[libcxx] Optimize rng::generate_n for segmented iterators (#165280)

Part of #102817.

This patch optimizes `rng::generate_n` for segmented iterators by
forwarding the implementation directly to `std::generate_n`.

- before

```
rng::generate_n(deque<int>)/32          21.7 ns         22.0 ns     32000000
rng::generate_n(deque<int>)/50          30.8 ns         30.7 ns     22400000
rng::generate_n(deque<int>)/1024         492 ns          488 ns      1120000
rng::generate_n(deque<int>)/8192        3938 ns         3924 ns       179200
```

- after

```
rng::generate_n(deque<int>)/32          11.0 ns         11.0 ns     64000000
rng::generate_n(deque<int>)/50          16.2 ns         16.1 ns     40727273
rng::generate_n(deque<int>)/1024         292 ns          286 ns      2240000
rng::generate_n(deque<int>)/8192        2291 ns         2302 ns       298667
```

Author: c8ef

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
 -------------------------

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

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);
   }
 };