __orig_n, _Generator __gen) {
+  return std::__generate_n(std::move(__first), __orig_n, __gen);
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif // _LIBCPP___ALGORITHM_GENERATE_N_H
diff --git a/libcxx/include/__algorithm/ranges_generate_n.h b/libcxx/include/__algorithm/ranges_generate_n.h
index a318994d0eaf8..0cc9ce7b1193b 100644
--- a/libcxx/include/__algorithm/ranges_generate_n.h
+++ b/libcxx/include/__algorithm/ranges_generate_n.h
@@ -9,6 +9,7 @@
 #ifndef _LIBCPP___ALGORITHM_RANGES_GENERATE_N_H
 #define _LIBCPP___ALGORITHM_RANGES_GENERATE_N_H
 
+#include <__algorithm/generate_n.h>
 #include <__concepts/constructible.h>
 #include <__concepts/invocable.h>
 #include <__config>
@@ -38,12 +39,7 @@ struct __generate_n {
     requires invocable<_Func&> && indirectly_writable<_OutIter, invoke_result_t<_Func&>>
   _LIBCPP_HIDE_FROM_ABI constexpr _OutIter
   operator()(_OutIter __first, iter_difference_t<_OutIter> __n, _Func __gen) const {
-    for (; __n > 0; --__n) {
-      *__first = __gen();
-      ++__first;
-    }
-
-    return __first;
+    return std::__generate_n(std::move(__first), __n, __gen);
   }
 };