Add a new task pool class to LLDB

The purpose of the class is to make it easy to execute tasks in parallel

Basic design goals:
* Have a very lightweight and easy to use interface where a list of
  lambdas can be executed in parallel
* Use a global thread pool to limit the number of threads used
  (std::async don't do it on Linux) and to eliminate the thread creation
  overhead
* Destroy the thread currently not in use to avoid the confusion caused
  by them during debugging LLDB

Possible future improvements:
* Possibility to cancel already added, but not yet started tasks
* Parallel for_each implementation
* Optimizations in the thread creation destroyation code

Differential revision: http://reviews.llvm.org/D13727

llvm-svn: 250820

lldb/source/Utility/TaskPool.cpp

@@ -0,0 +1,88 @@
+//===--------------------- TaskPool.cpp -------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "lldb/Utility/TaskPool.h"
+
+namespace
+{
+    class TaskPoolImpl
+    {
+    public:
+        static TaskPoolImpl&
+        GetInstance();
+
+        void
+        AddTask(std::function<void()>&& task_fn);
+
+    private:
+        TaskPoolImpl();
+
+        static void
+        Worker(TaskPoolImpl* pool);
+
+        std::queue<std::function<void()>> m_tasks;
+        std::mutex                        m_tasks_mutex;
+        uint32_t                          m_thread_count;
+    };
+
+} // end of anonymous namespace
+
+TaskPoolImpl&
+TaskPoolImpl::GetInstance()
+{
+    static TaskPoolImpl g_task_pool_impl;
+    return g_task_pool_impl;
+}
+
+void
+TaskPool::AddTaskImpl(std::function<void()>&& task_fn)
+{
+    TaskPoolImpl::GetInstance().AddTask(std::move(task_fn));
+}
+
+TaskPoolImpl::TaskPoolImpl() :
+    m_thread_count(0)
+{
+}
+
+void
+TaskPoolImpl::AddTask(std::function<void()>&& task_fn)
+{
+    static const uint32_t max_threads = std::thread::hardware_concurrency();
+
+    std::unique_lock<std::mutex> lock(m_tasks_mutex);
+    m_tasks.emplace(std::move(task_fn));
+    if (m_thread_count < max_threads)
+    {
+        m_thread_count++;
+        lock.unlock();
+
+        std::thread (Worker, this).detach();
+    }
+}
+
+void
+TaskPoolImpl::Worker(TaskPoolImpl* pool)
+{
+    while (true)
+    {
+        std::unique_lock<std::mutex> lock(pool->m_tasks_mutex);
+        if (pool->m_tasks.empty())
+        {
+            pool->m_thread_count--;
+            break;
+        }
+
+        std::function<void()> f = pool->m_tasks.front();
+        pool->m_tasks.pop();
+        lock.unlock();
+
+        f();
+    }
+}