ffc team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

On Tuesday April 26 2011 08:33:11 Garth N. Wells wrote:
> On 26/04/11 16:31, Johan Hake wrote:
> > On Tuesday April 26 2011 08:16:29 Garth N. Wells wrote:
> >> On 26/04/11 16:07, Anders Logg wrote:
> >>> On Tue, Apr 26, 2011 at 03:59:52PM +0100, Garth N. Wells wrote:
> >>>> On 26/04/11 15:55, Anders Logg wrote:
> >>>>> On Tue, Apr 26, 2011 at 03:45:22PM +0100, Garth N. Wells wrote:
> >>>>>> On 26/04/11 13:51, Anders Logg wrote:
> >>>>>>> On Tue, Apr 26, 2011 at 02:00:50PM +0200, Anders Logg wrote:
> >>>>>>>> It feels good that you trust me enough to handle it. ;-)
> >>>>>>>> 
> >>>>>>>> Will add it sometime this afternoon and then we can revisit the
> >>>>>>>> JIT compiler caching.
> >>>>>>> 
> >>>>>>> I'm getting confused here... Looking at preprocess.py in UFL, I see
> > 
> > this:
> >>>>>> It is confusing. Does the function 'preprocess' do anything that the
> >>>>>> old FormData class didn't? It would be easier to follow if Form just
> >>>>>> had a member function form_data() that computes and stores data
> >>>>>> (like it used to), or if Form had a 'preprocess' function. Having
> >>>>>> the function preprocess return a new form is really confusing.
> >>>>> 
> >>>>> I don't find that particularly confusing. It's the same as
> >>>>> 
> >>>>>   refined_mesh = refine(mesh)
> >>>> 
> >>>> Which is the whole problem. By creating a new object, FormData is
> >>>> thrown away. The preprocessing should just compute some more data,
> >>>> just like we *don't* do
> >>>> 
> >>>>   initialised_mesh = mesh.init(0)
> >>>> 
> >>>> What was wrong with Martin's original design that necessitated the
> >>>> change?
> >>> 
> >>> As I explained, I thought it was better to have an explicit call to
> >>> preprocess since that makes it clear that one makes a call to a
> >>> function which may take some time to execute (instead of just calling
> >>> a member function which seems to just return some data).
> >>> 
> >>> But as I say above: I added the caching back at some point (maybe even
> >>> the day after I removed it 2 years ago) so we don't need to discuss
> >>> why I removed it (as I realized myself I shouldn't have removed it and
> >>> added it back a long time ago).
> >>> 
> >>> What has me confused now is that the caching seems to be in place but
> >>> we still need the extra caching in FFC/DOLFIN and I don't see why.
> >> 
> >> Because preprocess returns a new form, e.g. define a form
> >> 
> >>   a = u*v*dx
> >>   jit(a)
> >> 
> >> Inside jit,
> >> 
> >>    a.form_data() is None:
> >>        b = preprocess(a) # b now has data attached, but a doesn't
> >>    
> >>    else:
> >>        b = a
> >> 
> >> Now 'b' has been preprocessed, and has form data attached, but 'a'
> >> doesn't. Calling 'jit(a)' again, the code will never enter the 'else'
> >> part of the clause because 'a' never gets any form data. Johan has added
> >> some code FFC that attaches the form data of 'b' to 'a', but it is a bit
> >> clumsy.
> > 
> > No, it was already attached. I just made ffc use it.
> 
> Didn't you add the line
> 
>     form._form_data = preprocessed_form.form_data()

No, I added:

  preprocessed_form = form.form_data()._form

I think the thing here is that form_data has always had a preprocessed form. 
Someone (lets not point fingers!) thought that was too much magic and added an 
explicit need to call:

  form = preprocess(form)

in jit_compiler(). This made the design more complicated and also introduced a 
cirucular dependency, as the return preprocessed form need to know of its 
form_data, but the form_data already had a reference to the preprocessed form. 
The latter is what I used in the one line I altered.

Johan
 
> ?
> 
> Garth
> 
> >> Better would be
> >> 
> >>     a.preprocess()
> >> 
> >> or
> >> 
> >>     a.form_data()
> > 
> > As already mentioned in a previous email, I suggest we only call
> > form_data(). This will return the form_data. The preprocessed form is
> > attached to the form_data and this is what is passed to the code
> > generator. I am pretty sure this is what was there from the beginning.
> > 
> > It is confusing to call:
> >   form = preprocess(form)
> > 
> > as the preprocessed form was never ment to be doing anything but being
> > passed to the code generator, AFAIK.
> > 
> > Johan
> > 
> >> Garth
> >> 
> >>> --
> >>> Anders
> >>> 
> >>>> Garth
> >>>> 
> >>>>>> Garth
> >>>>>> 
> >>>>>>> def preprocess(form, object_names={}, common_cell=None):
> >>>>>>>     ...
> >>>>>>>     
> >>>>>>>     # Check that form is not already preprocessed
> >>>>>>>     
> >>>>>>>     if form.form_data() is not None:
> >>>>>>>         debug("Form is already preprocessed. Not updating form
> >>>>>>>         data.") return form
> >>>>>>>     
> >>>>>>>     ...
> >>>>>>>     
> >>>>>>>     # Attach form data to form
> >>>>>>>     form._form_data = form_data
> >>>>>>>     
> >>>>>>>     # Attach preprocessed form to form data
> >>>>>>>     form_data._form = form
> >>>>>>> 
> >>>>>>> And when I look at the blamelist (bzr annotate), it looks like I
> >>>>>>> added those lines, so I must have come to my senses and added it
> >>>>>>> back at some point (way back). So in conclusion, calling
> >>>>>>> preprocess() should not taking any time.
> >>>>>>> 
> >>>>>>> What am I missing?
> >> 
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