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[Martin Sandve Alnæs <martinal@xxxxxxxxx>]

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Martin



On Wed, Feb 25, 2009 at 10:41 PM, Anders Logg <logg@xxxxxxxxx> wrote:
> Thanks! This works fine.
>
> --
> Anders
>
>
> On Wed, Feb 25, 2009 at 07:14:29PM +0100, Martin Sandve Alnæs wrote:
>> I've implemented this in UFL such that:
>>
>>     a = ...*dx
>>     ...
>>     fd = a.form_data()
>>
>> ensures that a FormData object is
>> computed once and only once.
>> So jit can do this:
>>
>> def jit(form):
>>     formdata = form.form_data()
>>     ...
>>
>> Martin
>>
>>
>>
>> On Tue, Feb 24, 2009 at 10:51 PM, Martin Sandve Alnæs
>> <martinal@xxxxxxxxx> wrote:
>> > On Tue, Feb 24, 2009 at 10:12 PM, Anders Logg <logg@xxxxxxxxx> wrote:
>> >> The FFC JIT compiler (and any other JIT compiler that DOLFIN uses)
>> >> must be able to return FormData as well as the compiled form. DOLFIN
>> >> uses this to extract coefficients and elements.
>> >
>> > Agreed.
>> >
>> >> Without caching, the JIT compiler gets the form, generates form data,
>> >> generates the C++ code which is sent to Instant. Instant then returns
>> >> the compiled form and the JIT compiler returns the form + form data to
>> >> DOLFIN.
>> >
>> > So far so good.
>> >
>> >> With caching, the JIT compiler gets the form, sends it to Instant (or
>> >> rather a wrapper that returns an appropriate hash and signature). Then
>> >> Instant returns the compiled form. This means that the form data is
>> >> missing.
>> >
>> > Which is the way it has to be for Instant to be an independent project.
>> >
>> >> FFC solves this by attaching the form data to the form:
>> >>
>> >>  form.form_data = form_data
>> >>
>> >> This way, the form data is available both when the in-memory cache is
>> >> used and when the disk cache is used. In the first case, the form has
>> >> been JIT compiled before so the object has form_data attached to
>> >> it. In the second case, the signature needs to be computed which means
>> >> that also the form data will be computed.
>> >>
>> >> Would it be possible/desirable to attach form data to a UFL form? Or
>> >> will it be necessary to keep a separate form data cache in the JIT
>> >
>> > Let me just think loudly.
>> >
>> > The thing is, the Form that the user passes in to jit won't be
>> > the same Form as Formdata contains. Derivatives must be
>> > expanded to know the right number of form arguments,
>> > and indices and arguments are renumbered to get a canonical
>> > repr independent of the order which forms are defined in the
>> > application.
>> >
>> > Unless this behaviour is changed, attaching form data to
>> > the form will mean that "a.form_data.form != a", but
>> > a.form_data.form still represents the same form, and
>> > FormData does nothing compiler- or representation-specific.
>> >
>> > What does this mean? Take this pseudocode:
>> >
>> > # make some form:
>> > a = ...*dx
>> >
>> > # compile some times with jit and then with different options to make
>> > it interesting:
>> > form1, formdata1 = jit(a)
>> > form2, formdata2 = jit(a)
>> > form3, formdata3 = jit(a, options = non_default_options)
>> > form2, formdata2 = jit(a)
>> >
>> > # instant ensures this:
>> > form1.__module__ == form2.__module__
>> > form1.__module__ == form4.__module__
>> >
>> > # and the different options should lead to this:
>> > form3.__module__ != form2.__module__
>> >
>> > # you want this:
>> > a.form_data == formdata1 # computed and attached to a in first jit call
>> > a.form_data == formdata2 # reused in second jit call
>> > a.form_data == formdata3 # if not, we'll get major problems
>> > a.form_data == formdata4
>> >
>> >
>> > I was sceptical at first, but I think this will work out.
>> > As long as "fd = FormData(a)" is a completely deterministic
>> > operation with no options that can change the result,
>> > attaching it to a poses no problems. In particular, FormData
>> > must be independent of any dolfin or form compiler options.
>> >
>> >> compiler. It sort of makes the in-memory cache of Instant pointless
>> >> (for form compilation) since the JIT compiler might then as well cache
>> >> the compiled form (as it did before this was added to Instant).
>> >
>> > Not true. In PyDOLFIN, forms with different coefficients will
>> > have different formdata (which dolfin may or may not keep
>> > a cache of as we discuss now), but still be the same
>> > compiled form, which instant caches separately.
>> >
>> > Martin
>> >
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