fd08af0a96
This patch enhances the performance of `std::distance` and `std::ranges::distance` for non-random-access segmented iterators, e.g., `std::join_view` iterators. The original implementation operates in linear time, `O(n)`, where `n` is the total number of elements. The optimized version reduces this to approximately `O(n / segment_size)` by leveraging segmented structure, where `segment_size` is the average size of each segment. The table below summarizes the peak performance improvements observed across different segment sizes, with the total element count `n` ranging up to `1 << 20` (1,048,576 elements), based on benchmark results. ``` ---------------------------------------------------------------------------------------- Container/n/segment_size std::distance std::ranges::distance ---------------------------------------------------------------------------------------- join_view(vector<vector<int>>)/1048576/256 401.6x 422.9x join_view(deque<deque<int>>)/1048576/256 112.1x 132.6x join_view(vector<vector<int>>)/1048576/1024 1669.2x 1559.1x join_view(deque<deque<int>>)/1048576/1024 487.7x 497.4x ``` ## Benchmarks #### Segment size = 1024 ``` ----------------------------------------------------------------------------------------- Benchmark Before After Speedup ----------------------------------------------------------------------------------------- std::distance(join_view(vector<vector<int>>))/50 38.8 ns 1.01 ns 38.4x std::distance(join_view(vector<vector<int>>))/1024 660 ns 1.02 ns 647.1x std::distance(join_view(vector<vector<int>>))/4096 2934 ns 1.98 ns 1481.8x std::distance(join_view(vector<vector<int>>))/8192 5751 ns 3.92 ns 1466.8x std::distance(join_view(vector<vector<int>>))/16384 11520 ns 7.06 ns 1631.7x std::distance(join_view(vector<vector<int>>))/65536 46367 ns 32.2 ns 1440.6x std::distance(join_view(vector<vector<int>>))/262144 182611 ns 114 ns 1601.9x std::distance(join_view(vector<vector<int>>))/1048576 737785 ns 442 ns 1669.2x std::distance(join_view(deque<deque<int>>))/50 53.1 ns 6.13 ns 8.7x std::distance(join_view(deque<deque<int>>))/1024 854 ns 7.53 ns 113.4x std::distance(join_view(deque<deque<int>>))/4096 3507 ns 14.7 ns 238.6x std::distance(join_view(deque<deque<int>>))/8192 7114 ns 17.6 ns 404.2x std::distance(join_view(deque<deque<int>>))/16384 13997 ns 30.7 ns 455.9x std::distance(join_view(deque<deque<int>>))/65536 55598 ns 114 ns 487.7x std::distance(join_view(deque<deque<int>>))/262144 214293 ns 480 ns 446.4x std::distance(join_view(deque<deque<int>>))/1048576 833000 ns 2183 ns 381.6x rng::distance(join_view(vector<vector<int>>))/50 39.1 ns 1.10 ns 35.5x rng::distance(join_view(vector<vector<int>>))/1024 689 ns 1.14 ns 604.4x rng::distance(join_view(vector<vector<int>>))/4096 2753 ns 2.15 ns 1280.5x rng::distance(join_view(vector<vector<int>>))/8192 5530 ns 4.61 ns 1199.6x rng::distance(join_view(vector<vector<int>>))/16384 10968 ns 7.97 ns 1376.2x rng::distance(join_view(vector<vector<int>>))/65536 46009 ns 35.3 ns 1303.4x rng::distance(join_view(vector<vector<int>>))/262144 190569 ns 124 ns 1536.9x rng::distance(join_view(vector<vector<int>>))/1048576 746724 ns 479 ns 1559.1x rng::distance(join_view(deque<deque<int>>))/50 51.6 ns 6.57 ns 7.9x rng::distance(join_view(deque<deque<int>>))/1024 826 ns 6.50 ns 127.1x rng::distance(join_view(deque<deque<int>>))/4096 3323 ns 12.5 ns 265.8x rng::distance(join_view(deque<deque<int>>))/8192 6619 ns 19.1 ns 346.5x rng::distance(join_view(deque<deque<int>>))/16384 13495 ns 33.2 ns 406.5x rng::distance(join_view(deque<deque<int>>))/65536 53668 ns 114 ns 470.8x rng::distance(join_view(deque<deque<int>>))/262144 236277 ns 475 ns 497.4x rng::distance(join_view(deque<deque<int>>))/1048576 914177 ns 2157 ns 423.8x ----------------------------------------------------------------------------------------- ``` #### Segment size = 256 ``` ----------------------------------------------------------------------------------------- Benchmark Before After Speedup ----------------------------------------------------------------------------------------- std::distance(join_view(vector<vector<int>>))/50 38.1 ns 1.02 ns 37.4x std::distance(join_view(vector<vector<int>>))/1024 689 ns 2.06 ns 334.5x std::distance(join_view(vector<vector<int>>))/4096 2815 ns 7.01 ns 401.6x std::distance(join_view(vector<vector<int>>))/8192 5507 ns 14.3 ns 385.1x std::distance(join_view(vector<vector<int>>))/16384 11050 ns 33.7 ns 327.9x std::distance(join_view(vector<vector<int>>))/65536 44197 ns 118 ns 374.6x std::distance(join_view(vector<vector<int>>))/262144 175793 ns 449 ns 391.5x std::distance(join_view(vector<vector<int>>))/1048576 703242 ns 2140 ns 328.7x std::distance(join_view(deque<deque<int>>))/50 50.2 ns 6.12 ns 8.2x std::distance(join_view(deque<deque<int>>))/1024 835 ns 11.4 ns 73.2x std::distance(join_view(deque<deque<int>>))/4096 3353 ns 32.9 ns 101.9x std::distance(join_view(deque<deque<int>>))/8192 6711 ns 64.2 ns 104.5x std::distance(join_view(deque<deque<int>>))/16384 13231 ns 118 ns 112.1x std::distance(join_view(deque<deque<int>>))/65536 53523 ns 556 ns 96.3x std::distance(join_view(deque<deque<int>>))/262144 219101 ns 2166 ns 101.2x std::distance(join_view(deque<deque<int>>))/1048576 880277 ns 15852 ns 55.5x rng::distance(join_view(vector<vector<int>>))/50 37.7 ns 1.13 ns 33.4x rng::distance(join_view(vector<vector<int>>))/1024 697 ns 2.14 ns 325.7x rng::distance(join_view(vector<vector<int>>))/4096 2804 ns 7.52 ns 373.0x rng::distance(join_view(vector<vector<int>>))/8192 5749 ns 15.2 ns 378.2x rng::distance(join_view(vector<vector<int>>))/16384 11742 ns 34.8 ns 337.4x rng::distance(join_view(vector<vector<int>>))/65536 47274 ns 116 ns 407.7x rng::distance(join_view(vector<vector<int>>))/262144 187774 ns 444 ns 422.9x rng::distance(join_view(vector<vector<int>>))/1048576 749724 ns 2109 ns 355.5x rng::distance(join_view(deque<deque<int>>))/50 53.0 ns 6.09 ns 8.7x rng::distance(join_view(deque<deque<int>>))/1024 895 ns 11.0 ns 81.4x rng::distance(join_view(deque<deque<int>>))/4096 3825 ns 30.6 ns 125.0x rng::distance(join_view(deque<deque<int>>))/8192 7550 ns 60.5 ns 124.8x rng::distance(join_view(deque<deque<int>>))/16384 14847 ns 112 ns 132.6x rng::distance(join_view(deque<deque<int>>))/65536 56888 ns 453 ns 125.6x rng::distance(join_view(deque<deque<int>>))/262144 231395 ns 2034 ns 113.8x rng::distance(join_view(deque<deque<int>>))/1048576 933093 ns 15012 ns 62.2x ----------------------------------------------------------------------------------------- ``` Addresses a subtask of #102817. --------- Co-authored-by: Louis Dionne <ldionne.2@gmail.com> Co-authored-by: A. Jiang <de34@live.cn>
123 lines
3.9 KiB
C++
123 lines
3.9 KiB
C++
// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___ITERATOR_DISTANCE_H
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#define _LIBCPP___ITERATOR_DISTANCE_H
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#include <__algorithm/for_each_segment.h>
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#include <__config>
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#include <__iterator/concepts.h>
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#include <__iterator/incrementable_traits.h>
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#include <__iterator/iterator_traits.h>
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#include <__iterator/segmented_iterator.h>
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#include <__ranges/access.h>
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#include <__ranges/concepts.h>
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#include <__ranges/size.h>
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#include <__type_traits/decay.h>
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#include <__type_traits/enable_if.h>
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#include <__type_traits/remove_cvref.h>
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#include <__utility/move.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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#if _LIBCPP_STD_VER >= 20
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template <class _Iter>
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using __iter_distance_t _LIBCPP_NODEBUG = std::iter_difference_t<_Iter>;
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#else
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template <class _Iter>
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using __iter_distance_t _LIBCPP_NODEBUG = typename iterator_traits<_Iter>::difference_type;
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#endif
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template <class _InputIter, class _Sent>
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inline _LIBCPP_HIDE_FROM_ABI
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_LIBCPP_CONSTEXPR_SINCE_CXX17 __iter_distance_t<_InputIter> __distance(_InputIter __first, _Sent __last) {
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__iter_distance_t<_InputIter> __r(0);
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for (; __first != __last; ++__first)
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++__r;
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return __r;
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}
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template <class _RandIter, __enable_if_t<__has_random_access_iterator_category<_RandIter>::value, int> = 0>
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inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 __iter_distance_t<_RandIter>
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__distance(_RandIter __first, _RandIter __last) {
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return __last - __first;
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}
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#if _LIBCPP_STD_VER >= 20
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template <class _SegmentedIter,
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__enable_if_t<!__has_random_access_iterator_category<_SegmentedIter>::value &&
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__is_segmented_iterator_v<_SegmentedIter>,
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int> = 0>
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inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 __iter_distance_t<_SegmentedIter>
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__distance(_SegmentedIter __first, _SegmentedIter __last) {
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__iter_distance_t<_SegmentedIter> __r(0);
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std::__for_each_segment(__first, __last, [&__r](auto __lfirst, auto __llast) {
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__r += std::__distance(__lfirst, __llast);
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});
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return __r;
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}
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#endif // _LIBCPP_STD_VER >= 20
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template <class _InputIter>
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inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 typename iterator_traits<_InputIter>::difference_type
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distance(_InputIter __first, _InputIter __last) {
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return std::__distance(__first, __last);
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}
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#if _LIBCPP_STD_VER >= 20
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// [range.iter.op.distance]
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namespace ranges {
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struct __distance {
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template <class _Ip, sentinel_for<_Ip> _Sp>
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requires(!sized_sentinel_for<_Sp, _Ip>)
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_LIBCPP_HIDE_FROM_ABI constexpr iter_difference_t<_Ip> operator()(_Ip __first, _Sp __last) const {
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return std::__distance(std::move(__first), std::move(__last));
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}
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template <class _Ip, sized_sentinel_for<decay_t<_Ip>> _Sp>
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_LIBCPP_HIDE_FROM_ABI constexpr iter_difference_t<_Ip> operator()(_Ip&& __first, _Sp __last) const {
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if constexpr (sized_sentinel_for<_Sp, __remove_cvref_t<_Ip>>) {
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return __last - __first;
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} else {
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return __last - decay_t<_Ip>(__first);
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}
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}
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template <range _Rp>
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_LIBCPP_HIDE_FROM_ABI constexpr range_difference_t<_Rp> operator()(_Rp&& __r) const {
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if constexpr (sized_range<_Rp>) {
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return static_cast<range_difference_t<_Rp>>(ranges::size(__r));
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} else {
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return operator()(ranges::begin(__r), ranges::end(__r));
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}
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}
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};
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inline namespace __cpo {
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inline constexpr auto distance = __distance{};
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} // namespace __cpo
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} // namespace ranges
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#endif // _LIBCPP_STD_VER >= 20
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_LIBCPP_END_NAMESPACE_STD
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_LIBCPP_POP_MACROS
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#endif // _LIBCPP___ITERATOR_DISTANCE_H
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