-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 <de...@ant.uni-bremen.de> 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 > > > Discuss-gnuradio@gnu.org > > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > > _______________________________________________ > > Discuss-gnuradio mailing list > > Discuss-gnuradio@gnu.org > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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