[libc++] Introduce __product_iterator_traits and optimise flat_map::insert

libcxx/include/CMakeLists.txt

@@ -497,6 +497,7 @@ set(files
   __iterator/ostreambuf_iterator.h
   __iterator/permutable.h
   __iterator/prev.h
+  __iterator/product_iterator.h
   __iterator/projected.h
   __iterator/ranges_iterator_traits.h
   __iterator/readable_traits.h

libcxx/include/__flat_map/key_value_iterator.h

@@ -13,7 +13,9 @@
 #include <__compare/three_way_comparable.h>
 #include <__concepts/convertible_to.h>
 #include <__config>
+#include <__cstddef/size_t.h>
 #include <__iterator/iterator_traits.h>
+#include <__iterator/product_iterator.h>
 #include <__memory/addressof.h>
 #include <__type_traits/conditional.h>
 #include <__utility/move.h>
@@ -57,6 +59,8 @@ struct __key_value_iterator {
   template <class, class, class, bool>
   friend struct __key_value_iterator;
 
+  friend struct __product_iterator_traits<__key_value_iterator>;
+
 public:
   using iterator_concept = random_access_iterator_tag;
   // `__key_value_iterator` only satisfy "Cpp17InputIterator" named requirements, because
@@ -167,6 +171,23 @@ struct __key_value_iterator {
   }
 };
 
+template <class _Owner, class _KeyContainer, class _MappedContainer, bool _Const>
+struct __product_iterator_traits<__key_value_iterator<_Owner, _KeyContainer, _MappedContainer, _Const>> {
+  static constexpr size_t __size = 2;
+
+  template <size_t _Nth>
+  _LIBCPP_HIDE_FROM_ABI static auto
+  __get_iterator_element(__key_value_iterator<_Owner, _KeyContainer, _MappedContainer, _Const> __it)
+    requires(_Nth <= 1)
+  {
+    if constexpr (_Nth == 0) {
+      return __it.__key_iter_;
+    } else {
+      return __it.__mapped_iter_;
+    }
+  }
+};
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_STD_VER >= 23

libcxx/include/__flat_map/utils.h

@@ -11,6 +11,7 @@
 #define _LIBCPP___FLAT_MAP_UTILS_H
 
 #include <__config>
+#include <__iterator/product_iterator.h>
 #include <__type_traits/container_traits.h>
 #include <__utility/exception_guard.h>
 #include <__utility/forward.h>
@@ -93,6 +94,25 @@ struct __flat_map_utils {
     }
     return __num_appended;
   }
+
+  template <class _Map, class _InputIterator>
+  _LIBCPP_HIDE_FROM_ABI static typename _Map::size_type
+  __append(_Map& __map, _InputIterator __first, _InputIterator __last)
+    requires __is_product_iterator_of_size<_InputIterator, 2>::value
+  {
+    auto __s1 = __map.__containers_.keys.size();
+    __map.__containers_.keys.insert(
+        __map.__containers_.keys.end(),
+        __product_iterator_traits<_InputIterator>::template __get_iterator_element<0>(__first),
+        __product_iterator_traits<_InputIterator>::template __get_iterator_element<0>(__last));
+
+    __map.__containers_.values.insert(
+        __map.__containers_.values.end(),
+        __product_iterator_traits<_InputIterator>::template __get_iterator_element<1>(__first),
+        __product_iterator_traits<_InputIterator>::template __get_iterator_element<1>(__last));
+
+    return __map.__containers_.keys.size() - __s1;
+  }
 };
 _LIBCPP_END_NAMESPACE_STD
 

libcxx/include/__iterator/product_iterator.h

@@ -0,0 +1,69 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___PRODUCT_ITERATOR_H
+#define _LIBCPP___PRODUCT_ITERATOR_H
+
+// Product iterators are iterators that contain two or more underlying iterators.
+//
+// For example, std::flat_map stores its data into two separate containers, and its iterator
+// is a proxy over two separate underlying iterators. The concept of product iterators
+// allows algorithms to operate over these underlying iterators separately, opening the
+// door to various optimizations.
+//
+// If __product_iterator_traits can be instantiated, the following functions and associated types must be provided:
+// - static constexpr size_t Traits::__size
+//   The number of underlying iterators inside the product iterator.
+//
+// - template <size_t _N>
+//   static auto Traits::__get_iterator_element(It __it)
+//   Returns the _Nth iterator element of the given product iterator.
+
+#include <__config>
+#include <__cstddef/size_t.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
+#include <__utility/declval.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Iterator>
+struct __product_iterator_traits;
+/* exposition-only:
+{
+  static constexpr size_t __size = ...;
+
+  template <size_t _N>
+  static auto __get_iterator_element(_Iterator);
+};
+*/
+
+template <class _Tp, size_t = 0>
+struct __is_product_iterator : false_type {};
+
+template <class _Tp>
+struct __is_product_iterator<_Tp, sizeof(__product_iterator_traits<_Tp>) * 0> : true_type {};
+
+template <class _Tp, size_t _Size, class = void>
+struct __is_product_iterator_of_size : false_type {};
+
+template <class _Tp, size_t _Size>
+struct __is_product_iterator_of_size<_Tp, _Size, __enable_if_t<__product_iterator_traits<_Tp>::__size == _Size> >
+    : true_type {};
+
+template <class _Iterator, size_t _Nth>
+using __product_iterator_element_t _LIBCPP_NODEBUG =
+    decltype(__product_iterator_traits<_Iterator>::__get_iterator_element<_Nth>(std::declval<_Iterator>()));
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___PRODUCT_ITERATOR_H

libcxx/include/__ranges/zip_view.h

@@ -23,6 +23,7 @@
 #include <__iterator/iter_move.h>
 #include <__iterator/iter_swap.h>
 #include <__iterator/iterator_traits.h>
+#include <__iterator/product_iterator.h>
 #include <__ranges/access.h>
 #include <__ranges/all.h>
 #include <__ranges/concepts.h>
@@ -251,6 +252,10 @@ class zip_view<_Views...>::__iterator : public __zip_view_iterator_category_base
 
   friend class zip_view<_Views...>;
 
+  using __is_zip_view_iterator _LIBCPP_NODEBUG = true_type;
+
+  friend struct __product_iterator_traits<__iterator>;
+
 public:
   using iterator_concept = decltype(ranges::__get_zip_view_iterator_tag<_Const, _Views...>());
   using value_type       = tuple<range_value_t<__maybe_const<_Const, _Views>>...>;
@@ -468,6 +473,18 @@ inline constexpr auto zip = __zip::__fn{};
 } // namespace views
 } // namespace ranges
 
+template <class _Iter>
+  requires _Iter::__is_zip_view_iterator::value
+struct __product_iterator_traits<_Iter> {
+  static constexpr size_t __size = tuple_size<decltype(std::declval<_Iter>().__current_)>::value;
+
+  template <size_t _Nth>
+    requires(_Nth < __size)
+  _LIBCPP_HIDE_FROM_ABI static constexpr auto __get_iterator_element(_Iter __it) {
+    return std::get<_Nth>(__it.__current_);
+  }
+};
+
 #endif // _LIBCPP_STD_VER >= 23
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/include/module.modulemap.in

@@ -1525,6 +1525,7 @@ module std [system] {
     }
     module permutable                 { header "__iterator/permutable.h" }
     module prev                       { header "__iterator/prev.h" }
+    module product_iterator           { header "__iterator/product_iterator.h" }
     module projected                  { header "__iterator/projected.h" }
     module ranges_iterator_traits     { header "__iterator/ranges_iterator_traits.h" }
     module readable_traits            { header "__iterator/readable_traits.h" }

libcxx/test/benchmarks/containers/associative/flat_map.bench.cpp

@@ -10,6 +10,7 @@
 
 #include <flat_map>
 #include <utility>
+#include <ranges>
 
 #include "associative_container_benchmarks.h"
 #include "../../GenerateInput.h"
@@ -26,9 +27,54 @@ struct support::adapt_operations<std::flat_map<K, V>> {
   static auto get_iterator(InsertionResult const& result) { return result.first; }
 };
 
+void product_iterator_benchmark_flat_map(benchmark::State& state) {
+  const std::size_t size = state.range(0);
+
+  using M = std::flat_map<int, int>;
+
+  const M source =
+      std::views::iota(0, static_cast<int>(size)) | std::views::transform([](int i) { return std::pair(i, i); }) |
+      std::ranges::to<std::flat_map<int, int>>();
+
+  for (auto _ : state) {
+    M m;
+    m.insert(std::sorted_unique, source.begin(), source.end());
+    benchmark::DoNotOptimize(m);
+    benchmark::ClobberMemory();
+  }
+}
+
+void product_iterator_benchmark_zip_view(benchmark::State& state) {
+  const std::size_t size = state.range(0);
+
+  using M = std::flat_map<int, int>;
+
+  const std::vector<int> keys   = std::views::iota(0, static_cast<int>(size)) | std::ranges::to<std::vector<int>>();
+  const std::vector<int> values = keys;
+
+  auto source = std::views::zip(keys, values);
+  for (auto _ : state) {
+    M m;
+    m.insert(std::sorted_unique, source.begin(), source.end());
+    benchmark::DoNotOptimize(m);
+    benchmark::ClobberMemory();
+  }
+}
+
 int main(int argc, char** argv) {
   support::associative_container_benchmarks<std::flat_map<int, int>>("std::flat_map<int, int>");
 
+  benchmark::RegisterBenchmark("flat_map::insert_product_iterator_flat_map", product_iterator_benchmark_flat_map)
+      ->Arg(32)
+      ->Arg(1024)
+      ->Arg(8192)
+      ->Arg(65536);
+  benchmark::RegisterBenchmark("flat_map::insert_product_iterator_zip", product_iterator_benchmark_zip_view)
+      ->Arg(32)
+      ->Arg(1024)
+      ->Arg(8192)
+      ->Arg(65536);
+
   benchmark::Initialize(&argc, argv);
   benchmark::RunSpecifiedBenchmarks();
   benchmark::Shutdown();

libcxx/test/std/containers/container.adaptors/flat.map/flat.map.modifiers/insert_iter_iter.pass.cpp

@@ -17,6 +17,7 @@
 #include <cassert>
 #include <functional>
 #include <deque>
+#include <ranges>
 
 #include "MinSequenceContainer.h"
 #include "../helpers.h"
@@ -75,12 +76,33 @@ void test() {
   M expected2{{0, 1}, {1, 1}, {2, 1}, {3, 1}, {4, 1}};
   assert(m == expected2);
 }
+
+void test_product_iterator() {
+  using M = std::flat_map<int, int>;
+  {
+    M m1{{1, 1}, {2, 1}, {3, 1}};
+    M m2{{4, 1}, {5, 1}, {6, 1}};
+    m1.insert(m2.begin(), m2.end());
+    M expected{{1, 1}, {2, 1}, {3, 1}, {4, 1}, {5, 1}, {6, 1}};
+    assert(m1 == expected);
+  }
+  {
+    std::vector<int> keys{1, 2, 3};
+    std::vector<int> values{1, 1, 1};
+    auto zv = std::views::zip(keys, values);
+    M m;
+    m.insert(zv.begin(), zv.end());
+    M expected{{1, 1}, {2, 1}, {3, 1}};
+    assert(m == expected);
+  }
+}
+
 int main(int, char**) {
   test<std::vector<int>, std::vector<double>>();
   test<std::deque<int>, std::vector<double>>();
   test<MinSequenceContainer<int>, MinSequenceContainer<double>>();
   test<std::vector<int, min_allocator<int>>, std::vector<double, min_allocator<double>>>();
-
+  test_product_iterator();
   {
     auto insert_func = [](auto& m, const auto& newValues) { m.insert(newValues.begin(), newValues.end()); };
     test_insert_range_exception_guarantee(insert_func);