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

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https://github.com/c8ef updated https://github.com/llvm/llvm-project/pull/164266

>From 42fb027bb64909f5427c71f72def909a64d6a64c Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Mon, 20 Oct 2025 23:01:34 +0800
Subject: [PATCH 1/3] [libcxx] Optimize std::generate_n for segmented iterators

---
 libcxx/docs/ReleaseNotes/22.rst                       |  4 ++--
 libcxx/include/__algorithm/generate_n.h               | 11 +++++------
 .../alg.generate/generate_n.pass.cpp                  | 11 +++++++++++
 3 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/libcxx/docs/ReleaseNotes/22.rst b/libcxx/docs/ReleaseNotes/22.rst
index ada8b413e1259..a7a079e930959 100644
--- a/libcxx/docs/ReleaseNotes/22.rst
+++ b/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
 -------------------------
diff --git a/libcxx/include/__algorithm/generate_n.h b/libcxx/include/__algorithm/generate_n.h
index f36403fd0f94a..7c73620bd08f5 100644
--- a/libcxx/include/__algorithm/generate_n.h
+++ b/libcxx/include/__algorithm/generate_n.h
@@ -9,8 +9,8 @@
 #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>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -21,11 +21,10 @@ _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);
+  return std::for_each_n(__first, __orig_n, [&](__iter_ref __element) {
+    std::forward<__iter_ref>(__element) = __gen();
+  });
 }
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.generate/generate_n.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.generate/generate_n.pass.cpp
index 13fd1cbf1f33c..525737ceacfbf 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.generate/generate_n.pass.cpp
+++ b/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());

>From e103e9c2ec11b9795d7f4da3677c43e8379958f8 Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Mon, 20 Oct 2025 23:24:25 +0800
Subject: [PATCH 2/3] [libcxx] Optimize std::generate_n for segmented iterators

---
 libcxx/include/__algorithm/generate_n.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/libcxx/include/__algorithm/generate_n.h b/libcxx/include/__algorithm/generate_n.h
index 7c73620bd08f5..48decaade35a0 100644
--- a/libcxx/include/__algorithm/generate_n.h
+++ b/libcxx/include/__algorithm/generate_n.h
@@ -11,6 +11,7 @@
 
 #include <__algorithm/for_each_n.h>
 #include <__config>
+#include <__functional/identity.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -22,9 +23,9 @@ 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) {
   using __iter_ref = decltype(*__first);
-  return std::for_each_n(__first, __orig_n, [&](__iter_ref __element) {
-    std::forward<__iter_ref>(__element) = __gen();
-  });
+  __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

>From b9f510d948ad845199ddabd9b2e5ece2ce5820ac Mon Sep 17 00:00:00 2001
From: c8ef <c8ef at outlook.com>
Date: Mon, 20 Oct 2025 23:34:38 +0800
Subject: [PATCH 3/3] [libcxx] Optimize std::generate_n for segmented iterators

---
 libcxx/include/__algorithm/generate_n.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/libcxx/include/__algorithm/generate_n.h b/libcxx/include/__algorithm/generate_n.h
index 48decaade35a0..e9da133f0570a 100644
--- a/libcxx/include/__algorithm/generate_n.h
+++ b/libcxx/include/__algorithm/generate_n.h
@@ -12,6 +12,7 @@
 #include <__algorithm/for_each_n.h>
 #include <__config>
 #include <__functional/identity.h>
+#include <__utility/forward.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header