Fixing the syntax error in Warp when bc list is empty. When empty, pe…

…rf is really bad, otherwise normal.

examples/cfd/flow_past_sphere_3d.py

@@ -128,7 +128,7 @@ def post_process(self, i):
 
         fields = {"u_magnitude": u_magnitude, "u_x": u[0], "u_y": u[1], "u_z": u[2], "rho": rho[0]}
 
-        save_fields_vtk(fields, timestep=i)
+        # save_fields_vtk(fields, timestep=i)
         save_image(fields["u_magnitude"][:, self.grid_shape[1] // 2, :], timestep=i)
 
 

xlb/operator/stepper/nse_stepper.py

@@ -14,6 +14,7 @@
 from xlb.operator.macroscopic import Macroscopic
 from xlb.operator.stepper import Stepper
 from xlb.operator.boundary_condition.boundary_condition import ImplementationStep
+from xlb.operator.boundary_condition import DoNothingBC as DummyBC
 
 
 class IncompressibleNavierStokesStepper(Stepper):
@@ -378,17 +379,24 @@ def warp_implementation(self, f_0, f_1, boundary_map, missing_mask, timestep):
             setattr(bc_struct, "id_" + bc_name, bc_to_id[bc_name])
             active_bc_list.append("id_" + bc_name)
 
-        # Setting the Struct attributes and active BC classes based on the BC class names
-        bc_fallback = self.boundary_conditions[0]
-        # TODO: what if self.boundary_conditions is an empty list e.g. when we have periodic BC all around!
+        # Check if boundary_conditions is an empty list (e.g. all periodic and no BC)
+        # TODO: There is a huge issue here with perf. when boundary_conditions list
+        #       is empty and is initialized with a dummy BC. If it is not empty, no perf
+        #       loss ocurrs. The following code at least prevents syntax error for periodic examples.
+        if self.boundary_conditions:
+            bc_dummy = self.boundary_conditions[0]
+        else:
+            bc_dummy = DummyBC()
+
+        # Setting the Struct attributes for inactive BC classes
         for var in vars(bc_struct):
             if var not in active_bc_list and not var.startswith("_"):
                 # set unassigned boundaries to the maximum integer in uint8
                 setattr(bc_struct, var, 255)
 
                 # Assing a fall-back BC for inactive BCs. This is just to ensure Warp codegen does not
                 # produce error when a particular BC is not used in an example.
-                setattr(self, var.replace("id_", ""), bc_fallback)
+                setattr(self, var.replace("id_", ""), bc_dummy)
 
         # Launch the warp kernel
         wp.launch(