ffc team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Sun, Oct 18, 2009 at 09:26:11PM +0200, Johan Hake wrote:
> On Sunday 18 October 2009 21:14:10 Garth N. Wells wrote:
> > Anders Logg wrote:
> > > On Sun, Oct 18, 2009 at 08:37:21PM +0200, Johan Hake wrote:
> > >> On Sunday 18 October 2009 20:31:41 Garth N. Wells wrote:
> > >>> Johan Hake wrote:
> > >>>> On Sunday 18 October 2009 20:07:41 Garth N. Wells wrote:
> > >>>>> On Oct 18 2009, Johan Hake wrote:
> > >>>>>> On Sunday 18 October 2009 18:21:23 Garth N. Wells wrote:
> > >>>>>>> Johan Hake wrote:
> > >>>>>>>> On Sunday 18 October 2009 16:43:28 Garth N. Wells wrote:
> > >>>>>>>>> Garth N. Wells wrote:
> > >>>>>>>>>> Johan Hake wrote:
> > >>>>>>>>>>> On Saturday 17 October 2009 21:08:14 Garth N. Wells wrote:
> > >>>>>>>>>>>> Garth N. Wells wrote:
> > >>>>>>>>>>>>> DOLFIN wrote:
> > >>>>>>>>>>>>>> One or more new changesets pushed to the primary dolfin
> > >>>>>>>>>>>>>> repository. A short summary of the last three changesets is
> > >>>>>>>>>>>>>> included below.
> > >>>>>>>>>>>>>>
> > >>>>>>>>>>>>>> changeset:   7378:e5c921e0293a
> > >>>>>>>>>>>>>> tag:         tip
> > >>>>>>>>>>>>>> user:        "Johan Hake <hake@xxxxxxxxx>"
> > >>>>>>>>>>>>>> date:        Sat Oct 17 15:45:36 2009 +0200
> > >>>>>>>>>>>>>> files:       site-packages/dolfin/function.py
> > >>>>>>>>>>>>>> site-packages/dolfin/functionspace.py description:
> > >>>>>>>>>>>>>> Make Mixed FunctionSpace access more consistant.
> > >>>>>>>>>>>>>>   - All methods are now defined in FunctionSpaceBase.
> > >>>>>>>>>>>>>>   - We now do not save any spaces in MixedFunctionSpace
> > >>>>>>>>>>>>>
> > >>>>>>>>>>>>> This change broke my code. See below.
> > >>>>>>>>>>>>
> > >>>>>>>>>>>> Seems that the problem arises with spaces which are
> > >>>>>>>>>>>> restricted,
> > >>>>>>>>>>>>
> > >>>>>>>>>>>>      V = FunctionSpace(mesh, "CG", 1, "facet")
> > >>>>>>>>>>>
> > >>>>>>>>>>> This is an error in the generated FFC code.
> > >>>>>>>>>>>    ufc::finite_element::signature()
> > >>>>>>>>>>> should return a string that can be executed in a ufl namespace
> > >>>>>>>>>>> and then generate the corresponding ufl.FiniteElement.
> > >>>>>>>>>>>
> > >>>>>>>>>>> For a restricted element the signature returns:
> > >>>>>>>>>>>
> > >>>>>>>>>>>    "FiniteElement('Lagrange', 'triangle', 1)|_{<interval of
> > >>>>>>>>>>> degree 1>}"
> > >>>>>>>>>>>
> > >>>>>>>>>>> where it should return:
> > >>>>>>>>>>>
> > >>>>>>>>>>>    "ElementRestriction(FiniteElement('Lagrange',
> > >>>>>>>>>>> Cell('triangle', 1, Space(2)), 1), Cell('interval', 1,
> > >>>>>>>>>>> Space(1)))"
> > >>>>>>>>>
> > >>>>>>>>> I've had a look, and while I don't yet follow where UFL defines
> > >>>>>>>>> its signatures,
> > >>>>>>>>
> > >>>>>>>>   repr(ulf_object)
> > >>>>>>>>
> > >>>>>>>> returns the uniqe signature of an ufl_object.
> > >>>>>>>>
> > >>>>>>>>> things are dangerous because FFC formats its own signature
> > >>>>>>>>> strings, see line 227 of
> > >>>>>>>>>
> > >>>>>>>>>      ffc/ffc/fem/finiteelement.py
> > >>>>>>>>
> > >>>>>>>> Yes, this is dangerous, at least if we want to use them as we do
> > >>>>>>>> in PyDOLFIN. However taking repr of the corresponding ufl object
> > >>>>>>>> is a well defined method that ufl use internally, when for example
> > >>>>>>>> creating a unique string representation of a form.
> > >>>>>>>>
> > >>>>>>>>> We stopped using signature strings in DOLFIN because it gave us
> > >>>>>>>>> all sorts of problems. Is it desirable to have PyDOLFIN depend on
> > >>>>>>>>> the generated strings? Can it be avoided?
> > >>>>>>>>
> > >>>>>>>> It is a very nice way of constructing an ufl object when we have
> > >>>>>>>> the compiled version. As the convention of repr(object) is that:
> > >>>>>>>>
> > >>>>>>>>   new_object = eval(repr(object))
> > >>>>>>>>
> > >>>>>>>> should return a new object of the same kind.
> > >>>>>>>>
> > >>>>>>>> So when we have a SubSpace with its compiled FiniteElement, it is
> > >>>>>>>> easy to just call its signature method of its element to generate
> > >>>>>>>> the corresponding ufl element, which is used to construct a full
> > >>>>>>>> fledged dolfin.FunctionSpace.
> > >>>>>>>>
> > >>>>>>>> Not sure how this could be done another way.
> > >>>>>>>
> > >>>>>>> Can't we get the sub-element from the original UFL function?
> > >>>>>>
> > >>>>>> Not when we return a SubSpace which is a compiled C++ structure. To
> > >>>>>> be able to construct the ufl.FiniteElement (done in the class
> > >>>>>> FunctionSpaceFromCpp in functionspace.py) we use the signature of
> > >>>>>> the cpp.FiniteElement.
> > >>>>>>
> > >>>>>>> If I do
> > >>>>>>>
> > >>>>>>>     (u0, u1) = pde.solve().split()
> > >>>>>>>
> > >>>>>>> are u0 and u1 UFL Functions, or just cpp Functions?
> > >>>>>>
> > >>>>>> They should be both. Their FunctionSpaces (self._V) are constructed
> > >>>>>> using the the FunctionSpace.sub(i) (operator[]) method, which
> > >>>>>> returns the compiled SubSpace I am talking about above.
> > >>>>>
> > >>>>> OK, but if we have
> > >>>>>
> > >>>>>   U = pde.solve()
> > >>>>>
> > >>>>> and U is a UFL Function, can't the UFL finiteelement for U be
> > >>>>> accessed, and then the UFL sub-element(s) accessed and then compiled?
> > >>>>
> > >>>> Yes, this should be possible! (Did not think of getting the sub
> > >>>> element from a mixed ufl.element :P)
> > >>>>
> > >>>> However we do not have to compile them, as we needed to go the other
> > >>>> way, from compiled to UFL.
> > >>>
> > >>> OK. Now the situation is clear to me.
> > >>>
> > >>>> I still think the signature() -> UFL object is a neat feature!
> > >>>
> > >>> The problem is that there is nothing that says that a form compiler
> > >>> that uses UFL and produces UFC-compliant code must return the UFL
> > >>> signature (repr) in ufc::finite_element::signature().
> > >>
> > >> True. However that is something we discussed a year ago when we
> > >> implemented the transition to FunctionSpace in PyDOLFIN. I thought this
> > >> went into the ufc documentation, but I now see that this is not the
> > >> case. For now...
> > >>
> > >> Lets get a blueprint at ufc and here what folks says.
> > >>
> > >> Johan
> > >
> > > How about adding an optional function to the UFC interface for
> > > returning the UFL string:
> > >
> > >   virtual std::string ufl_repr() const { return ""; }
> > >
> > > This function can then be implemented optionally by form compilers
> > > that rely on UFL. We should not tie the UFC interface to UFL.
>
> Ok.
>
> > The really clever way would be to have PyDOLFIN create a sub-class of
> > ufc::finite_element which implements ufl_repr().
>
> It is not PyDOLFIN that compile the ulf.form. This is done in the jit function
> of the formcompiler together with the ufc.build_module function.
>
> Not sure how easy (or correct) it is to interfere with the jit function.
>
> Johan

It seems better to add it to the interface so that DOLFIN can check
whether the ufl_repr function is implemented or not. If it returns and
empty string, then it can return an appropriate error message.

--
Anders


> > Garth
> >
> >
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> >
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