[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]

================
@@ -0,0 +1,223 @@
+//===----------------------------------------------------------------------===//
+//
+// 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();
----------------
ldionne wrote:

```suggestion
  std::abort();
```

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