s_mpi_allreduce_integer_sum integer overflow #1601 (#1605)

Co-authored-by: Spencer Bryngelson <sbryngelson@gmail.com>

Author: danieljvickers

src/common/m_mpi_common.fpp

@@ -367,13 +367,13 @@ contains
     !> Reduce a local integer value to its global sum across all MPI ranks.
     impure subroutine s_mpi_allreduce_integer_sum(var_loc, var_glb)
 
-        integer, intent(in)  :: var_loc
-        integer, intent(out) :: var_glb
+        integer(kind=8), intent(in)  :: var_loc
+        integer(kind=8), intent(out) :: var_glb
 
 #ifdef MFC_MPI
         integer :: ierr  !< Generic flag used to identify and report MPI errors
 
-        call MPI_ALLREDUCE(var_loc, var_glb, 1, MPI_INTEGER, MPI_SUM, MPI_COMM_WORLD, ierr)
+        call MPI_ALLREDUCE(var_loc, var_glb, 1, MPI_INTEGER8, MPI_SUM, MPI_COMM_WORLD, ierr)
 #else
         var_glb = var_loc
 #endif

src/simulation/m_ibm.fpp

@@ -72,8 +72,8 @@ contains
     !> Initializes the values of various IBM variables, such as ghost points and image points.
     impure subroutine s_ibm_setup()
 
-        integer :: i, j, k
-        integer :: max_num_gps
+        integer         :: i, j, k
+        integer(kind=8) :: max_num_gps
 
         call nvtxStartRange("SETUP-IBM-MODULE")
 
@@ -119,10 +119,10 @@ contains
         ! find the number of ghost points and set them to be the maximum total across ranks
         call s_find_num_ghost_points(num_gps)
         if (moving_immersed_boundary_flag) then
-            call s_mpi_allreduce_integer_sum(num_gps, max_num_gps)
-            max_num_gps = min(max_num_gps*2, (m + 1)*(n + 1)*(p + 1))
+            call s_mpi_allreduce_integer_sum(int(num_gps, 8), max_num_gps)
+            max_num_gps = min(max_num_gps*2_8, int(m + 1, 8)*int(n + 1, 8)*int(p + 1, 8))
         else
-            max_num_gps = num_gps
+            max_num_gps = int(num_gps, 8)
         end if
 
         ! set the size of the ghost point arrays to be the amount of points total, plus a factor of 2 buffer
@@ -1071,7 +1071,8 @@ contains
     subroutine s_compute_centroid_offset(ib_marker)
 
         integer, intent(in)      :: ib_marker
-        integer                  :: i, j, k, num_cells, num_cells_local, decoded_gbl_id
+        integer                  :: i, j, k, num_cells_local, decoded_gbl_id
+        integer(kind=8)          :: num_cells
         real(wp), dimension(1:3) :: center_of_mass, center_of_mass_local
 
         ! Offset only needs to be computes for specific geometries
@@ -1098,7 +1099,7 @@ contains
             end do
 
             ! reduce the mass contribution over all MPI ranks and compute COM
-            call s_mpi_allreduce_integer_sum(num_cells_local, num_cells)
+            call s_mpi_allreduce_integer_sum(int(num_cells_local, 8), num_cells)
             if (num_cells /= 0) then
                 call s_mpi_allreduce_sum(center_of_mass_local(1), center_of_mass(1))
                 call s_mpi_allreduce_sum(center_of_mass_local(2), center_of_mass(2))