[libc++] Optimize std::{,ranges}::{fill,fill_n} for segmented iterators

libcxx/docs/ReleaseNotes/21.rst

@@ -88,6 +88,10 @@ Improvements and New Features
 
 - ``ctype::tolower`` and ``ctype::toupper`` have been optimized, resulting in a 2x performance improvement.
 
+- 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.
+
 Deprecations and Removals
 -------------------------
 

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 +24,42 @@ _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>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__fill(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value, forward_iterator_tag) {
+template <class _ForwardIterator, class _Sentinel, class _Tp>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _ForwardIterator
+__fill(_ForwardIterator __first, _Sentinel __last, const _Tp& __value) {
   for (; __first != __last; ++__first)
     *__first = __value;
+  return __first;
 }
 
-template <class _RandomAccessIterator, class _Tp>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
-__fill(_RandomAccessIterator __first, _RandomAccessIterator __last, const _Tp& __value, random_access_iterator_tag) {
-  std::fill_n(__first, __last - __first, __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 _RandomAccessIterator
+__fill(_RandomAccessIterator __first, _RandomAccessIterator __last, const _Tp& __value) {
+  return std::__fill_n(__first, __last - __first, __value);
+}
+
+#ifndef _LIBCPP_CXX03_LANG
+template <class _SegmentedIterator,
+          class _Tp,
+          __enable_if_t<__is_segmented_iterator<_SegmentedIterator>::value, int> = 0>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _SegmentedIterator
+__fill(_SegmentedIterator __first, _SegmentedIterator __last, const _Tp& __value) {
+  using __local_iterator_t = typename __segmented_iterator_traits<_SegmentedIterator>::__local_iterator;
+  std::__for_each_segment(__first, __last, [&](__local_iterator_t __lfirst, __local_iterator_t __llast) {
+    std::__fill(__lfirst, __llast, __value);
+  });
+  return __last;
 }
+#endif // !_LIBCPP_CXX03_LANG
 
 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

libcxx/include/__algorithm/fill_n.h

@@ -9,10 +9,16 @@
 #ifndef _LIBCPP___ALGORITHM_FILL_N_H
 #define _LIBCPP___ALGORITHM_FILL_N_H
 
+#include <__algorithm/for_each_n_segment.h>
 #include <__algorithm/min.h>
 #include <__config>
 #include <__fwd/bit_reference.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/segmented_iterator.h>
 #include <__memory/pointer_traits.h>
+#include <__type_traits/conjunction.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/negation.h>
 #include <__utility/convert_to_integral.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -26,9 +32,38 @@ _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>
+template <class _OutputIterator,
+          class _Size,
+          class _Tp
+#ifndef _LIBCPP_CXX03_LANG
+          ,
+          __enable_if_t<!_And<__is_segmented_iterator<_OutputIterator>,
+                              __has_random_access_local_iterator<_OutputIterator> >::value,
+                        int> = 0
+#endif
+          >
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator
-__fill_n(_OutputIterator __first, _Size __n, const _Tp& __value);
+__fill_n(_OutputIterator __first, _Size __n, const _Tp& __value) {
+  for (; __n > 0; ++__first, (void)--__n)
+    *__first = __value;
+  return __first;
+}
+
+#ifndef _LIBCPP_CXX03_LANG
+template < class _OutputIterator,
+           class _Size,
+           class _Tp,
+           __enable_if_t<_And<__is_segmented_iterator<_OutputIterator>,
+                              __has_random_access_local_iterator<_OutputIterator> >::value,
+                         int> = 0>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _OutputIterator
+__fill_n(_OutputIterator __first, _Size __n, const _Tp& __value) {
+  using __local_iterator_t = typename __segmented_iterator_traits<_OutputIterator>::__local_iterator;
+  return std::__for_each_n_segment(__first, __n, [&](__local_iterator_t __lfirst, __local_iterator_t __llast) {
+    std::__fill_n(__lfirst, __llast - __lfirst, __value);
+  });
+}
+#endif // !_LIBCPP_CXX03_LANG
 
 template <bool _FillVal, class _Cp>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
@@ -68,14 +103,6 @@ __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 _OutputIterator, class _Size, class _Tp>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator
 fill_n(_OutputIterator __first, _Size __n, const _Tp& __value) {

libcxx/include/__algorithm/ranges_fill.h

@@ -9,12 +9,14 @@
 #ifndef _LIBCPP___ALGORITHM_RANGES_FILL_H
 #define _LIBCPP___ALGORITHM_RANGES_FILL_H
 
-#include <__algorithm/ranges_fill_n.h>
+#include <__algorithm/fill.h>
+#include <__algorithm/fill_n.h>
 #include <__config>
 #include <__iterator/concepts.h>
 #include <__ranges/access.h>
 #include <__ranges/concepts.h>
 #include <__ranges/dangling.h>
+#include <__utility/move.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -31,12 +33,11 @@ namespace ranges {
 struct __fill {
   template <class _Type, output_iterator<const _Type&> _Iter, sentinel_for<_Iter> _Sent>
   _LIBCPP_HIDE_FROM_ABI constexpr _Iter operator()(_Iter __first, _Sent __last, const _Type& __value) const {
-    if constexpr (random_access_iterator<_Iter> && sized_sentinel_for<_Sent, _Iter>) {
-      return ranges::fill_n(__first, __last - __first, __value);
+    if constexpr (sized_sentinel_for<_Sent, _Iter>) {
+      auto __n = __last - __first;
+      return std::__fill_n(std::move(__first), __n, __value);
     } else {
-      for (; __first != __last; ++__first)
-        *__first = __value;
-      return __first;
+      return std::__fill(std::move(__first), std::move(__last), __value);
     }
   }
 

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 +95,13 @@ TEST_CONSTEXPR_CXX20 bool test_vector_bool(std::size_t N) {
   return true;
 }
 
+/*TEST_CONSTEXPR_CXX26*/ void test_deque() { // TODO: Mark as TEST_CONSTEXPR_CXX26 once std::deque is constexpr
+  std::deque<int> in(20);
+  std::deque<int> expected(in.size(), 42);
+  std::fill(in.begin(), in.end(), 42);
+  assert(in == expected);
+}
+
 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 +147,20 @@ TEST_CONSTEXPR_CXX20 bool test() {
     }
   }
 
+  if (!TEST_IS_CONSTANT_EVALUATED) // TODO: Use TEST_STD_AT_LEAST_26_OR_RUNTIME_EVALUATED when std::deque is made constexpr
+    test_deque();
+
+#if TEST_STD_VER >= 20
+  { // Verify that join_view of vectors work properly.
+    std::vector<std::vector<int>> v{{1, 2}, {1, 2, 3}, {}, {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
+
   return true;
 }
 

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"
@@ -126,6 +128,13 @@ TEST_CONSTEXPR_CXX20 bool test_vector_bool(std::size_t N) {
   return true;
 }
 
+/*TEST_CONSTEXPR_CXX26*/ void test_deque() { // TODO: Mark as TEST_CONSTEXPR_CXX26 once std::deque is constexpr
+  std::deque<int> in(20);
+  std::deque<int> expected(in.size(), 42);
+  std::fill_n(in.begin(), in.size(), 42);
+  assert(in == expected);
+}
+
 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>());
@@ -221,6 +230,20 @@ TEST_CONSTEXPR_CXX20 bool test() {
     }
   }
 
+  if (!TEST_IS_CONSTANT_EVALUATED) // TODO: Use TEST_STD_AT_LEAST_26_OR_RUNTIME_EVALUATED when std::deque is made constexpr
+    test_deque();
+
+#if TEST_STD_VER >= 20
+  {
+    std::vector<std::vector<int>> v{{1, 2}, {1, 2, 3}, {}, {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
+
   return true;
 }
 

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 +129,13 @@ constexpr bool test_vector_bool(std::size_t N) {
 }
 #endif
 
+/*TEST_CONSTEXPR_CXX26*/ void test_deque() { // TODO: Mark as TEST_CONSTEXPR_CXX26 once std::deque is constexpr
+  std::deque<int> in(20);
+  std::deque<int> expected(in.size(), 42);
+  std::ranges::fill(in, 42);
+  assert(in == expected);
+}
+
 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 +235,20 @@ constexpr bool test() {
   }
 #endif
 
+  if (!TEST_IS_CONSTANT_EVALUATED) // TODO: Use TEST_STD_AT_LEAST_26_OR_RUNTIME_EVALUATED when std::deque is made constexpr
+    test_deque();
+
+#if TEST_STD_VER >= 20
+  {
+    std::vector<std::vector<int>> v{{1, 2}, {1, 2, 3}, {}, {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);
+  }
+#endif
+
   return true;
 }
 

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 +102,13 @@ constexpr bool test_vector_bool(std::size_t N) {
 }
 #endif
 
+/*TEST_CONSTEXPR_CXX26*/ void test_deque() { // TODO: Mark as TEST_CONSTEXPR_CXX26 once std::deque is constexpr
+  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);
+}
+
 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 +183,20 @@ constexpr bool test() {
   }
 #endif
 
+  if (!TEST_IS_CONSTANT_EVALUATED) // TODO: Use TEST_STD_AT_LEAST_26_OR_RUNTIME_EVALUATED when std::deque is made constexpr
+    test_deque();
+
+#if TEST_STD_VER >= 20
+  {
+    std::vector<std::vector<int>> v{{1, 2}, {1, 2, 3}, {}, {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);
+  }
+#endif
+
   return true;
 }