[libcxx-commits] [libcxx] [libcxx][algorithm] Optimize std::stable_sort via radix sort algorithm (PR #104683)

[Nikolas Klauser via libcxx-commits]

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In-Reply-To: <llvm.org/llvm/llvm-project/pull/104683 at github.com>


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+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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_RADIX_SORT_H
+#define _LIBCPP___ALGORITHM_RADIX_SORT_H
+
+#include <__algorithm/copy.h>
+#include <__algorithm/for_each.h>
+#include <__bit/countl.h>
+#include <__config>
+#include <__iterator/distance.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/move_iterator.h>
+#include <__iterator/next.h>
+#include <__iterator/reverse_iterator.h>
+#include <__numeric/partial_sum.h>
+#include <__type_traits/decay.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/invoke.h>
+#include <__type_traits/is_assignable.h>
+#include <__type_traits/is_integral.h>
+#include <__type_traits/is_unsigned.h>
+#include <__type_traits/make_unsigned.h>
+#include <__utility/forward.h>
+#include <__utility/integer_sequence.h>
+#include <__utility/move.h>
+#include <__utility/pair.h>
+#include <climits>
+#include <cstdint>
+#include <initializer_list>
+#include <limits>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 14
+
+inline _LIBCPP_HIDE_FROM_ABI void __variadic_expansion_dummy(initializer_list<int>) {}
+
+#  define _EXPAND_VARIADIC(expression) std::__variadic_expansion_dummy({(expression, 0)...})
+
+template <typename _Iterator>
+_LIBCPP_HIDE_FROM_ABI constexpr auto __move_assign_please(_Iterator __i)
+    -> enable_if_t<is_move_assignable<typename iterator_traits<_Iterator>::value_type>::value,
+                   move_iterator<_Iterator> > {
+  return std::make_move_iterator(std::move(__i));
+}
+
+template <typename _Iterator>
+_LIBCPP_HIDE_FROM_ABI constexpr auto __move_assign_please(_Iterator __i)
+    -> enable_if_t<not is_move_assignable<typename iterator_traits<_Iterator>::value_type>::value, _Iterator> {
+  return __i;
+}
+
+template <typename _UnsignedInteger>
+_LIBCPP_HIDE_FROM_ABI constexpr _UnsignedInteger __intlog2(_UnsignedInteger __n) {
+  static_assert(is_integral<_UnsignedInteger>::value, "Must be an integral type");
+  static_assert(is_unsigned<_UnsignedInteger>::value, "Must be unsigned");
+
+  return numeric_limits<_UnsignedInteger>::digits - 1 - std::__countl_zero(__n);
+}
+
+template <typename _InputIterator, typename _OutputIterator>
+_LIBCPP_HIDE_FROM_ABI pair<_OutputIterator, typename iterator_traits<_InputIterator>::value_type>
+__partial_sum_max(_InputIterator __first, _InputIterator __last, _OutputIterator __result) {
+  if (__first == __last)
+    return {__result, 0};
+
+  auto __max                                                 = *__first;
+  typename iterator_traits<_InputIterator>::value_type __sum = *__first;
+  *__result                                                  = __sum;
+
+  while (++__first != __last) {
+    if (__max < *__first) {
+      __max = *__first;
+    }
+    __sum       = std::move(__sum) + *__first;
+    *++__result = __sum;
+  }
+  return {++__result, __max};
+}
+
+template <typename _Value, typename _Map, typename _Radix>
+struct __radix_sort_traits {
+  using image_type = decay_t<invoke_result_t<_Map, _Value> >;
+  static_assert(is_integral<image_type>::value, "");
+  static_assert(is_unsigned<image_type>::value, "");
+
+  using radix_type = decay_t<invoke_result_t<_Radix, image_type> >;
+  static_assert(is_integral<radix_type>::value, "");
+
+  constexpr static auto __radix_value_range = numeric_limits<radix_type>::max() + 1;
+  constexpr static auto __radix_size        = std::__intlog2<uint64_t>(__radix_value_range);
+  constexpr static auto __radix_count       = sizeof(image_type) * CHAR_BIT / __radix_size;
+};
+
+template <typename _Value, typename _Map>
+struct __counting_sort_traits {
+  using image_type = decay_t<invoke_result_t<_Map, _Value> >;
+  static_assert(is_integral<image_type>::value, "");
+  static_assert(is_unsigned<image_type>::value, "");
+
+  constexpr static const auto __value_range = numeric_limits<image_type>::max() + 1;
+  constexpr static auto __radix_size        = std::__intlog2<uint64_t>(__value_range);
+};
+
+template <typename _Radix>
+_LIBCPP_HIDE_FROM_ABI auto __nth_radix(size_t __radix_number, _Radix __radix) {
+  return [__radix_number, __radix = std::move(__radix)](auto __n) {
+    using value_type = decltype(__n);
+    static_assert(is_integral<value_type>::value, "");
+    static_assert(is_unsigned<value_type>::value, "");
+    using traits = __counting_sort_traits<value_type, _Radix>;
+
+    return __radix(static_cast<value_type>(__n >> traits::__radix_size * __radix_number));
+  };
+}
+
+template <typename _ForwardIterator, typename _Map, typename _RandomAccessIterator>
+_LIBCPP_HIDE_FROM_ABI void
+__count(_ForwardIterator __first, _ForwardIterator __last, _Map __map, _RandomAccessIterator __counters) {
----------------
philnik777 wrote:

This is a rather generic name and readers would probably expect this to be an implementation detail of `std::count`. In fact, you only happen to call this overload because the first and second parameters are the same. I wouldn't be surprised if this additional overload caused `std::count` to break.

https://github.com/llvm/llvm-project/pull/104683