[libcxx] Optimize std::generate_n for segmented iterators

libcxx/docs/ReleaseNotes/22.rst

@@ -75,8 +75,8 @@ 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`` 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
 -------------------------

libcxx/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 @@ _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) {
-  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

libcxx/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 @@ test()
     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());