include/llvm/Support/Parallel.h

   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