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Fieldsplit: aggressively expand Forms to detect zero blocks #4707

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Fieldsplit: aggressively expand Forms to detect zero blocks #4707

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12 changes: 8 additions & 4 deletions firedrake/formmanipulation.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3,9 +3,9 @@
import collections

from ufl import as_tensor, as_vector, split
from ufl.classes import Zero, FixedIndex, ListTensor, ZeroBaseForm
from ufl.classes import FixedIndex, Form, ListTensor, Zero, ZeroBaseForm
from ufl.algorithms.map_integrands import map_integrand_dags
from ufl.algorithms import expand_derivatives
from ufl.algorithms import expand_derivatives, expand_indices
from ufl.corealg.map_dag import MultiFunction, map_expr_dags

from pyop2 import MixedDat
Expand Down Expand Up @@ -78,9 +78,13 @@ def split(self, form, argument_indices):
assert (len(idx) == 1 for idx in self.blocks.values())
assert (idx[0] == 0 for idx in self.blocks.values())
return form
# TODO find a way to distinguish empty Forms avoiding expand_derivatives
f = map_integrand_dags(self, form)
if expand_derivatives(f).empty():

# TODO find a better way to distinguish empty Forms
f_expanded = expand_derivatives(f)
if isinstance(f_expanded, Form):
f_expanded = expand_indices(f_expanded)
if f_expanded.empty():
# Get ZeroBaseForm with the right shape
f = ZeroBaseForm(tuple(map(self._subspace_argument, form.arguments())))
return f
Expand Down
12 changes: 12 additions & 0 deletions tests/firedrake/regression/test_split.py
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Original file line number Diff line number Diff line change
Expand Up @@ -125,3 +125,15 @@ def test_split_coefficient_not_argument():
as_vector([TestFunction(V), 0])),
w, wr)
assert J00.signature() == expect.signature()


def test_split_zero_block():
mesh = UnitSquareMesh(1, 1)
V = FunctionSpace(mesh, "DG", 0)
Z = V * V * V * V
J = inner(TrialFunction(Z), TestFunction(Z))*dx
splitter = ExtractSubBlock()

J00 = splitter.split(J, (0, 1))

assert isinstance(J00, ZeroBaseForm)
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