[libcxx-commits] [libcxx] [libc++] Optimize {std, ranges}::{fill, fill_n} for segmented iterators (PR #132665)

Peng Liu via libcxx-commits libcxx-commits at lists.llvm.org
Mon Mar 24 18:12:00 PDT 2025


https://github.com/winner245 updated https://github.com/llvm/llvm-project/pull/132665

>From 86092be2f55aebb64594a82e7c63799ce8988083 Mon Sep 17 00:00:00 2001
From: Peng Liu <winner245 at hotmail.com>
Date: Sun, 23 Mar 2025 22:28:29 -0400
Subject: [PATCH 1/2] Optimize {std,ranges}::{fill,fill_n} for segmented
 iterators

---
 libcxx/include/CMakeLists.txt                 |  1 +
 libcxx/include/__algorithm/fill.h             | 40 ++++++++++++++--
 libcxx/include/__algorithm/fill_n.h           | 30 ++++++++----
 libcxx/include/__fwd/fill.h                   | 46 +++++++++++++++++++
 libcxx/include/module.modulemap               |  1 +
 .../alg.fill/fill.pass.cpp                    | 26 +++++++++++
 .../alg.fill/fill_n.pass.cpp                  | 26 +++++++++++
 .../alg.fill/ranges.fill.pass.cpp             | 22 +++++++++
 .../alg.fill/ranges.fill_n.pass.cpp           | 22 +++++++++
 libcxx/utils/generate_iwyu_mapping.py         |  2 +
 10 files changed, 202 insertions(+), 14 deletions(-)
 create mode 100644 libcxx/include/__fwd/fill.h

diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index a021b9bb44d67..1ac582dcc1614 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -435,6 +435,7 @@ set(files
   __fwd/byte.h
   __fwd/complex.h
   __fwd/deque.h
+  __fwd/fill.h
   __fwd/format.h
   __fwd/fstream.h
   __fwd/functional.h
diff --git a/libcxx/include/__algorithm/fill.h b/libcxx/include/__algorithm/fill.h
index 1ce3eadb013d0..395b6a5a1e358 100644
--- a/libcxx/include/__algorithm/fill.h
+++ b/libcxx/include/__algorithm/fill.h
@@ -10,8 +10,11 @@
 #define _LIBCPP___ALGORITHM_FILL_H
 
 #include <__algorithm/fill_n.h>
+#include <__algorithm/for_each_segment.h>
 #include <__config>
 #include <__iterator/iterator_traits.h>
+#include <__iterator/segmented_iterator.h>
+#include <__type_traits/enable_if.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -21,23 +24,50 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 // fill isn't specialized for std::memset, because the compiler already optimizes the loop to a call to std::memset.
 
-template <class _ForwardIterator, class _Tp>
+template <class _ForwardIterator,
+          class _Sentinel,
+          class _Tp,
+          __enable_if_t<__has_forward_iterator_category<_ForwardIterator>::value, int> >
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__fill(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value, forward_iterator_tag) {
+__fill(_ForwardIterator __first, _Sentinel __last, const _Tp& __value) {
   for (; __first != __last; ++__first)
     *__first = __value;
 }
 
-template <class _RandomAccessIterator, class _Tp>
+template <class _OutIter, class _Tp>
+struct _FillSegment {
+  using _Traits _LIBCPP_NODEBUG = __segmented_iterator_traits<_OutIter>;
+
+  const _Tp& __value_;
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit _FillSegment(const _Tp& __value) : __value_(__value) {}
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
+  operator()(typename _Traits::__local_iterator __lfirst, typename _Traits::__local_iterator __llast) {
+    std::__fill(__lfirst, __llast, __value_);
+  }
+};
+
+template <class _RandomAccessIterator,
+          class _Tp,
+          __enable_if_t<__has_random_access_iterator_category<_RandomAccessIterator>::value &&
+                            !__is_segmented_iterator<_RandomAccessIterator>::value,
+                        int> >
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__fill(_RandomAccessIterator __first, _RandomAccessIterator __last, const _Tp& __value, random_access_iterator_tag) {
+__fill(_RandomAccessIterator __first, _RandomAccessIterator __last, const _Tp& __value) {
   std::fill_n(__first, __last - __first, __value);
 }
 
+template <class _SegmentedIterator, class _Tp, __enable_if_t<__is_segmented_iterator<_SegmentedIterator>::value, int> >
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+__fill(_SegmentedIterator __first, _SegmentedIterator __last, const _Tp& __value) {
+  std::__for_each_segment(__first, __last, _FillSegment<_SegmentedIterator, _Tp>(__value));
+}
+
 template <class _ForwardIterator, class _Tp>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
 fill(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value) {
-  std::__fill(__first, __last, __value, typename iterator_traits<_ForwardIterator>::iterator_category());
+  std::__fill(__first, __last, __value);
 }
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__algorithm/fill_n.h b/libcxx/include/__algorithm/fill_n.h
index 0da78e1f38c4c..72fd8d0f9a988 100644
--- a/libcxx/include/__algorithm/fill_n.h
+++ b/libcxx/include/__algorithm/fill_n.h
@@ -12,7 +12,12 @@
 #include <__algorithm/min.h>
 #include <__config>
 #include <__fwd/bit_reference.h>
+#include <__fwd/fill.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/next.h>
+#include <__iterator/segmented_iterator.h>
 #include <__memory/pointer_traits.h>
+#include <__type_traits/enable_if.h>
 #include <__utility/convert_to_integral.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -26,9 +31,13 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 // fill_n isn't specialized for std::memset, because the compiler already optimizes the loop to a call to std::memset.
 
-template <class _OutputIterator, class _Size, class _Tp>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator
-__fill_n(_OutputIterator __first, _Size __n, const _Tp& __value);
+template <class _OutIter, class _Size, class _Tp, __enable_if_t<!__is_segmented_iterator<_OutIter>::value, int> = 0>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutIter
+__fill_n(_OutIter __first, _Size __n, const _Tp& __value) {
+  for (; __n > 0; ++__first, (void)--__n)
+    *__first = __value;
+  return __first;
+}
 
 template <bool _FillVal, class _Cp>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
@@ -68,12 +77,15 @@ __fill_n(__bit_iterator<_Cp, false> __first, _Size __n, const bool& __value) {
   return __first + __n;
 }
 
-template <class _OutputIterator, class _Size, class _Tp>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator
-__fill_n(_OutputIterator __first, _Size __n, const _Tp& __value) {
-  for (; __n > 0; ++__first, (void)--__n)
-    *__first = __value;
-  return __first;
+template < class _OutIter,
+           class _Size,
+           class _Tp,
+           __enable_if_t<__is_segmented_iterator<_OutIter>::value && __has_forward_iterator_category<_OutIter>::value,
+                         int> = 0>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _OutIter __fill_n(_OutIter __first, _Size __n, const _Tp& __value) {
+  _OutIter __last = std::next(__first, __n);
+  std::__fill(__first, __last, __value);
+  return __last;
 }
 
 template <class _OutputIterator, class _Size, class _Tp>
diff --git a/libcxx/include/__fwd/fill.h b/libcxx/include/__fwd/fill.h
new file mode 100644
index 0000000000000..1c13229f6c4de
--- /dev/null
+++ b/libcxx/include/__fwd/fill.h
@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FWD_FILL_H
+#define _LIBCPP___FWD_FILL_H
+
+#include <__config>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/segmented_iterator.h>
+#include <__type_traits/enable_if.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _ForwardIterator,
+          class _Sentinel,
+          class _Tp,
+          __enable_if_t<__has_forward_iterator_category<_ForwardIterator>::value, int> = 0>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+__fill(_ForwardIterator __first, _Sentinel __last, const _Tp& __value);
+
+template <class _RandomAccessIterator,
+          class _Tp,
+          __enable_if_t<__has_random_access_iterator_category<_RandomAccessIterator>::value &&
+                            !__is_segmented_iterator<_RandomAccessIterator>::value,
+                        int> = 0>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+__fill(_RandomAccessIterator __first, _RandomAccessIterator __last, const _Tp& __value);
+
+template <class _SegmentedIterator,
+          class _Tp,
+          __enable_if_t<__is_segmented_iterator<_SegmentedIterator>::value, int> = 0>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+__fill(_SegmentedIterator __first, _SegmentedIterator __last, const _Tp& __value);
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___FWD_FILL_H
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
index b42f945a6832c..422dbf802fe30 100644
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -435,6 +435,7 @@ module std [system] {
     module for_each_n                             { header "__algorithm/for_each_n.h" }
     module for_each_segment                       { header "__algorithm/for_each_segment.h" }
     module for_each                               { header "__algorithm/for_each.h" }
+    module fwd                                    { header "__fwd/fill.h" }
     module generate_n                             { header "__algorithm/generate_n.h" }
     module generate                               { header "__algorithm/generate.h" }
     module half_positive                          { header "__algorithm/half_positive.h" }
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill.pass.cpp
index 9b403db85ebf9..0b87f9038e618 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill.pass.cpp
@@ -17,6 +17,8 @@
 #include <array>
 #include <cassert>
 #include <cstddef>
+#include <deque>
+#include <ranges>
 #include <vector>
 
 #include "sized_allocator.h"
@@ -93,6 +95,27 @@ TEST_CONSTEXPR_CXX20 bool test_vector_bool(std::size_t N) {
   return true;
 }
 
+/*TEST_CONSTEXPR_CXX23*/ void
+test_segmented_iterator() { // TODO: Mark this test as TEST_CONSTEXPR_CXX23 when std::deque is constexpr
+  {                         // std::deque iterator
+    std::deque<int> in(20);
+    std::deque<int> expected(in.size(), 42);
+    std::fill(in.begin(), in.end(), 42);
+    assert(in == expected);
+  }
+
+#if TEST_STD_VER >= 20
+  { // join_view iterator
+    std::vector<std::vector<int>> v{{1, 2}, {1, 2, 3}, {0, 0}, {3, 4, 5}, {6}, {7, 8, 9, 6}, {0, 1, 2, 3, 0, 1, 2}};
+    auto jv = std::ranges::join_view(v);
+    std::fill(jv.begin(), jv.end(), 42);
+    for (const auto& vec : v)
+      for (auto n : vec)
+        assert(n == 42);
+  }
+#endif
+}
+
 TEST_CONSTEXPR_CXX20 bool test() {
   types::for_each(types::forward_iterator_list<char*>(), Test<char>());
   types::for_each(types::forward_iterator_list<int*>(), Test<int>());
@@ -138,6 +161,9 @@ TEST_CONSTEXPR_CXX20 bool test() {
     }
   }
 
+  if (!TEST_IS_CONSTANT_EVALUATED) // TODO: Use TEST_STD_AT_LEAST_23_OR_RUNTIME_EVALUATED when std::deque is made constexpr
+    test_segmented_iterator();
+
   return true;
 }
 
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill_n.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill_n.pass.cpp
index 4dda8714d2cfa..3d397bc6c9f06 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill_n.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/fill_n.pass.cpp
@@ -17,6 +17,8 @@
 #include <array>
 #include <cassert>
 #include <cstddef>
+#include <deque>
+#include <ranges>
 #include <vector>
 
 #include "sized_allocator.h"
@@ -176,6 +178,27 @@ TEST_CONSTEXPR_CXX20 void test_struct_array() {
   }
 }
 
+/*TEST_CONSTEXPR_CXX23*/ void
+test_segmented_iterator() { // TODO: Mark this test as TEST_CONSTEXPR_CXX23 when std::deque is constexpr
+  {                         // std::deque iterator
+    std::deque<int> in(20);
+    std::deque<int> expected(in.size(), 42);
+    std::fill_n(in.begin(), in.size(), 42);
+    assert(in == expected);
+  }
+
+#if TEST_STD_VER >= 20
+  { // join_view iterator
+    std::vector<std::vector<int>> v{{1, 2}, {1, 2, 3}, {0, 0}, {3, 4, 5}, {6}, {7, 8, 9, 6}, {0, 1, 2, 3, 0, 1, 2}};
+    auto jv = std::ranges::join_view(v);
+    std::fill_n(jv.begin(), std::distance(jv.begin(), jv.end()), 42);
+    for (const auto& vec : v)
+      for (auto n : vec)
+        assert(n == 42);
+  }
+#endif
+}
+
 TEST_CONSTEXPR_CXX20 bool test() {
   types::for_each(types::forward_iterator_list<char*>(), Test<char>());
   types::for_each(types::forward_iterator_list<int*>(), Test<int>());
@@ -225,6 +248,9 @@ TEST_CONSTEXPR_CXX20 bool test() {
     }
   }
 
+  if (!TEST_IS_CONSTANT_EVALUATED) // TODO: Use TEST_STD_AT_LEAST_23_OR_RUNTIME_EVALUATED when std::deque is made constexpr
+    test_segmented_iterator();
+
   return true;
 }
 
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill.pass.cpp
index 61a659fb0028c..e610fd3c3cb06 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill.pass.cpp
@@ -18,6 +18,7 @@
 #include <algorithm>
 #include <array>
 #include <cassert>
+#include <deque>
 #include <ranges>
 #include <string>
 #include <vector>
@@ -128,6 +129,24 @@ constexpr bool test_vector_bool(std::size_t N) {
 }
 #endif
 
+/*TEST_CONSTEXPR_CXX23*/ void
+test_segmented_range() { // TODO: Mark this test as TEST_CONSTEXPR_CXX23 when std::deque is constexpr
+  {                      // std::deque
+    std::deque<int> in(20);
+    std::deque<int> expected(in.size(), 42);
+    std::ranges::fill(in, 42);
+    assert(in == expected);
+  }
+  { // join_view
+    std::vector<std::vector<int>> v{{1, 2}, {1, 2, 3}, {0, 0}, {3, 4, 5}, {6}, {7, 8, 9, 6}, {0, 1, 2, 3, 0, 1, 2}};
+    auto jv = std::ranges::join_view(v);
+    std::ranges::fill(jv, 42);
+    for (const auto& vec : v)
+      for (auto n : vec)
+        assert(n == 42);
+  }
+}
+
 constexpr bool test() {
   test_iterators<cpp17_output_iterator<int*>, sentinel_wrapper<cpp17_output_iterator<int*>>>();
   test_iterators<cpp20_output_iterator<int*>, sentinel_wrapper<cpp20_output_iterator<int*>>>();
@@ -227,6 +246,9 @@ constexpr bool test() {
   }
 #endif
 
+  if (!TEST_IS_CONSTANT_EVALUATED) // TODO: Use TEST_STD_AT_LEAST_23_OR_RUNTIME_EVALUATED when std::deque is made constexpr
+    test_segmented_range();
+
   return true;
 }
 
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill_n.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill_n.pass.cpp
index 2d6e24a03e0b3..2c7b0fe091c04 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill_n.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.fill/ranges.fill_n.pass.cpp
@@ -16,6 +16,7 @@
 #include <algorithm>
 #include <array>
 #include <cassert>
+#include <deque>
 #include <ranges>
 #include <string>
 #include <vector>
@@ -101,6 +102,24 @@ constexpr bool test_vector_bool(std::size_t N) {
 }
 #endif
 
+/*TEST_CONSTEXPR_CXX23*/ void
+test_segmented_range() { // TODO: Mark this test as TEST_CONSTEXPR_CXX23 when std::deque is constexpr
+  {                      // std::deque
+    std::deque<int> in(20);
+    std::deque<int> expected(in.size(), 42);
+    std::ranges::fill_n(std::ranges::begin(in), std::ranges::size(in), 42);
+    assert(in == expected);
+  }
+  { // join_view
+    std::vector<std::vector<int>> v{{1, 2}, {1, 2, 3}, {0, 0}, {3, 4, 5}, {6}, {7, 8, 9, 6}, {0, 1, 2, 3, 0, 1, 2}};
+    auto jv = std::ranges::join_view(v);
+    std::ranges::fill_n(std::ranges::begin(jv), std::ranges::distance(jv), 42);
+    for (const auto& vec : v)
+      for (auto n : vec)
+        assert(n == 42);
+  }
+}
+
 constexpr bool test() {
   test_iterators<cpp17_output_iterator<int*>, sentinel_wrapper<cpp17_output_iterator<int*>>>();
   test_iterators<cpp20_output_iterator<int*>, sentinel_wrapper<cpp20_output_iterator<int*>>>();
@@ -175,6 +194,9 @@ constexpr bool test() {
   }
 #endif
 
+  if (!TEST_IS_CONSTANT_EVALUATED) // TODO: Use TEST_STD_AT_LEAST_23_OR_RUNTIME_EVALUATED when std::deque is made constexpr
+    test_segmented_range();
+
   return true;
 }
 
diff --git a/libcxx/utils/generate_iwyu_mapping.py b/libcxx/utils/generate_iwyu_mapping.py
index ab3e3f14e14d8..bf7cdfd4bfb08 100644
--- a/libcxx/utils/generate_iwyu_mapping.py
+++ b/libcxx/utils/generate_iwyu_mapping.py
@@ -41,6 +41,8 @@ def IWYU_mapping(header: str) -> typing.Optional[typing.List[str]]:
         return ["atomic", "mutex", "semaphore", "thread"]
     elif header == "__tree":
         return ["map", "set"]
+    elif header == "__fwd/fill.h":
+        return ["algorithm"]
     elif header == "__fwd/byte.h":
         return ["cstddef"]
     elif header == "__fwd/pair.h":

>From d89bca5a648f57266cbd725332fcef9ce2937f3f Mon Sep 17 00:00:00 2001
From: Peng Liu <winner245 at hotmail.com>
Date: Mon, 24 Mar 2025 20:30:47 -0400
Subject: [PATCH 2/2] Fix circular reference

---
 libcxx/include/CMakeLists.txt                 |   2 +-
 libcxx/include/__algorithm/fill.h             |  46 +-----
 .../include/__algorithm/fill_fill_n_common.h  | 155 ++++++++++++++++++
 libcxx/include/__algorithm/fill_n.h           |  71 +-------
 libcxx/include/__algorithm/ranges_fill_n.h    |   2 +-
 libcxx/include/__fwd/fill.h                   |  46 ------
 libcxx/include/module.modulemap               |   2 +-
 libcxx/utils/generate_iwyu_mapping.py         |   2 -
 8 files changed, 160 insertions(+), 166 deletions(-)
 create mode 100644 libcxx/include/__algorithm/fill_fill_n_common.h
 delete mode 100644 libcxx/include/__fwd/fill.h

diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index 1ac582dcc1614..0ab06d02a1718 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -16,6 +16,7 @@ set(files
   __algorithm/equal.h
   __algorithm/equal_range.h
   __algorithm/fill.h
+  __algorithm/fill_fill_n_common.h
   __algorithm/fill_n.h
   __algorithm/find.h
   __algorithm/find_end.h
@@ -435,7 +436,6 @@ set(files
   __fwd/byte.h
   __fwd/complex.h
   __fwd/deque.h
-  __fwd/fill.h
   __fwd/format.h
   __fwd/fstream.h
   __fwd/functional.h
diff --git a/libcxx/include/__algorithm/fill.h b/libcxx/include/__algorithm/fill.h
index 395b6a5a1e358..1f7890ceb336c 100644
--- a/libcxx/include/__algorithm/fill.h
+++ b/libcxx/include/__algorithm/fill.h
@@ -9,12 +9,8 @@
 #ifndef _LIBCPP___ALGORITHM_FILL_H
 #define _LIBCPP___ALGORITHM_FILL_H
 
-#include <__algorithm/fill_n.h>
-#include <__algorithm/for_each_segment.h>
+#include <__algorithm/fill_fill_n_common.h>
 #include <__config>
-#include <__iterator/iterator_traits.h>
-#include <__iterator/segmented_iterator.h>
-#include <__type_traits/enable_if.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -24,46 +20,6 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 // fill isn't specialized for std::memset, because the compiler already optimizes the loop to a call to std::memset.
 
-template <class _ForwardIterator,
-          class _Sentinel,
-          class _Tp,
-          __enable_if_t<__has_forward_iterator_category<_ForwardIterator>::value, int> >
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__fill(_ForwardIterator __first, _Sentinel __last, const _Tp& __value) {
-  for (; __first != __last; ++__first)
-    *__first = __value;
-}
-
-template <class _OutIter, class _Tp>
-struct _FillSegment {
-  using _Traits _LIBCPP_NODEBUG = __segmented_iterator_traits<_OutIter>;
-
-  const _Tp& __value_;
-
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit _FillSegment(const _Tp& __value) : __value_(__value) {}
-
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
-  operator()(typename _Traits::__local_iterator __lfirst, typename _Traits::__local_iterator __llast) {
-    std::__fill(__lfirst, __llast, __value_);
-  }
-};
-
-template <class _RandomAccessIterator,
-          class _Tp,
-          __enable_if_t<__has_random_access_iterator_category<_RandomAccessIterator>::value &&
-                            !__is_segmented_iterator<_RandomAccessIterator>::value,
-                        int> >
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__fill(_RandomAccessIterator __first, _RandomAccessIterator __last, const _Tp& __value) {
-  std::fill_n(__first, __last - __first, __value);
-}
-
-template <class _SegmentedIterator, class _Tp, __enable_if_t<__is_segmented_iterator<_SegmentedIterator>::value, int> >
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__fill(_SegmentedIterator __first, _SegmentedIterator __last, const _Tp& __value) {
-  std::__for_each_segment(__first, __last, _FillSegment<_SegmentedIterator, _Tp>(__value));
-}
-
 template <class _ForwardIterator, class _Tp>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
 fill(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value) {
diff --git a/libcxx/include/__algorithm/fill_fill_n_common.h b/libcxx/include/__algorithm/fill_fill_n_common.h
new file mode 100644
index 0000000000000..90ebbf7123cdb
--- /dev/null
+++ b/libcxx/include/__algorithm/fill_fill_n_common.h
@@ -0,0 +1,155 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ALGORITHM_FILL_FILL_N_COMMON_H
+#define _LIBCPP___ALGORITHM_FILL_FILL_N_COMMON_H
+
+#include <__algorithm/for_each_segment.h>
+#include <__algorithm/min.h>
+#include <__config>
+#include <__fwd/bit_reference.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/next.h>
+#include <__iterator/segmented_iterator.h>
+#include <__memory/pointer_traits.h>
+#include <__type_traits/enable_if.h>
+#include <__utility/convert_to_integral.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 _ForwardIterator,
+          class _Sentinel,
+          class _Tp,
+          __enable_if_t<__has_forward_iterator_category<_ForwardIterator>::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+__fill(_ForwardIterator __first, _Sentinel __last, const _Tp& __value);
+
+template <class _RandomAccessIterator,
+          class _Tp,
+          __enable_if_t<__has_random_access_iterator_category<_RandomAccessIterator>::value &&
+                            !__is_segmented_iterator<_RandomAccessIterator>::value,
+                        int> = 0>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+__fill(_RandomAccessIterator __first, _RandomAccessIterator __last, const _Tp& __value);
+
+template <class _SegmentedIterator,
+          class _Tp,
+          __enable_if_t<__is_segmented_iterator<_SegmentedIterator>::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+__fill(_SegmentedIterator __first, _SegmentedIterator __last, const _Tp& __value);
+
+template <class _OutIter, class _Size, class _Tp, __enable_if_t<!__is_segmented_iterator<_OutIter>::value, int> = 0>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutIter
+__fill_n(_OutIter __first, _Size __n, const _Tp& __value) {
+  for (; __n > 0; ++__first, (void)--__n)
+    *__first = __value;
+  return __first;
+}
+
+template <bool _FillVal, class _Cp>
+_LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
+__fill_n_bool(__bit_iterator<_Cp, false> __first, typename __size_difference_type_traits<_Cp>::size_type __n) {
+  using _It            = __bit_iterator<_Cp, false>;
+  using __storage_type = typename _It::__storage_type;
+
+  const int __bits_per_word = _It::__bits_per_word;
+  // do first partial word
+  if (__first.__ctz_ != 0) {
+    __storage_type __clz_f = static_cast<__storage_type>(__bits_per_word - __first.__ctz_);
+    __storage_type __dn    = std::min(__clz_f, __n);
+    std::__fill_masked_range(std::__to_address(__first.__seg_), __clz_f - __dn, __first.__ctz_, _FillVal);
+    __n -= __dn;
+    ++__first.__seg_;
+  }
+  // do middle whole words
+  __storage_type __nw = __n / __bits_per_word;
+  std::__fill_n(std::__to_address(__first.__seg_), __nw, _FillVal ? static_cast<__storage_type>(-1) : 0);
+  __n -= __nw * __bits_per_word;
+  // do last partial word
+  if (__n > 0) {
+    __first.__seg_ += __nw;
+    std::__fill_masked_range(std::__to_address(__first.__seg_), __bits_per_word - __n, 0u, _FillVal);
+  }
+}
+
+template <class _Cp, class _Size>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_iterator<_Cp, false>
+__fill_n(__bit_iterator<_Cp, false> __first, _Size __n, const bool& __value) {
+  if (__n > 0) {
+    if (__value)
+      std::__fill_n_bool<true>(__first, __n);
+    else
+      std::__fill_n_bool<false>(__first, __n);
+  }
+  return __first + __n;
+}
+
+template < class _OutIter,
+           class _Size,
+           class _Tp,
+           __enable_if_t<__is_segmented_iterator<_OutIter>::value && __has_forward_iterator_category<_OutIter>::value,
+                         int> = 0>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _OutIter
+__fill_n(_OutIter __first, _Size __n, const _Tp& __value) {
+  _OutIter __last = std::next(__first, __n);
+  std::__fill(__first, __last, __value);
+  return __last;
+}
+
+template <class _ForwardIterator,
+          class _Sentinel,
+          class _Tp,
+          __enable_if_t<__has_forward_iterator_category<_ForwardIterator>::value, int> >
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+__fill(_ForwardIterator __first, _Sentinel __last, const _Tp& __value) {
+  for (; __first != __last; ++__first)
+    *__first = __value;
+}
+
+template <class _OutIter, class _Tp>
+struct _FillSegment {
+  using _Traits _LIBCPP_NODEBUG = __segmented_iterator_traits<_OutIter>;
+
+  const _Tp& __value_;
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit _FillSegment(const _Tp& __value) : __value_(__value) {}
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
+  operator()(typename _Traits::__local_iterator __lfirst, typename _Traits::__local_iterator __llast) {
+    std::__fill(__lfirst, __llast, __value_);
+  }
+};
+
+template <class _RandomAccessIterator,
+          class _Tp,
+          __enable_if_t<__has_random_access_iterator_category<_RandomAccessIterator>::value &&
+                            !__is_segmented_iterator<_RandomAccessIterator>::value,
+                        int> >
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+__fill(_RandomAccessIterator __first, _RandomAccessIterator __last, const _Tp& __value) {
+  std::__fill_n(__first, __last - __first, __value);
+}
+
+template <class _SegmentedIterator, class _Tp, __enable_if_t<__is_segmented_iterator<_SegmentedIterator>::value, int> >
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
+__fill(_SegmentedIterator __first, _SegmentedIterator __last, const _Tp& __value) {
+  std::__for_each_segment(__first, __last, _FillSegment<_SegmentedIterator, _Tp>(__value));
+}
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___ALGORITHM_FILL_FILL_N_COMMON_H
diff --git a/libcxx/include/__algorithm/fill_n.h b/libcxx/include/__algorithm/fill_n.h
index 72fd8d0f9a988..9940c385874fc 100644
--- a/libcxx/include/__algorithm/fill_n.h
+++ b/libcxx/include/__algorithm/fill_n.h
@@ -9,85 +9,18 @@
 #ifndef _LIBCPP___ALGORITHM_FILL_N_H
 #define _LIBCPP___ALGORITHM_FILL_N_H
 
-#include <__algorithm/min.h>
+#include <__algorithm/fill_fill_n_common.h>
 #include <__config>
-#include <__fwd/bit_reference.h>
-#include <__fwd/fill.h>
-#include <__iterator/iterator_traits.h>
-#include <__iterator/next.h>
-#include <__iterator/segmented_iterator.h>
-#include <__memory/pointer_traits.h>
-#include <__type_traits/enable_if.h>
 #include <__utility/convert_to_integral.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
-_LIBCPP_PUSH_MACROS
-#include <__undef_macros>
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 // fill_n isn't specialized for std::memset, because the compiler already optimizes the loop to a call to std::memset.
 
-template <class _OutIter, class _Size, class _Tp, __enable_if_t<!__is_segmented_iterator<_OutIter>::value, int> = 0>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutIter
-__fill_n(_OutIter __first, _Size __n, const _Tp& __value) {
-  for (; __n > 0; ++__first, (void)--__n)
-    *__first = __value;
-  return __first;
-}
-
-template <bool _FillVal, class _Cp>
-_LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
-__fill_n_bool(__bit_iterator<_Cp, false> __first, typename __size_difference_type_traits<_Cp>::size_type __n) {
-  using _It            = __bit_iterator<_Cp, false>;
-  using __storage_type = typename _It::__storage_type;
-
-  const int __bits_per_word = _It::__bits_per_word;
-  // do first partial word
-  if (__first.__ctz_ != 0) {
-    __storage_type __clz_f = static_cast<__storage_type>(__bits_per_word - __first.__ctz_);
-    __storage_type __dn    = std::min(__clz_f, __n);
-    std::__fill_masked_range(std::__to_address(__first.__seg_), __clz_f - __dn, __first.__ctz_, _FillVal);
-    __n -= __dn;
-    ++__first.__seg_;
-  }
-  // do middle whole words
-  __storage_type __nw = __n / __bits_per_word;
-  std::__fill_n(std::__to_address(__first.__seg_), __nw, _FillVal ? static_cast<__storage_type>(-1) : 0);
-  __n -= __nw * __bits_per_word;
-  // do last partial word
-  if (__n > 0) {
-    __first.__seg_ += __nw;
-    std::__fill_masked_range(std::__to_address(__first.__seg_), __bits_per_word - __n, 0u, _FillVal);
-  }
-}
-
-template <class _Cp, class _Size>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_iterator<_Cp, false>
-__fill_n(__bit_iterator<_Cp, false> __first, _Size __n, const bool& __value) {
-  if (__n > 0) {
-    if (__value)
-      std::__fill_n_bool<true>(__first, __n);
-    else
-      std::__fill_n_bool<false>(__first, __n);
-  }
-  return __first + __n;
-}
-
-template < class _OutIter,
-           class _Size,
-           class _Tp,
-           __enable_if_t<__is_segmented_iterator<_OutIter>::value && __has_forward_iterator_category<_OutIter>::value,
-                         int> = 0>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _OutIter __fill_n(_OutIter __first, _Size __n, const _Tp& __value) {
-  _OutIter __last = std::next(__first, __n);
-  std::__fill(__first, __last, __value);
-  return __last;
-}
-
 template <class _OutputIterator, class _Size, class _Tp>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator
 fill_n(_OutputIterator __first, _Size __n, const _Tp& __value) {
@@ -96,6 +29,4 @@ fill_n(_OutputIterator __first, _Size __n, const _Tp& __value) {
 
 _LIBCPP_END_NAMESPACE_STD
 
-_LIBCPP_POP_MACROS
-
 #endif // _LIBCPP___ALGORITHM_FILL_N_H
diff --git a/libcxx/include/__algorithm/ranges_fill_n.h b/libcxx/include/__algorithm/ranges_fill_n.h
index 1276f13680a9f..b3c70de877e59 100644
--- a/libcxx/include/__algorithm/ranges_fill_n.h
+++ b/libcxx/include/__algorithm/ranges_fill_n.h
@@ -9,7 +9,7 @@
 #ifndef _LIBCPP___ALGORITHM_RANGES_FILL_N_H
 #define _LIBCPP___ALGORITHM_RANGES_FILL_N_H
 
-#include <__algorithm/fill_n.h>
+#include <__algorithm/fill_fill_n_common.h>
 #include <__config>
 #include <__iterator/concepts.h>
 #include <__iterator/incrementable_traits.h>
diff --git a/libcxx/include/__fwd/fill.h b/libcxx/include/__fwd/fill.h
deleted file mode 100644
index 1c13229f6c4de..0000000000000
--- a/libcxx/include/__fwd/fill.h
+++ /dev/null
@@ -1,46 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef _LIBCPP___FWD_FILL_H
-#define _LIBCPP___FWD_FILL_H
-
-#include <__config>
-#include <__iterator/iterator_traits.h>
-#include <__iterator/segmented_iterator.h>
-#include <__type_traits/enable_if.h>
-
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
-
-_LIBCPP_BEGIN_NAMESPACE_STD
-
-template <class _ForwardIterator,
-          class _Sentinel,
-          class _Tp,
-          __enable_if_t<__has_forward_iterator_category<_ForwardIterator>::value, int> = 0>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__fill(_ForwardIterator __first, _Sentinel __last, const _Tp& __value);
-
-template <class _RandomAccessIterator,
-          class _Tp,
-          __enable_if_t<__has_random_access_iterator_category<_RandomAccessIterator>::value &&
-                            !__is_segmented_iterator<_RandomAccessIterator>::value,
-                        int> = 0>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__fill(_RandomAccessIterator __first, _RandomAccessIterator __last, const _Tp& __value);
-
-template <class _SegmentedIterator,
-          class _Tp,
-          __enable_if_t<__is_segmented_iterator<_SegmentedIterator>::value, int> = 0>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__fill(_SegmentedIterator __first, _SegmentedIterator __last, const _Tp& __value);
-
-_LIBCPP_END_NAMESPACE_STD
-
-#endif // _LIBCPP___FWD_FILL_H
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
index 422dbf802fe30..b0ce1c2e0103f 100644
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -424,6 +424,7 @@ module std [system] {
     module count                                  { header "__algorithm/count.h" }
     module equal_range                            { header "__algorithm/equal_range.h" }
     module equal                                  { header "__algorithm/equal.h" }
+    module fill_fill_n_common                     { header "__algorithm/fill_fill_n_common.h" }
     module fill_n                                 { header "__algorithm/fill_n.h" }
     module fill                                   { header "__algorithm/fill.h" }
     module find_end                               { header "__algorithm/find_end.h" }
@@ -435,7 +436,6 @@ module std [system] {
     module for_each_n                             { header "__algorithm/for_each_n.h" }
     module for_each_segment                       { header "__algorithm/for_each_segment.h" }
     module for_each                               { header "__algorithm/for_each.h" }
-    module fwd                                    { header "__fwd/fill.h" }
     module generate_n                             { header "__algorithm/generate_n.h" }
     module generate                               { header "__algorithm/generate.h" }
     module half_positive                          { header "__algorithm/half_positive.h" }
diff --git a/libcxx/utils/generate_iwyu_mapping.py b/libcxx/utils/generate_iwyu_mapping.py
index bf7cdfd4bfb08..ab3e3f14e14d8 100644
--- a/libcxx/utils/generate_iwyu_mapping.py
+++ b/libcxx/utils/generate_iwyu_mapping.py
@@ -41,8 +41,6 @@ def IWYU_mapping(header: str) -> typing.Optional[typing.List[str]]:
         return ["atomic", "mutex", "semaphore", "thread"]
     elif header == "__tree":
         return ["map", "set"]
-    elif header == "__fwd/fill.h":
-        return ["algorithm"]
     elif header == "__fwd/byte.h":
         return ["cstddef"]
     elif header == "__fwd/pair.h":



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