Allow thread-pool.h to work without threads

thread-pool.h requires CXX_STD_THREAD in order to even be included.
However, there's no deep reason for this, and during review we found
that one patch in the new DWARF indexer series unconditionally
requires the thread pool.

Because the thread pool already allows a task to be run in the calling
thread (for example if it is configured to have no threads in the
pool), it seemed straightforward to make this code ok to use when host
threads aren't available at all.

This patch implements this idea.  I built it on a thread-less host
(mingw, before my recent configure patch) and verified that the result
builds.

After the thread-pool change, parallel-for.h no longer needs any
CXX_STD_THREAD checks at all, so this patch removes these as well.

diff --git a/gdb/maint.c b/gdb/maint.c
index 3f3dad5bd79424af75056da8fd116b4daf0051aa..60e183efdd1bda68b5955224b36fb2e1b20dabce 100644 (file)
--- a/gdb/maint.c
+++ b/gdb/maint.c
@@ -41,16 +41,13 @@
 #include "maint.h"
 #include "gdbsupport/selftest.h"
 #include "inferior.h"
+#include "gdbsupport/thread-pool.h"
 
 #include "cli/cli-decode.h"
 #include "cli/cli-utils.h"
 #include "cli/cli-setshow.h"
 #include "cli/cli-cmds.h"
 
-#if CXX_STD_THREAD
-#include "gdbsupport/thread-pool.h"
-#endif
-
 static void maintenance_do_deprecate (const char *, int);
 
 #ifndef _WIN32

diff --git a/gdbsupport/parallel-for.h b/gdbsupport/parallel-for.h
index ca03094bf1215e83a76333eb47e3e66a07dacb95..915814e485e169c6c4d8ee4801cb1d394b87d3a2 100644 (file)
--- a/gdbsupport/parallel-for.h
+++ b/gdbsupport/parallel-for.h
@@ -21,10 +21,7 @@
 #define GDBSUPPORT_PARALLEL_FOR_H
 
 #include <algorithm>
-#if CXX_STD_THREAD
-#include <thread>
 #include "gdbsupport/thread-pool.h"
-#endif
 
 namespace gdb
 {
@@ -41,7 +38,6 @@ template<class RandomIt, class RangeFunction>
 void
 parallel_for_each (RandomIt first, RandomIt last, RangeFunction callback)
 {
-#if CXX_STD_THREAD
   /* So we can use a local array below.  */
   const size_t local_max = 16;
   size_t n_threads = std::min (thread_pool::g_thread_pool->thread_count (),
@@ -70,15 +66,12 @@ parallel_for_each (RandomIt first, RandomIt last, RangeFunction callback)
          first = end;
        }
     }
-#endif /* CXX_STD_THREAD */
 
   /* Process all the remaining elements in the main thread.  */
   callback (first, last);
 
-#if CXX_STD_THREAD
   for (int i = 0; i < n_actual_threads; ++i)
     futures[i].wait ();
-#endif /* CXX_STD_THREAD */
 }
 
 }

diff --git a/gdbsupport/thread-pool.cc b/gdbsupport/thread-pool.cc
index ce19ef23af53ffd439f1d9a2f44312e2818985f0..7d446952cc7154083353b0c9d1eaa7621b7e1083 100644 (file)
--- a/gdbsupport/thread-pool.cc
+++ b/gdbsupport/thread-pool.cc
@@ -18,10 +18,10 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include "common-defs.h"
+#include "gdbsupport/thread-pool.h"
 
 #if CXX_STD_THREAD
 
-#include "gdbsupport/thread-pool.h"
 #include "gdbsupport/alt-stack.h"
 #include "gdbsupport/block-signals.h"
 #include <algorithm>
@@ -67,6 +67,7 @@ set_thread_name (int (*set_name) (const char *), const char *name)
 }
 
 #endif /* USE_PTHREAD_SETNAME_NP */
+#endif /* CXX_STD_THREAD */
 
 namespace gdb
 {
@@ -93,6 +94,7 @@ thread_pool::~thread_pool ()
 void
 thread_pool::set_thread_count (size_t num_threads)
 {
+#if CXX_STD_THREAD
   std::lock_guard<std::mutex> guard (m_tasks_mutex);
 
   /* If the new size is larger, start some new threads.  */
@@ -127,6 +129,9 @@ thread_pool::set_thread_count (size_t num_threads)
     }
 
   m_thread_count = num_threads;
+#else
+  /* No threads available, simply ignore the request.  */
+#endif /* CXX_STD_THREAD */
 }
 
 std::future<void>
@@ -135,20 +140,24 @@ thread_pool::post_task (std::function<void ()> &&func)
   std::packaged_task<void ()> t (std::move (func));
   std::future<void> f = t.get_future ();
 
-  if (m_thread_count == 0)
-    {
-      /* Just execute it now.  */
-      t ();
-    }
-  else
+#if CXX_STD_THREAD
+  if (m_thread_count != 0)
     {
       std::lock_guard<std::mutex> guard (m_tasks_mutex);
       m_tasks.emplace (std::move (t));
       m_tasks_cv.notify_one ();
     }
+  else
+#endif
+    {
+      /* Just execute it now.  */
+      t ();
+    }
   return f;
 }
 
+#if CXX_STD_THREAD
+
 void
 thread_pool::thread_function ()
 {
@@ -182,6 +191,6 @@ thread_pool::thread_function ()
     }
 }
 
-}
-
 #endif /* CXX_STD_THREAD */
+
+} /* namespace gdb */

diff --git a/gdbsupport/thread-pool.h b/gdbsupport/thread-pool.h
index 682106645255d5a15bccd6f05bf87583dce31c78..2672e4d739a90c7eac0eec2c51edacce61502424 100644 (file)
--- a/gdbsupport/thread-pool.h
+++ b/gdbsupport/thread-pool.h
@@ -21,11 +21,13 @@
 #define GDBSUPPORT_THREAD_POOL_H
 
 #include <queue>
-#include <thread>
 #include <vector>
 #include <functional>
+#if CXX_STD_THREAD
+#include <thread>
 #include <mutex>
 #include <condition_variable>
+#endif
 #include <future>
 #include "gdbsupport/gdb_optional.h"
 
@@ -53,7 +55,11 @@ public:
   /* Return the number of executing threads.  */
   size_t thread_count () const
   {
+#if CXX_STD_THREAD
     return m_thread_count;
+#else
+    return 0;
+#endif
   }
 
   /* Post a task to the thread pool.  A future is returned, which can
@@ -64,6 +70,7 @@ private:
 
   thread_pool () = default;
 
+#if CXX_STD_THREAD
   /* The callback for each worker thread.  */
   void thread_function ();
 
@@ -83,6 +90,7 @@ private:
      between the main thread and the worker threads.  */
   std::condition_variable m_tasks_cv;
   std::mutex m_tasks_mutex;
+#endif /* CXX_STD_THREAD */
 };
 
 }