Fix FieldPoyntingFlux diagnostic for GPU (BLAST-WarpX#5780)

The FieldPoyntingFlux reduced diagnostic uses `amrex::ReduceOps` to
calculate the diagnostic. The loops are structured so that the
calculation is only done on the boundaries, by limiting the boxes to
only include the boundary cells. For any blocks in the interior, the box
will be empty. The `amrex::ReduceOps` on an empty box works Ok on the
CPU but crashes on the GPU. The solution is to only call the reduction
when the box is not empty.

Author: dpgrote

Source/Diagnostics/ReducedDiags/FieldPoyntingFlux.cpp

@@ -254,31 +254,34 @@ void FieldPoyntingFlux::ComputePoyntingFlux ()
             };
 
             // Compute E x B
-            reduce_ops.eval(box, reduce_data,
-                [=] AMREX_GPU_DEVICE (int i, int j, int k) -> amrex::GpuTuple<amrex::Real>
-                {
-                    amrex::Real Ex_cc = 0._rt, Ey_cc = 0._rt, Ez_cc = 0._rt;
-                    amrex::Real Bx_cc = 0._rt, By_cc = 0._rt, Bz_cc = 0._rt;
-
-                    if (normal_dir == 1 || normal_dir == 2) {
-                        Ex_cc = ablastr::coarsen::sample::Interp(Ex_arr, Ex_stag, cc, cr, i, j, k, comp);
-                        Bx_cc = ablastr::coarsen::sample::Interp(Bx_arr, Bx_stag, cc, cr, i, j, k, comp);
-                    }
-
-                    if (normal_dir == 0 || normal_dir == 2) {
-                        Ey_cc = ablastr::coarsen::sample::Interp(Ey_arr, Ey_stag, cc, cr, i, j, k, comp);
-                        By_cc = ablastr::coarsen::sample::Interp(By_arr, By_stag, cc, cr, i, j, k, comp);
-                    }
-                    if (normal_dir == 0 || normal_dir == 1) {
-                        Ez_cc = ablastr::coarsen::sample::Interp(Ez_arr, Ez_stag, cc, cr, i, j, k, comp);
-                        Bz_cc = ablastr::coarsen::sample::Interp(Bz_arr, Bz_stag, cc, cr, i, j, k, comp);
-                    }
-
-                    amrex::Real const af = area_factor(i,j,k);
-                    if      (normal_dir == 0) { return af*(Ey_cc * Bz_cc - Ez_cc * By_cc); }
-                    else if (normal_dir == 1) { return af*(Ez_cc * Bx_cc - Ex_cc * Bz_cc); }
-                    else                      { return af*(Ex_cc * By_cc - Ey_cc * Bx_cc); }
-                });
+            // On GPU, reduce_ops doesn't work with empty boxes.
+            if (box.ok()) {
+                reduce_ops.eval(box, reduce_data,
+                    [=] AMREX_GPU_DEVICE (int i, int j, int k) -> amrex::GpuTuple<amrex::Real>
+                    {
+                        amrex::Real Ex_cc = 0._rt, Ey_cc = 0._rt, Ez_cc = 0._rt;
+                        amrex::Real Bx_cc = 0._rt, By_cc = 0._rt, Bz_cc = 0._rt;
+
+                        if (normal_dir == 1 || normal_dir == 2) {
+                            Ex_cc = ablastr::coarsen::sample::Interp(Ex_arr, Ex_stag, cc, cr, i, j, k, comp);
+                            Bx_cc = ablastr::coarsen::sample::Interp(Bx_arr, Bx_stag, cc, cr, i, j, k, comp);
+                        }
+
+                        if (normal_dir == 0 || normal_dir == 2) {
+                            Ey_cc = ablastr::coarsen::sample::Interp(Ey_arr, Ey_stag, cc, cr, i, j, k, comp);
+                            By_cc = ablastr::coarsen::sample::Interp(By_arr, By_stag, cc, cr, i, j, k, comp);
+                        }
+                        if (normal_dir == 0 || normal_dir == 1) {
+                            Ez_cc = ablastr::coarsen::sample::Interp(Ez_arr, Ez_stag, cc, cr, i, j, k, comp);
+                            Bz_cc = ablastr::coarsen::sample::Interp(Bz_arr, Bz_stag, cc, cr, i, j, k, comp);
+                        }
+
+                        amrex::Real const af = area_factor(i,j,k);
+                        if      (normal_dir == 0) { return af*(Ey_cc * Bz_cc - Ez_cc * By_cc); }
+                        else if (normal_dir == 1) { return af*(Ez_cc * Bx_cc - Ex_cc * Bz_cc); }
+                        else                      { return af*(Ex_cc * By_cc - Ey_cc * Bx_cc); }
+                    });
+            }
         }
 
         int const sign = (face().isLow() ? -1 : 1);