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[libc++] Backport segmented iterator optimization for std::for_each to C++11 (#134960)
Previously, the segmented iterator optimization for `std::for_each` was restricted to C++23 and later due to its dependency on `__movable_box`, which is not available in earlier standards. This patch eliminates that restriction, enabling consistent optimizations starting from C++11. By backporting this enhancement, we improve performance across older standards and create opportunities to extend similar optimizations to other algorithms by forwarding their calls to `std::for_each`.
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‎libcxx/docs/ReleaseNotes/21.rst

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@@ -67,6 +67,9 @@ Improvements and New Features
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- The ``std::stable_sort`` algorithm uses radix sort for floating-point types now, which can improve the performance
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up to 10x, depending on type of sorted elements and the initial state of the sorted array.
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- The segmented iterator optimization for ``std::for_each`` has been backported to C++11. Previously it was only available
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in C++23 and later.
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Deprecations and Removals
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-------------------------
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‎libcxx/include/__algorithm/for_each.h

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#include <__algorithm/for_each_segment.h>
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#include <__config>
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#include <__iterator/segmented_iterator.h>
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#include <__ranges/movable_box.h>
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#include <__utility/in_place.h>
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#include <__utility/move.h>
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#include <__type_traits/enable_if.h>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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#endif
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_LIBCPP_PUSH_MACROS
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#include <__undef_macros>
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_LIBCPP_BEGIN_NAMESPACE_STD
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template <class _InputIterator, class _Function>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Function
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for_each(_InputIterator __first, _InputIterator __last, _Function __f) {
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template <class _InputIterator, class _Sent, class _Func>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __for_each(_InputIterator __first, _Sent __last, _Func& __f) {
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for (; __first != __last; ++__first)
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__f(*__first);
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return __f;
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}
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// __movable_box is available in C++20, but is actually a copyable-box, so optimization is only correct in C++23
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#if _LIBCPP_STD_VER >= 23
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template <class _SegmentedIterator, class _Function>
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requires __is_segmented_iterator<_SegmentedIterator>::value
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_LIBCPP_HIDE_FROM_ABI constexpr _Function
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for_each(_SegmentedIterator __first, _SegmentedIterator __last, _Function __func) {
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ranges::__movable_box<_Function> __wrapped_func(in_place, std::move(__func));
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std::__for_each_segment(__first, __last, [&](auto __lfirst, auto __llast) {
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__wrapped_func =
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ranges::__movable_box<_Function>(in_place, std::for_each(__lfirst, __llast, std::move(*__wrapped_func)));
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#ifndef _LIBCPP_CXX03_LANG
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template <class _SegmentedIterator,
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class _Function,
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__enable_if_t<__is_segmented_iterator<_SegmentedIterator>::value, int> = 0>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
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__for_each(_SegmentedIterator __first, _SegmentedIterator __last, _Function& __func) {
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using __local_iterator_t = typename __segmented_iterator_traits<_SegmentedIterator>::__local_iterator;
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std::__for_each_segment(__first, __last, [&](__local_iterator_t __lfirst, __local_iterator_t __llast) {
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std::__for_each(__lfirst, __llast, __func);
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});
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return std::move(*__wrapped_func);
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}
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#endif // _LIBCPP_STD_VER >= 23
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#endif // !_LIBCPP_CXX03_LANG
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_LIBCPP_END_NAMESPACE_STD
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template <class _InputIterator, class _Function>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Function
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for_each(_InputIterator __first, _InputIterator __last, _Function __f) {
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std::__for_each(__first, __last, __f);
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return __f;
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}
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_LIBCPP_POP_MACROS
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___ALGORITHM_FOR_EACH_H

‎libcxx/include/algorithm

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# include <cstring>
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# include <iterator>
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# include <memory>
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# include <optional>
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# include <stdexcept>
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# include <type_traits>
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# include <utility>

‎libcxx/include/array

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# include <cstdlib>
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# include <iterator>
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# include <new>
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# include <optional>
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# include <type_traits>
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# include <utility>
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# endif

‎libcxx/include/bitset

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# if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
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# include <concepts>
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# include <cstdlib>
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# include <optional>
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# include <type_traits>
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# endif
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#endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)

‎libcxx/include/codecvt

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# include <limits>
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# include <mutex>
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# include <new>
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# include <optional>
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# include <stdexcept>
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# include <type_traits>
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# include <typeinfo>

‎libcxx/include/condition_variable

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# include <initializer_list>
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# include <iosfwd>
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# include <new>
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# include <optional>
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# include <stdexcept>
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# include <system_error>
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# include <type_traits>

‎libcxx/include/ios

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# include <limits>
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# include <mutex>
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# include <new>
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# include <optional>
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# include <stdexcept>
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# include <system_error>
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# include <type_traits>

‎libcxx/include/locale

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# include <cstdarg>
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# include <iterator>
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# include <mutex>
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# include <optional>
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# include <stdexcept>
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# include <type_traits>
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# include <typeinfo>

‎libcxx/include/streambuf

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# if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
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# include <cstdint>
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# include <optional>
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# endif
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#endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
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‎libcxx/include/string

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# include <cstdlib>
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# include <iterator>
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# include <new>
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# include <optional>
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# include <type_traits>
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# include <typeinfo>
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# include <utility>

‎libcxx/include/string_view

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# include <concepts>
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# include <cstdlib>
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# include <iterator>
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# include <optional>
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# include <type_traits>
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# endif
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#endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)

‎libcxx/include/system_error

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# include <cstdint>
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# include <cstring>
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# include <limits>
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# include <optional>
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# include <type_traits>
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# endif
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#endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)

‎libcxx/include/vector

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# if _LIBCPP_HAS_LOCALIZATION
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# include <locale>
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# endif
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# include <optional>
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# include <string>
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# include <string_view>
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# include <tuple>

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