[libc++] Add benchmarks for partitioning algorithms

libcxx/include/module.modulemap

@@ -687,6 +687,7 @@ module std [system] {
     }
     module ranges_partition {
       header "__algorithm/ranges_partition.h"
+      export std.ranges.subrange // return type
     }
     module ranges_pop_heap {
       header "__algorithm/ranges_pop_heap.h"
@@ -786,6 +787,7 @@ module std [system] {
     }
     module ranges_stable_partition {
       header "__algorithm/ranges_stable_partition.h"
+      export std.ranges.subrange // return type
     }
     module ranges_stable_sort {
       header "__algorithm/ranges_stable_sort.h"

libcxx/test/benchmarks/algorithms/partitions/is_partitioned.bench.cpp

@@ -0,0 +1,96 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <deque>
+#include <iterator>
+#include <list>
+#include <string>
+#include <vector>
+
+#include "benchmark/benchmark.h"
+#include "../../GenerateInput.h"
+
+auto compute_median(auto first, auto last) {
+  std::vector v(first, last);
+  auto middle = v.begin() + v.size() / 2;
+  std::nth_element(v.begin(), middle, v.end());
+  return *middle;
+}
+
+int main(int argc, char** argv) {
+  auto std_is_partitioned = [](auto first, auto last, auto pred) { return std::is_partitioned(first, last, pred); };
+
+  auto bm = []<class Container, bool Partitioned>(std::string name, auto is_partitioned) {
+    benchmark::RegisterBenchmark(
+        name,
+        [is_partitioned](auto& st) {
+          std::size_t const size = st.range(0);
+          using ValueType        = typename Container::value_type;
+          Container c;
+          std::generate_n(std::back_inserter(c), size, [] { return Generate<ValueType>::random(); });
+
+          // Partition the container in two equally-sized halves, ensuring the median
+          // value appears in the left half. Note that the median value isn't located
+          // in the middle -- this isn't std::nth_element.
+          ValueType median = compute_median(c.begin(), c.end());
+          auto pred        = [median](auto const& element) { return element <= median; };
+          std::partition(c.begin(), c.end(), pred);
+          assert(std::is_partitioned(c.begin(), c.end(), pred));
+
+          if constexpr (!Partitioned) {
+            // De-partition the container by swapping the element containing the median
+            // value with the last one.
+            auto median_it = std::find(c.begin(), c.end(), median);
+            auto last_it   = std::next(c.begin(), c.size() - 1);
+            std::iter_swap(median_it, last_it);
+            assert(!std::is_partitioned(c.begin(), c.end(), pred));
+          }
+
+          for ([[maybe_unused]] auto _ : st) {
+            benchmark::DoNotOptimize(c);
+            auto result = is_partitioned(c.begin(), c.end(), pred);
+            benchmark::DoNotOptimize(result);
+          }
+        })
+        ->Arg(32)
+        ->Arg(50) // non power-of-two
+        ->Arg(1024)
+        ->Arg(8192);
+  };
+
+  // std::is_partitioned
+  bm.operator()<std::vector<int>, true>("std::is_partitioned(vector<int>) (partitioned)", std_is_partitioned);
+  bm.operator()<std::vector<int>, false>("std::is_partitioned(vector<int>) (unpartitioned)", std_is_partitioned);
+
+  bm.operator()<std::deque<int>, true>("std::is_partitioned(deque<int>) (partitioned)", std_is_partitioned);
+  bm.operator()<std::deque<int>, false>("std::is_partitioned(deque<int>) (unpartitioned)", std_is_partitioned);
+
+  bm.operator()<std::list<int>, true>("std::is_partitioned(list<int>) (partitioned)", std_is_partitioned);
+  bm.operator()<std::list<int>, false>("std::is_partitioned(list<int>) (unpartitioned)", std_is_partitioned);
+
+  // ranges::is_partitioned
+  bm.operator()<std::vector<int>, true>("rng::is_partitioned(vector<int>) (partitioned)", std::ranges::is_partitioned);
+  bm.operator()<std::vector<int>, false>(
+      "rng::is_partitioned(vector<int>) (unpartitioned)", std::ranges::is_partitioned);
+
+  bm.operator()<std::deque<int>, true>("rng::is_partitioned(deque<int>) (partitioned)", std::ranges::is_partitioned);
+  bm.operator()<std::deque<int>, false>("rng::is_partitioned(deque<int>) (unpartitioned)", std::ranges::is_partitioned);
+
+  bm.operator()<std::list<int>, true>("rng::is_partitioned(list<int>) (partitioned)", std::ranges::is_partitioned);
+  bm.operator()<std::list<int>, false>("rng::is_partitioned(list<int>) (unpartitioned)", std::ranges::is_partitioned);
+
+  benchmark::Initialize(&argc, argv);
+  benchmark::RunSpecifiedBenchmarks();
+  benchmark::Shutdown();
+  return 0;
+}

libcxx/test/benchmarks/algorithms/partitions/partition.bench.cpp

@@ -0,0 +1,133 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <deque>
+#include <iterator>
+#include <list>
+#include <string>
+#include <vector>
+
+#include "benchmark/benchmark.h"
+#include "../../GenerateInput.h"
+
+auto compute_median(auto first, auto last) {
+  std::vector v(first, last);
+  auto middle = v.begin() + v.size() / 2;
+  std::nth_element(v.begin(), middle, v.end());
+  return *middle;
+}
+
+int main(int argc, char** argv) {
+  auto std_partition = [](auto first, auto last, auto pred) { return std::partition(first, last, pred); };
+
+  // Benchmark {std,ranges}::partition on a fully unpartitionned sequence, i.e. a lot of elements
+  // have to be moved around in order to partition the range.
+  {
+    auto bm = []<class Container>(std::string name, auto partition) {
+      benchmark::RegisterBenchmark(
+          name,
+          [partition](auto& st) {
+            std::size_t const size = st.range(0);
+            using ValueType        = typename Container::value_type;
+            Container c;
+            std::generate_n(std::back_inserter(c), size, [] { return Generate<ValueType>::random(); });
+
+            ValueType median = compute_median(c.begin(), c.end());
+            auto pred1       = [median](auto const& element) { return element < median; };
+            auto pred2       = [median](auto const& element) { return element > median; };
+            bool toggle      = false;
+
+            for ([[maybe_unused]] auto _ : st) {
+              benchmark::DoNotOptimize(c);
+              // By toggling the predicate, we have to move almost all elements in the sequence
+              // to restore the partition.
+              if (toggle) {
+                auto result = partition(c.begin(), c.end(), pred1);
+                benchmark::DoNotOptimize(result);
+              } else {
+                auto result = partition(c.begin(), c.end(), pred2);
+                benchmark::DoNotOptimize(result);
+              }
+              toggle = !toggle;
+            }
+          })
+          ->Arg(32)
+          ->Arg(50) // non power-of-two
+          ->Arg(1024)
+          ->Arg(8192);
+    };
+
+    // std::partition
+    bm.operator()<std::vector<int>>("std::partition(vector<int>) (dense)", std_partition);
+    bm.operator()<std::deque<int>>("std::partition(deque<int>) (dense)", std_partition);
+    bm.operator()<std::list<int>>("std::partition(list<int>) (dense)", std_partition);
+
+    // ranges::partition
+    bm.operator()<std::vector<int>>("rng::partition(vector<int>) (dense)", std::ranges::partition);
+    bm.operator()<std::deque<int>>("rng::partition(deque<int>) (dense)", std::ranges::partition);
+    bm.operator()<std::list<int>>("rng::partition(list<int>) (dense)", std::ranges::partition);
+  }
+
+  // Benchmark {std,ranges}::partition on a mostly partitioned sequence, i.e. only 10% of the elements
+  // have to be moved around in order to partition the range.
+  {
+    auto bm = []<class Container>(std::string name, auto partition) {
+      benchmark::RegisterBenchmark(
+          name,
+          [partition](auto& st) {
+            std::size_t const size = st.range(0);
+            using ValueType        = typename Container::value_type;
+            Container c;
+            std::generate_n(std::back_inserter(c), size, [] { return Generate<ValueType>::random(); });
+            ValueType median = compute_median(c.begin(), c.end());
+            auto pred        = [median](auto const& element) { return element < median; };
+            std::partition(c.begin(), c.end(), pred);
+
+            // Between iterations, we swap 5% of the elements to the left of the median with 5% of the elements
+            // to the right of the median. This ensures that the range is slightly unpartitioned.
+            auto median_it = std::partition_point(c.begin(), c.end(), pred);
+            auto low       = std::next(c.begin(), std::distance(c.begin(), median_it) - (size / 20));
+            auto high      = std::next(median_it, size / 20);
+            auto shuffle   = [&] { std::swap_ranges(low, median_it, high); };
+            shuffle();
+            assert(!std::is_partitioned(c.begin(), c.end(), pred));
+
+            for ([[maybe_unused]] auto _ : st) {
+              benchmark::DoNotOptimize(c);
+              auto result = partition(c.begin(), c.end(), pred);
+              benchmark::DoNotOptimize(result);
+              shuffle();
+            }
+          })
+          ->Arg(32)
+          ->Arg(50) // non power-of-two
+          ->Arg(1024)
+          ->Arg(8192);
+    };
+
+    // std::partition
+    bm.operator()<std::vector<int>>("std::partition(vector<int>) (sparse)", std_partition);
+    bm.operator()<std::deque<int>>("std::partition(deque<int>) (sparse)", std_partition);
+    bm.operator()<std::list<int>>("std::partition(list<int>) (sparse)", std_partition);
+
+    // ranges::partition
+    bm.operator()<std::vector<int>>("rng::partition(vector<int>) (sparse)", std::ranges::partition);
+    bm.operator()<std::deque<int>>("rng::partition(deque<int>) (sparse)", std::ranges::partition);
+    bm.operator()<std::list<int>>("rng::partition(list<int>) (sparse)", std::ranges::partition);
+  }
+
+  benchmark::Initialize(&argc, argv);
+  benchmark::RunSpecifiedBenchmarks();
+  benchmark::Shutdown();
+  return 0;
+}