[libcxx] Optimize `std::generate_n` for segmented iterators (#164266)

Part of #102817.

This is a natural follow-up to #163006. We are forwarding
`std::generate_n` to `std::__for_each_n` (`std::for_each_n` needs
c++17), resulting in improved performance for segmented iterators.

before:

```
std::generate_n(deque<int>)/32          17.5 ns         17.3 ns     40727273
std::generate_n(deque<int>)/50          25.7 ns         25.5 ns     26352941
std::generate_n(deque<int>)/1024         490 ns          487 ns      1445161
std::generate_n(deque<int>)/8192        3908 ns         3924 ns       179200
```

after:

```
std::generate_n(deque<int>)/32          11.1 ns         11.0 ns     64000000
std::generate_n(deque<int>)/50          16.1 ns         16.0 ns     44800000
std::generate_n(deque<int>)/1024         291 ns          292 ns      2357895
std::generate_n(deque<int>)/8192        2269 ns         2250 ns       298667
```
GitOrigin-RevId: c06ae43e26aa5cd472d0b25d5904c44d20e84067

docs/ReleaseNotes/22.rst

diff --git a/docs/ReleaseNotes/22.rst b/docs/ReleaseNotes/22.rst
index e95cbc0..25d33a9 100644
--- a/docs/ReleaseNotes/22.rst
+++ b/docs/ReleaseNotes/22.rst
@@ -76,8 +76,8 @@
 - 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`` algorithm has 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`` 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.
 
 Deprecations and Removals
 -------------------------

include/__algorithm/generate_n.h

diff --git a/include/__algorithm/generate_n.h b/include/__algorithm/generate_n.h
index f36403f..e9da133 100644
--- a/include/__algorithm/generate_n.h
+++ b/include/__algorithm/generate_n.h
@@ -9,8 +9,10 @@
 #ifndef _LIBCPP___ALGORITHM_GENERATE_N_H
 #define _LIBCPP___ALGORITHM_GENERATE_N_H
 
+#include <__algorithm/for_each_n.h>
 #include <__config>
-#include <__utility/convert_to_integral.h>
+#include <__functional/identity.h>
+#include <__utility/forward.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -21,11 +23,10 @@
 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) {
-  typedef decltype(std::__convert_to_integral(__orig_n)) _IntegralSize;
-  _IntegralSize __n = __orig_n;
-  for (; __n > 0; ++__first, (void)--__n)
-    *__first = __gen();
-  return __first;
+  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);
 }
 
 _LIBCPP_END_NAMESPACE_STD

test/std/algorithms/alg.modifying.operations/alg.generate/generate_n.pass.cpp

diff --git a/test/std/algorithms/alg.modifying.operations/alg.generate/generate_n.pass.cpp b/test/std/algorithms/alg.modifying.operations/alg.generate/generate_n.pass.cpp
index 13fd1cb..525737c 100644
--- a/test/std/algorithms/alg.modifying.operations/alg.generate/generate_n.pass.cpp
+++ b/test/std/algorithms/alg.modifying.operations/alg.generate/generate_n.pass.cpp
@@ -16,6 +16,7 @@
 
 #include <algorithm>
 #include <cassert>
+#include <deque>
 
 #include "test_iterators.h"
 #include "test_macros.h"
@@ -71,12 +72,22 @@
     test2<Iter, long double>();
 }
 
+void deque_test() {
+  int sizes[] = {0, 1, 2, 1023, 1024, 1025, 2047, 2048, 2049};
+  for (const int size : sizes) {
+    std::deque<int> d(size);
+    std::generate_n(d.begin(), size, gen_test());
+    assert(std::all_of(d.begin(), d.end(), [](int x) { return x == 2; }));
+  }
+}
+
 int main(int, char**)
 {
     test<forward_iterator<int*> >();
     test<bidirectional_iterator<int*> >();
     test<random_access_iterator<int*> >();
     test<int*>();
+    deque_test();
 
 #if TEST_STD_VER > 17
     static_assert(test_constexpr());