[PATCH 1/4] Avoid submitting empty tasks in parallel_for_each

[Tom Tromey <tromey@adacore.com>]

I found that parallel_for_each would submit empty tasks to the thread
pool.  For example, this can happen if the number of tasks is smaller
than the number of available threads.  In the DWARF reader, this
resulted in the cooked index containing empty sub-indices.  This patch
arranges to instead shrink the result vector and process the trailing
entries in the calling thread.
---
 gdbsupport/parallel-for.h | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/gdbsupport/parallel-for.h b/gdbsupport/parallel-for.h
index acd9137efbd..ed460f31f63 100644
--- a/gdbsupport/parallel-for.h
+++ b/gdbsupport/parallel-for.h
@@ -70,6 +70,12 @@ struct par_for_accumulator
     return result;
   }
 
+  /* Resize the results to N.  */
+  void resize (size_t n)
+  {
+    m_futures.resize (n);
+  }
+
 private:
   
   /* A vector of futures coming from the tasks run in the
@@ -108,6 +114,12 @@ struct par_for_accumulator<void>
       }
   }
 
+  /* Resize the results to N.  */
+  void resize (size_t n)
+  {
+    m_futures.resize (n);
+  }
+
 private:
 
   std::vector<gdb::future<void>> m_futures;
@@ -232,6 +244,17 @@ parallel_for_each (unsigned n, RandomIt first, RandomIt last,
 	  end = j;
 	  remaining_size -= chunk_size;
 	}
+
+      if (end == last)
+	{
+	  /* We're about to dispatch the last batch of elements, which
+	     we normally process in the main thread.  So just truncate
+	     the result list here.  */
+	  count = i;
+	  results.resize (count);
+	  break;
+	}
+
       if (parallel_for_each_debug)
 	{
 	  debug_printf (_("Parallel for: elements on worker thread %i\t: %zu"),
-- 
2.34.3