From d13d16c4d8e976ebd00353e04ca8a761b8850a11 Mon Sep 17 00:00:00 2001
From: Christian Biesinger <cbiesinger@google.com>
Date: Tue, 8 Oct 2019 17:25:26 -0500
Subject: Switch to futures

---
 gdb/gdbsupport/parallel-for.h | 26 ++++++++------------------
 gdb/gdbsupport/thread_pool.c  |  2 +-
 gdb/gdbsupport/thread_pool.h  | 10 +++++++---
 3 files changed, 16 insertions(+), 22 deletions(-)

diff --git a/gdb/gdbsupport/parallel-for.h b/gdb/gdbsupport/parallel-for.h
index 6f4235da363..11af33d61f1 100644
--- a/gdb/gdbsupport/parallel-for.h
+++ b/gdb/gdbsupport/parallel-for.h
@@ -68,9 +68,7 @@ parallel_for_each (RandomIt first, RandomIt last, RangeFunction callback)
     parallel_for_pool.start (n_threads);
   }
 
-  std::mutex mtx;
-  std::condition_variable cv;
-  int num_finished = 0;
+  std::future<void> futures[n_threads];
 
   size_t n_elements = last - first;
   if (n_threads > 1 && 2 * n_threads <= n_elements)
@@ -79,12 +77,9 @@ parallel_for_each (RandomIt first, RandomIt last, RangeFunction callback)
       for (int i = 0; i < n_threads; ++i)
 	{
 	  RandomIt end = first + elts_per_thread;
-	  parallel_for_pool.post_task ([&, first, end] () {
-				       callback (first, end);
-				       std::unique_lock<std::mutex> lck (mtx);
-				       num_finished++;
-				       cv.notify_all ();
-				       });
+	  futures[i] = parallel_for_pool.post_task ([&, first, end] () {
+						     callback (first, end);
+						     });
 	  first = end;
 	}
     }
@@ -94,15 +89,10 @@ parallel_for_each (RandomIt first, RandomIt last, RangeFunction callback)
 
   /* Process all the remaining elements in the main thread.  */
   callback (first, last);
-  if (n_threads)
-    {
-      for (;;) {
-	std::unique_lock<std::mutex> lck (mtx);
-	if (num_finished == n_threads)
-	  break;
-	cv.wait (lck);
-      }
-    }
+#ifdef CXX_STD_THREAD
+  for (size_t i = 0; i < n_threads; ++i)
+    futures[i].wait ();
+#endif /* CXX_STD_THREAD */
 }
 
 }
diff --git a/gdb/gdbsupport/thread_pool.c b/gdb/gdbsupport/thread_pool.c
index 03fef956df7..a78edeaef7a 100644
--- a/gdb/gdbsupport/thread_pool.c
+++ b/gdb/gdbsupport/thread_pool.c
@@ -40,7 +40,7 @@ thread_pool::thread_function ()
 	  break;
 	if (m_tasks.empty ())
 	  continue;
-	t = m_tasks.front();
+	t = std::move (m_tasks.front());
 	m_tasks.pop();
       }
       t ();
diff --git a/gdb/gdbsupport/thread_pool.h b/gdb/gdbsupport/thread_pool.h
index 77760a95042..9717263309f 100644
--- a/gdb/gdbsupport/thread_pool.h
+++ b/gdb/gdbsupport/thread_pool.h
@@ -8,6 +8,7 @@
 #include <atomic>
 #include <mutex>
 #include <condition_variable>
+#include <future>
 
 namespace gdb {
 
@@ -20,11 +21,14 @@ class thread_pool {
 
   void start(size_t num_threads);
 
-  typedef std::function<void ()> task;
-  void post_task(task t) {
+  typedef std::packaged_task<void()> task;
+  std::future<void> post_task(std::function<void ()> func) {
+    task t(func);
+    std::future<void> f = t.get_future();
     std::lock_guard<std::mutex> guard (m_tasks_mutex);
-    m_tasks.push (t);
+    m_tasks.push (std::move (t));
     m_tasks_cv.notify_one ();
+    return f;
   }
 
  private:
-- 
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