Re: [Discuss-gnuradio] ffast-math

[Albin Stigö]

Interesting! I'm experimenting with this right now. NEON on armv7 is
not always IEEE754 compliant by the way.

--Albin

On Mon, Sep 2, 2019 at 5:28 PM Müller, Marcus (CEL) <address@hidden> wrote:
>
> -ffast-math is allowed to reorder calculations, e.g. it can do
>     a*f + b*f + c*f as (a+b+c)*f (or vice versa)
> which of course isn't necessarily the same result, and can lead to
> numerical instability.
>
> The problem is really that for some kernels, things like zero-signage
> and NaN / Inf handling / rounding / saturation are negligible, for
> others not. We've had plenty of fun with kernels where the SIMD
> implementation actually did the right thing, but the plain C
> implementation misbehaved, for example, because it didn't use the same
> FPU flags; I'd be kind of hesitant to add more "might or might not act
> as expected" code...
>
> Best regards,
> Marcus
>
> On Mon, 2019-09-02 at 16:52 +0200, Albin Stigö wrote:
> > ffast-math disables signed zero by design I think...
> >
> > I'm particularly interested in problems of numeric stability and loss of 
> > dynamic range.
> >
> >
> > --Albin
> >
> > On Mon, Sep 2, 2019, 15:27 Johannes Demel <address@hidden> wrote:
> > > Hi Albin,
> > >
> > > one of my students reported a little oddity about `-ffast-math`. Assume
> > > you want to set a float to -0, i.e. set the sign bit only. In this case
> > > `-ffast-math` seems to remove the sign bit.
> > >
> > > In VOLK this might be an issue with `_mm256_conjugate_ps` in
> > > `include/volk/volk_avx_intrinsics.h`. I didn't check for other potential
> > > error sources.
> > >
> > > Cheers
> > > Johannes
> > >
> > > Am 01.09.19 um 16:10 schrieb Albin Stigö:
> > > > Anyone has experience with the real world impact of using gcc
> > > > -ffast-math with SDR in general, and GNURadio/Volk in particular?
> > > >
> > > >
> > > > --Albin SM6WJM
> > > >
> > > > _______________________________________________
> > > > Discuss-gnuradio mailing list
> > > > address@hidden
> > > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> > > >
> > > _______________________________________________
> > > Discuss-gnuradio mailing list
> > > address@hidden
> > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> >
> > _______________________________________________
> > Discuss-gnuradio mailing list
> > address@hidden
> > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio