Hi,
Right, the compiler try to factorize a multiplication by 0 ! Another way to
see the problem it to compare :
process= (_*cos(1/2*ma.PI)^3,_,_*cos(-1/2*ma.PI)^3);
and
process= (_*cos(1/2*ma.PI)^3,_,_*cos(-1/2*ma.PI)^3) :> _;
in simple and double precision.
Clearly, the factorization rule needs to be fixed
Thanks for discovering the problem !
Yann
Yann Orlarey
Directeur scientifique
www.grame.fr
2016-09-24 18:33 GMT+02:00 Julius Smith <j...@ccrma.stanford.edu>:
> This looks like numerical failure in the optimization by the Faust
> compiler. You can see it approaching this situation in the following
> simplified case:
>
> process=(_*cos(ma.PI/2),_,_*cos(0-ma.PI/2)):>_;
>
> which compiles to
>
> output0[i] = (FAUSTFLOAT)(6.123234e-17f * (((1.633124e+16f *
> (float)input1[i]) + (float)input0[i]) + (float)input2[i]));
>
> Since cos(PI/2) = 0, the input coefficients are approximately 0,1,0,
> which become (numerically) eps,1,eps, where eps = 6.123234e-17f in
> this particular case. Factoring out eps gives 1/eps for the
> coefficient of input1, which became infinity in the previous example
> (numerical bug... 1/eps should possibly go to MAX_FLOAT, but not inf).
> If there were a third signal multiplied by eps, then this factoring
> would save a multiply, but in this case there are two multiplies
> either way.
>
> I would personally like to be able to set a magnitude threshold below
> which numbers are flushed to zero. Then the compiler would see
> coefficients (0,1,0) in this case and not try to factor out the 0.
>
> - Julius
>
> On Fri, Sep 23, 2016 at 7:26 AM, Stéphane Letz <l...@grame.fr> wrote:
> > The generated C++ code is incorrect (see this "(inf.0f *
> (float)input1[i]))) » subexpression), so probably a bug in the compiler.
> >
> > Stéphane
> >
> >
> >> Le 23 sept. 2016 à 13:28, Pierre Lecomte <pierre.leco...@gadz.org> a
> écrit :
> >>
> >> import("stdfaust.lib");
> >> process=(_*cos(1/2*ma.PI)^3,_,_*cos(-1/2*ma.PI)^3):>_;
> >
> >
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> >
> >
>
>
>
> --
> Julius O. Smith III <j...@ccrma.stanford.edu>
> Professor of Music and, by courtesy, Electrical Engineering
> CCRMA, Stanford University
> http://ccrma.stanford.edu/~jos/
>
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