ffc team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Thu, Sep 03, 2009 at 05:09:03PM +0100, Garth N. Wells wrote:
>
>
> Kristian Oelgaard wrote:
> >Quoting Anders Logg <logg@xxxxxxxxx>:
> >
> >>I just fixed a bug in UFL for lhs/rhs when applied to mixed problems
> >>and noticed some strange results for the generated quadrature
> >>code. Nothing serious, but a bit annoying.
> >>
> >>There are two different demos in UFL: Stokes.ufl and StokesEquation.ufl.
> >>They should give the same results. The results are the same for tensor
> >>representation, but differ for quadrature. The difference is in the
> >>following code:
> >>
> >>1. Code generated from Stokes.ufl
> >>
> >>  // Number of operations for primary indices = 90
> >>  for (unsigned int j = 0; j < 15; j++)
> >>  {
> >>    // Number of operations to compute entry = 6
> >>    A[j] += (FE0_C0[ip][j]*F0 + FE0_C1[ip][j]*F1)*W9[ip]*det;
> >>  }
> >>
> >>2. Code generated from StokesEquation.ufl
> >>
> >>  // Number of operations for primary indices = 120
> >>  for (unsigned int j = 0; j < 15; j++)
> >>  {
> >>    // Number of operations to compute entry = 8
> >>    A[j] += (FE0_C0[ip][j]*F0 + FE0_C1[ip][j]*F1)*-1*-1*W9[ip]*det;
> >>  }
> >>
> >>So we get an extra (-1)*(-1) which I guess should be possible to remove?
> >
> >Not really, the tree-format of the integrand that the quadrature transformer
> >receives contains:
> >
> >Product
> >(
> > IntValue(-1, (), (),{})
> > Product
> > (
> >  IntValue(-1, (), (),{})
> >  Sum
> >  (
> >[snip]
> >
> >one '-1' from F and one '-1' when applying rhs(F).
> >The quadrature transformer (without optimisation) is not aware of any values, it
> >just translates UFL objects into strings (you can use the -O option and they
> >will be removed).
> >To fix this 'bug' we should let UFL handle float values when expanding indices
> >and derivatives. Note that this would probably mean that:
> >
> >-1*x*-1*y --> x*y, OK
> >2*(x + y) --> 2*x + 2*y, maybe not so nice
> >
> >In short, it is possible to remove but I don't know if it is worth the effort,
> >if you want the same code in both cases use the -O option.
> >
>
> A bug report was created on this:
>
>     https://bugs.launchpad.net/ffc/+bug/423382
>
> It would be good to post the discussion there too.
>
> Garth

Already done.

--
Anders

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