gdbsupport/thread-pool.h

   1 /* Thread pool
   2
   3    Copyright (C) 2019-2024 Free Software Foundation, Inc.
   4
   5    This file is part of GDB.
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20 #ifndef GDBSUPPORT_THREAD_POOL_H
  21 #define GDBSUPPORT_THREAD_POOL_H
  22
  23 #include <queue>
  24 #include <vector>
  25 #include <functional>
  26 #include <chrono>
  27 #if CXX_STD_THREAD
  28 #include <thread>
  29 #include <mutex>
  30 #include <condition_variable>
  31 #include <future>
  32 #endif
  33 #include <optional>
  34
  35 namespace gdb
  36 {
  37
  38 #if CXX_STD_THREAD
  39
  40 /* Simply use the standard future.  */
  41 template<typename T>
  42 using future = std::future<T>;
  43
  44 /* ... and the standard future_status.  */
  45 using future_status = std::future_status;
  46
  47 #else /* CXX_STD_THREAD */
  48
  49 /* A compatibility enum for std::future_status.  This is just the
  50    subset needed by gdb.  */
  51 enum class future_status
  52 {
  53   ready,
  54   timeout,
  55 };
  56
  57 /* A compatibility wrapper for std::future.  Once <thread> and
  58    <future> are available in all GCC builds -- should that ever happen
  59    -- this can be removed.  GCC does not implement threading for
  60    MinGW, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93687.
  61
  62    Meanwhile, in this mode, there are no threads.  Tasks submitted to
  63    the thread pool are invoked immediately and their result is stored
  64    here.  The base template here simply wraps a T and provides some
  65    std::future compatibility methods.  The provided methods are chosen
  66    based on what GDB needs presently.  */
  67
  68 template<typename T>
  69 class future
  70 {
  71 public:
  72
  73   explicit future (T value)
  74     : m_value (std::move (value))
  75   {
  76   }
  77
  78   future () = default;
  79   future (future &&other) = default;
  80   future (const future &other) = delete;
  81   future &operator= (future &&other) = default;
  82   future &operator= (const future &other) = delete;
  83
  84   void wait () const { }
  85
  86   template<class Rep, class Period>
  87   future_status wait_for (const std::chrono::duration<Rep,Period> &duration)
  88     const
  89   {
  90     return future_status::ready;
  91   }
  92
  93   T get () { return std::move (m_value); }
  94
  95 private:
  96
  97   T m_value;
  98 };
  99
 100 /* A specialization for void.  */
 101
 102 template<>
 103 class future<void>
 104 {
 105 public:
 106   void wait () const { }
 107
 108   template<class Rep, class Period>
 109   future_status wait_for (const std::chrono::duration<Rep,Period> &duration)
 110     const
 111   {
 112     return future_status::ready;
 113   }
 114
 115   void get () { }
 116 };
 117
 118 #endif /* CXX_STD_THREAD */
 119
 120
 121 /* A thread pool.
 122
 123    There is a single global thread pool, see g_thread_pool.  Tasks can
 124    be submitted to the thread pool.  They will be processed in worker
 125    threads as time allows.  */
 126 class thread_pool
 127 {
 128 public:
 129   /* The sole global thread pool.  */
 130   static thread_pool *g_thread_pool;
 131
 132   ~thread_pool ();
 133   DISABLE_COPY_AND_ASSIGN (thread_pool);
 134
 135   /* Set the thread count of this thread pool.  By default, no threads
 136      are created -- the thread count must be set first.  */
 137   void set_thread_count (size_t num_threads);
 138
 139   /* Return the number of executing threads.  */
 140   size_t thread_count () const
 141   {
 142 #if CXX_STD_THREAD
 143     return m_thread_count;
 144 #else
 145     return 0;
 146 #endif
 147   }
 148
 149   /* Post a task to the thread pool.  A future is returned, which can
 150      be used to wait for the result.  */
 151   future<void> post_task (std::function<void ()> &&func)
 152   {
 153 #if CXX_STD_THREAD
 154     std::packaged_task<void ()> task (std::move (func));
 155     future<void> result = task.get_future ();
 156     do_post_task (std::packaged_task<void ()> (std::move (task)));
 157     return result;
 158 #else
 159     func ();
 160     return {};
 161 #endif /* CXX_STD_THREAD */
 162   }
 163
 164   /* Post a task to the thread pool.  A future is returned, which can
 165      be used to wait for the result.  */
 166   template<typename T>
 167   future<T> post_task (std::function<T ()> &&func)
 168   {
 169 #if CXX_STD_THREAD
 170     std::packaged_task<T ()> task (std::move (func));
 171     future<T> result = task.get_future ();
 172     do_post_task (std::packaged_task<void ()> (std::move (task)));
 173     return result;
 174 #else
 175     return future<T> (func ());
 176 #endif /* CXX_STD_THREAD */
 177   }
 178
 179 private:
 180
 181   thread_pool () = default;
 182
 183 #if CXX_STD_THREAD
 184   /* The callback for each worker thread.  */
 185   void thread_function ();
 186
 187   /* Post a task to the thread pool.  A future is returned, which can
 188      be used to wait for the result.  */
 189   void do_post_task (std::packaged_task<void ()> &&func);
 190
 191   /* The current thread count.  */
 192   size_t m_thread_count = 0;
 193
 194   /* A convenience typedef for the type of a task.  */
 195   typedef std::packaged_task<void ()> task_t;
 196
 197   /* The tasks that have not been processed yet.  An optional is used
 198      to represent a task.  If the optional is empty, then this means
 199      that the receiving thread should terminate.  If the optional is
 200      non-empty, then it is an actual task to evaluate.  */
 201   std::queue<std::optional<task_t>> m_tasks;
 202
 203   /* A condition variable and mutex that are used for communication
 204      between the main thread and the worker threads.  */
 205   std::condition_variable m_tasks_cv;
 206   std::mutex m_tasks_mutex;
 207   bool m_sized_at_least_once = false;
 208 #endif /* CXX_STD_THREAD */
 209 };
 210
 211 }
 212
 213 #endif /* GDBSUPPORT_THREAD_POOL_H */