[libcxx-commits] [libcxx] [libc++] Introduce one-sided binary search for set_intersection so that it can be completed in sub-linear time for a large class of inputs. (PR #75230)

[Louis Dionne via libcxx-commits]

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+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include <algorithm>
+#include <iterator>
+#include <set>
+#include <stdlib.h>
+#include <vector>
+
+#include "common.h"
+#include "test_iterators.h"
+
+namespace {
+
+// types of containers we'll want to test, covering interesting iterator types
+struct VectorContainer {
+  template <typename... Args>
+  using type = std::vector<Args...>;
+
+  static constexpr const char* Name = "Vector";
+};
+
+struct SetContainer {
+  template <typename... Args>
+  using type = std::set<Args...>;
+
+  static constexpr const char* Name = "Set";
+};
+
+using AllContainerTypes = std::tuple<VectorContainer, SetContainer>;
+
+// set_intersection performance may depend on where matching values lie
+enum class OverlapPosition {
+  None,
+  Front,
+  // performance-wise, matches at the back are identical to ones at the front
+  Interlaced,
+};
+
+struct AllOverlapPositions : EnumValuesAsTuple<AllOverlapPositions, OverlapPosition, 3> {
+  static constexpr const char* Names[] = {"None", "Front", "Interlaced"};
+};
+
+// functor that moves elements from an iterator range into a new Container instance
+template <typename Container>
+struct MoveInto {
+  template <class It>
+  [[nodiscard]] static Container operator()(It first, It last) {
+    Container out;
+    std::move(first, last, std::inserter(out, out.begin()));
+    return out;
+  }
+};
+
+// lightweight wrapping around fillValues() which puts a little effort into
+// making that would be contiguous when sorted non-contiguous in memory
+template <typename T>
+std::vector<T> getVectorOfRandom(size_t N) {
+  std::vector<T> v;
+  fillValues(v, N, Order::Random);
+  sortValues(v, Order::Random);
+  return std::vector<T>(v);
+}
+
+// forward_iterator wrapping which, for each increment, moves the underlying iterator forward Stride elements
+template <typename Wrapped>
+struct StridedFwdIt {
+  Wrapped base_;
+  unsigned stride_;
+
+  using iterator_category = std::forward_iterator_tag;
+  using difference_type   = typename Wrapped::difference_type;
+  using value_type        = typename Wrapped::value_type;
+  using pointer           = typename Wrapped::pointer;
+  using reference         = typename Wrapped::reference;
+
+  StridedFwdIt(Wrapped base, unsigned stride) : base_(base), stride_(stride) { assert(stride_ != 0); }
+
+  StridedFwdIt operator++() {
+    for (unsigned i = 0; i < stride_; ++i)
+      ++base_;
+    return *this;
+  }
+  StridedFwdIt operator++(int) {
+    auto tmp = *this;
+    ++*this;
+    return tmp;
+  }
+  value_type& operator*() { return *base_; }
+  const value_type& operator*() const { return *base_; }
+  value_type& operator->() { return *base_; }
+  const value_type& operator->() const { return *base_; }
+  bool operator==(const StridedFwdIt& o) const { return base_ == o.base_; }
+  bool operator!=(const StridedFwdIt& o) const { return !operator==(o); }
+};
+template <typename Wrapped>
+StridedFwdIt(Wrapped, unsigned) -> StridedFwdIt<Wrapped>;
+
+// realistically, data won't all be nicely contiguous in a container
+// we'll go through some effort to ensure that it's shuffled through memory
+template <class Container>
+std::pair<Container, Container> genCacheUnfriendlyData(size_t size1, size_t size2, OverlapPosition pos) {
+  using ValueType = typename Container::value_type;
+  const MoveInto<Container> move_into;
+  const auto src_size        = pos == OverlapPosition::None ? size1 + size2 : std::max(size1, size2);
+  std::vector<ValueType> src = getVectorOfRandom<ValueType>(src_size);
+
+  if (pos == OverlapPosition::None) {
+    std::sort(src.begin(), src.end());
+    return std::make_pair(move_into(src.begin(), src.begin() + size1), move_into(src.begin() + size1, src.end()));
+  }
+
+  // all other overlap types will have to copy some part of the data, but if
+  // we copy after sorting it will likely have high cache locality, so we sort
+  // each copy separately
+  auto copy = src;
+  std::sort(src.begin(), src.end());
+  std::sort(copy.begin(), copy.end());
+
+  switch (pos) {
+  case OverlapPosition::None:
+    break;
+
+  case OverlapPosition::Front:
+    return std::make_pair(move_into(src.begin(), src.begin() + size1), move_into(copy.begin(), copy.begin() + size2));
+
+  case OverlapPosition::Interlaced:
+    const auto stride1 = size1 < size2 ? size2 / size1 : 1;
+    const auto stride2 = size2 < size1 ? size1 / size2 : 1;
+    return std::make_pair(move_into(StridedFwdIt(src.begin(), stride1), StridedFwdIt(src.end(), stride1)),
+                          move_into(StridedFwdIt(copy.begin(), stride2), StridedFwdIt(copy.end(), stride2)));
+  }
+  abort();
+  return std::pair<Container, Container>();
+}
+
+// use environment variable to enable additional counters: instrumentation will
+// impact CPU utilisation, let's give the user the option
+static const bool TRACK_COUNTERS = getenv("TRACK_COUNTERS") != nullptr;
+
+template <class ValueType, class Container, class Overlap>
+struct SetIntersection {
+  using ContainerType = typename Container::template type<Value<ValueType>>;
+  size_t size1_;
+  size_t size2_;
+
+  SetIntersection(size_t size1, size_t size2) : size1_(size1), size2_(size2) {}
+
+  bool skip() const noexcept {
+    // let's save some time and skip simmetrical runs
+    return size1_ < size2_;
+  }
+
+  void run(benchmark::State& state) const {
+    state.PauseTiming();
+    auto input = genCacheUnfriendlyData<ContainerType>(size1_, size2_, Overlap());
+    std::vector<Value<ValueType>> out(std::min(size1_, size2_));
+
+    const auto BATCH_SIZE = std::max(size_t{512}, (2 * TestSetElements) / (size1_ + size2_));
+    state.ResumeTiming();
+
+    for (const auto& _ : state) {
+      while (state.KeepRunningBatch(BATCH_SIZE)) {
+        for (unsigned i = 0; i < BATCH_SIZE; ++i) {
+          const auto& [c1, c2] = input;
+          if (TRACK_COUNTERS) {
----------------
ldionne wrote:

I think this is a really interesting idea (being able to count the number of x/y/z operations in an algorithm). However, concretely this changes a lot of things in the benchmark. For example, we could potentially be using a different overload (e.g. many algorithms are optimized for raw pointers). So what is sold as being "just additional counters" in reality may have a deeper impact on what's being benchmarked. For that reason, I think these benchmarks may not be the right medium for counting operations. I'd remove this option.

I'd encourage you to think about how to solve this for other algorithms as well, since this seems like a useful facility to have. I think we'd want to have "the big picture" here, for example whether we want to standardize the counters that we gather across all algorithms, whether we want a common utility to do it, etc.

https://github.com/llvm/llvm-project/pull/75230