Fix the FormSum memory leak

firedrake/assemble.py

@@ -5,6 +5,7 @@
 import itertools
 from itertools import product
 import numbers
+import numpy as np
 
 import cachetools
 import finat
@@ -469,8 +470,14 @@ def base_form_assembly_visitor(self, expr, tensor, *args):
                 return sum(weight * arg for weight, arg in zip(expr.weights(), args))
             elif all(isinstance(op, firedrake.Cofunction) for op in args):
                 V, = set(a.function_space() for a in args)
-                res = sum([w*op.dat for (op, w) in zip(args, expr.weights())])
-                return firedrake.Cofunction(V, res)
+                result = firedrake.Cofunction(V)
+                for op, w in zip(args, expr.weights()):
+                    for dat_result, dat_op in zip(result.dat.split, op.dat.split):
+                        np.add(
+                            dat_result.data_ro_with_halos,
+                            w * dat_op.data_ro_with_halos,
+                            out=dat_result.data_wo_with_halos)
+                return result
             elif all(isinstance(op, ufl.Matrix) for op in args):
                 res = tensor.petscmat if tensor else PETSc.Mat()
                 is_set = False