From 9b4c1ed4b5960a7238edcbed804b1bf02df2034d Mon Sep 17 00:00:00 2001
From: Timo Heister <timo.heister@gmail.com>
Date: Sun, 16 Feb 2020 23:10:10 -0500
Subject: [PATCH] Fix overflow in tria.n_global_active_cells()

for large computations.
---
 source/distributed/tria_base.cc | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/source/distributed/tria_base.cc b/source/distributed/tria_base.cc
index 87b76f80da..189be2a997 100644
--- a/source/distributed/tria_base.cc
+++ b/source/distributed/tria_base.cc
@@ -217,9 +217,12 @@ namespace parallel
         if (cell->subdomain_id() == my_subdomain)
           ++number_cache.n_locally_owned_active_cells;
 
+    // Potentially cast to a 64 bit type before accumulating to avoid overflow:
     number_cache.n_global_active_cells =
-      Utilities::MPI::sum(number_cache.n_locally_owned_active_cells,
+      Utilities::MPI::sum(static_cast<types::global_dof_index>(
+                            number_cache.n_locally_owned_active_cells),
                           this->mpi_communicator);
+
     number_cache.n_global_levels =
       Utilities::MPI::max(this->n_levels(), this->mpi_communicator);
   }
-- 
2.39.5