1//===- llvm/Support/Parallel.h - Parallel algorithms ----------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#ifndef LLVM_SUPPORT_PARALLEL_H
10#define LLVM_SUPPORT_PARALLEL_H
11
12#include "llvm/ADT/STLExtras.h"
13#include "llvm/Config/llvm-config.h"
14#include "llvm/Support/MathExtras.h"
15
16#include <algorithm>
17#include <condition_variable>
18#include <functional>
19#include <mutex>
20
21namespace llvm {
22
23namespace parallel {
24struct sequential_execution_policy {};
25struct parallel_execution_policy {};
26
27template <typename T>
28struct is_execution_policy
29    : public std::integral_constant<
30          bool, llvm::is_one_of<T, sequential_execution_policy,
31                                parallel_execution_policy>::value> {};
32
33constexpr sequential_execution_policy seq{};
34constexpr parallel_execution_policy par{};
35
36namespace detail {
37
38#if LLVM_ENABLE_THREADS
39
40class Latch {
41  uint32_t Count;
42  mutable std::mutex Mutex;
43  mutable std::condition_variable Cond;
44
45public:
46  explicit Latch(uint32_t Count = 0) : Count(Count) {}
47  ~Latch() { sync(); }
48
49  void inc() {
50    std::lock_guard<std::mutex> lock(Mutex);
51    ++Count;
52  }
53
54  void dec() {
55    std::lock_guard<std::mutex> lock(Mutex);
56    if (--Count == 0)
57      Cond.notify_all();
58  }
59
60  void sync() const {
61    std::unique_lock<std::mutex> lock(Mutex);
62    Cond.wait(lock, [&] { return Count == 0; });
63  }
64};
65
66class TaskGroup {
67  Latch L;
68  bool Parallel;
69
70public:
71  TaskGroup();
72  ~TaskGroup();
73
74  void spawn(std::function<void()> f);
75
76  void sync() const { L.sync(); }
77};
78
79const ptrdiff_t MinParallelSize = 1024;
80
81/// Inclusive median.
82template <class RandomAccessIterator, class Comparator>
83RandomAccessIterator medianOf3(RandomAccessIterator Start,
84                               RandomAccessIterator End,
85                               const Comparator &Comp) {
86  RandomAccessIterator Mid = Start + (std::distance(Start, End) / 2);
87  return Comp(*Start, *(End - 1))
88             ? (Comp(*Mid, *(End - 1)) ? (Comp(*Start, *Mid) ? Mid : Start)
89                                       : End - 1)
90             : (Comp(*Mid, *Start) ? (Comp(*(End - 1), *Mid) ? Mid : End - 1)
91                                   : Start);
92}
93
94template <class RandomAccessIterator, class Comparator>
95void parallel_quick_sort(RandomAccessIterator Start, RandomAccessIterator End,
96                         const Comparator &Comp, TaskGroup &TG, size_t Depth) {
97  // Do a sequential sort for small inputs.
98  if (std::distance(Start, End) < detail::MinParallelSize || Depth == 0) {
99    llvm::sort(Start, End, Comp);
100    return;
101  }
102
103  // Partition.
104  auto Pivot = medianOf3(Start, End, Comp);
105  // Move Pivot to End.
106  std::swap(*(End - 1), *Pivot);
107  Pivot = std::partition(Start, End - 1, [&Comp, End](decltype(*Start) V) {
108    return Comp(V, *(End - 1));
109  });
110  // Move Pivot to middle of partition.
111  std::swap(*Pivot, *(End - 1));
112
113  // Recurse.
114  TG.spawn([=, &Comp, &TG] {
115    parallel_quick_sort(Start, Pivot, Comp, TG, Depth - 1);
116  });
117  parallel_quick_sort(Pivot + 1, End, Comp, TG, Depth - 1);
118}
119
120template <class RandomAccessIterator, class Comparator>
121void parallel_sort(RandomAccessIterator Start, RandomAccessIterator End,
122                   const Comparator &Comp) {
123  TaskGroup TG;
124  parallel_quick_sort(Start, End, Comp, TG,
125                      llvm::Log2_64(std::distance(Start, End)) + 1);
126}
127
128template <class IterTy, class FuncTy>
129void parallel_for_each(IterTy Begin, IterTy End, FuncTy Fn) {
130  // TaskGroup has a relatively high overhead, so we want to reduce
131  // the number of spawn() calls. We'll create up to 1024 tasks here.
132  // (Note that 1024 is an arbitrary number. This code probably needs
133  // improving to take the number of available cores into account.)
134  ptrdiff_t TaskSize = std::distance(Begin, End) / 1024;
135  if (TaskSize == 0)
136    TaskSize = 1;
137
138  TaskGroup TG;
139  while (TaskSize < std::distance(Begin, End)) {
140    TG.spawn([=, &Fn] { std::for_each(Begin, Begin + TaskSize, Fn); });
141    Begin += TaskSize;
142  }
143  std::for_each(Begin, End, Fn);
144}
145
146template <class IndexTy, class FuncTy>
147void parallel_for_each_n(IndexTy Begin, IndexTy End, FuncTy Fn) {
148  ptrdiff_t TaskSize = (End - Begin) / 1024;
149  if (TaskSize == 0)
150    TaskSize = 1;
151
152  TaskGroup TG;
153  IndexTy I = Begin;
154  for (; I + TaskSize < End; I += TaskSize) {
155    TG.spawn([=, &Fn] {
156      for (IndexTy J = I, E = I + TaskSize; J != E; ++J)
157        Fn(J);
158    });
159  }
160  for (IndexTy J = I; J < End; ++J)
161    Fn(J);
162}
163
164#endif
165
166template <typename Iter>
167using DefComparator =
168    std::less<typename std::iterator_traits<Iter>::value_type>;
169
170} // namespace detail
171
172// sequential algorithm implementations.
173template <class Policy, class RandomAccessIterator,
174          class Comparator = detail::DefComparator<RandomAccessIterator>>
175void sort(Policy policy, RandomAccessIterator Start, RandomAccessIterator End,
176          const Comparator &Comp = Comparator()) {
177  static_assert(is_execution_policy<Policy>::value,
178                "Invalid execution policy!");
179  llvm::sort(Start, End, Comp);
180}
181
182template <class Policy, class IterTy, class FuncTy>
183void for_each(Policy policy, IterTy Begin, IterTy End, FuncTy Fn) {
184  static_assert(is_execution_policy<Policy>::value,
185                "Invalid execution policy!");
186  std::for_each(Begin, End, Fn);
187}
188
189template <class Policy, class IndexTy, class FuncTy>
190void for_each_n(Policy policy, IndexTy Begin, IndexTy End, FuncTy Fn) {
191  static_assert(is_execution_policy<Policy>::value,
192                "Invalid execution policy!");
193  for (IndexTy I = Begin; I != End; ++I)
194    Fn(I);
195}
196
197// Parallel algorithm implementations, only available when LLVM_ENABLE_THREADS
198// is true.
199#if LLVM_ENABLE_THREADS
200template <class RandomAccessIterator,
201          class Comparator = detail::DefComparator<RandomAccessIterator>>
202void sort(parallel_execution_policy policy, RandomAccessIterator Start,
203          RandomAccessIterator End, const Comparator &Comp = Comparator()) {
204  detail::parallel_sort(Start, End, Comp);
205}
206
207template <class IterTy, class FuncTy>
208void for_each(parallel_execution_policy policy, IterTy Begin, IterTy End,
209              FuncTy Fn) {
210  detail::parallel_for_each(Begin, End, Fn);
211}
212
213template <class IndexTy, class FuncTy>
214void for_each_n(parallel_execution_policy policy, IndexTy Begin, IndexTy End,
215                FuncTy Fn) {
216  detail::parallel_for_each_n(Begin, End, Fn);
217}
218#endif
219
220} // namespace parallel
221} // namespace llvm
222
223#endif // LLVM_SUPPORT_PARALLEL_H
224