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 
21 namespace llvm {
22 
23 namespace parallel {
24 struct sequential_execution_policy {};
25 struct parallel_execution_policy {};
26 
27 template <typename T>
28 struct is_execution_policy
29     : public std::integral_constant<
30           bool, llvm::is_one_of<T, sequential_execution_policy,
31                                 parallel_execution_policy>::value> {};
32 
33 constexpr sequential_execution_policy seq{};
34 constexpr parallel_execution_policy par{};
35 
36 namespace detail {
37 
38 #if LLVM_ENABLE_THREADS
39 
40 class Latch {
41   uint32_t Count;
42   mutable std::mutex Mutex;
43   mutable std::condition_variable Cond;
44 
45 public:
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 
66 class TaskGroup {
67   Latch L;
68   bool Parallel;
69 
70 public:
71   TaskGroup();
72   ~TaskGroup();
73 
74   void spawn(std::function<void()> f);
75 
76   void sync() const { L.sync(); }
77 };
78 
79 const ptrdiff_t MinParallelSize = 1024;
80 
81 /// Inclusive median.
82 template <class RandomAccessIterator, class Comparator>
83 RandomAccessIterator 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 
94 template <class RandomAccessIterator, class Comparator>
95 void 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 
120 template <class RandomAccessIterator, class Comparator>
121 void 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 
128 template <class IterTy, class FuncTy>
129 void 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 
146 template <class IndexTy, class FuncTy>
147 void 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 
166 template <typename Iter>
167 using DefComparator =
168     std::less<typename std::iterator_traits<Iter>::value_type>;
169 
170 } // namespace detail
171 
172 // sequential algorithm implementations.
173 template <class Policy, class RandomAccessIterator,
174           class Comparator = detail::DefComparator<RandomAccessIterator>>
175 void 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 
182 template <class Policy, class IterTy, class FuncTy>
183 void 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 
189 template <class Policy, class IndexTy, class FuncTy>
190 void 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
200 template <class RandomAccessIterator,
201           class Comparator = detail::DefComparator<RandomAccessIterator>>
202 void sort(parallel_execution_policy policy, RandomAccessIterator Start,
203           RandomAccessIterator End, const Comparator &Comp = Comparator()) {
204   detail::parallel_sort(Start, End, Comp);
205 }
206 
207 template <class IterTy, class FuncTy>
208 void for_each(parallel_execution_policy policy, IterTy Begin, IterTy End,
209               FuncTy Fn) {
210   detail::parallel_for_each(Begin, End, Fn);
211 }
212 
213 template <class IndexTy, class FuncTy>
214 void 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