Direct use of compute_form_data leads to FFC failures

[James R. Maddison]

Consider the following example:

from dolfin import *
mesh = UnitIntervalMesh(10)
space = FunctionSpace(mesh, "CG", 1)
c = Constant(1.0)
test = TestFunction(space)
F = Function(space)
form = inner(test, c * F) * dx
print form.form_data()          # None
print form.compute_form_data()  # The form data

Attempting to assemble the form leads to an error:

Traceback (most recent call last):
  File "./ufl_bug_form_data", line 12, in <module>
    v = assemble(form)
  File "/usr/lib/python2.7/dist-packages/dolfin/fem/assembling.py", line 169, in assemble
    common_cell=common_cell)
  File "/usr/lib/python2.7/dist-packages/dolfin/fem/form.py", line 56, in __init__
    common_cell)
  File "/usr/lib/python2.7/dist-packages/dolfin/compilemodules/jit.py", line 66, in mpi_jit
    return local_jit(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/dolfin/compilemodules/jit.py", line 154, in jit
    return jit_compile(form, parameters=p, common_cell=common_cell)
  File "/usr/lib/python2.7/dist-packages/ffc/jitcompiler.py", line 77, in jit
    return jit_form(ufl_object, parameters, common_cell)
  File "/usr/lib/python2.7/dist-packages/ffc/jitcompiler.py", line 159, in jit_form
    element_mapping=element_mapping)
  File "/usr/lib/python2.7/dist-packages/ufl/form.py", line 127, in compute_form_data
    element_mapping=element_mapping)
  File "/usr/lib/python2.7/dist-packages/ufl/algorithms/formdata.py", line 72, in validate
    "Found non-matching element mappings in form data validation.")
  File "/usr/lib/python2.7/dist-packages/ufl/assertions.py", line 37, in ufl_assert
    if not condition: error(*message)
  File "/usr/lib/python2.7/dist-packages/ufl/log.py", line 154, in error
    raise self._exception_type(self._format_raw(*message))
ufl.log.UFLException: Found non-matching element mappings in form data validation.

This seems to be caused by the two calls to compute_form_data having different arguments.

[Martin Sandve Alnæs]

compute_form_data is not meant for direct use.

[Martin Sandve Alnæs]

Not intended for users, and not fixable without major code rewriting and breaking the user interface.

[James R. Maddison]

That was more-or-less my expected response. I have some obscure cases where I need direct access to form data, and it led to this opaque bug on updating my code for FEniCS 1.2.

[Martin Sandve Alnæs]

You might get the same results if you make all your Constant and Expression objects fully defined from a ufl point of view by defining the cell of the Constant:

c = Constant(0.0, cell=mesh.ufl_cell())

and the element of the Expression

e = Expression("0.0", ufl_element=V)

as these are the things that ffc "fills in" later.

You could also call preprocess directly:

from ufl.algorithms.preprocess import preprocess
form_data = preprocess(form)

although this is a similarly unintended use that may break later, you'll get around the ffc failures reported in this issue.

[James R. Maddison]

Great, thanks. preprocess seems to return the information I need.