22 Nov
2018
22 Nov
'18
10:28 p.m.
Am 22.11.18 um 21:07 schrieb Fons Adriaensen:
On Thu, Nov 22, 2018 at 06:57:15PM +0100, Hermann
Meyer wrote:
In guitarix nearly all DSP is generated by FAUST.
However, problems with NAN's and INF's when use -ffinite-math-only
occurs only when we save preset values to file, and only sometimes then.
Without -ffinite-math-only never ever.
Am 22.11.18 um 18:30 schrieb Will Godfrey:
-ffast-math should be perfectly OK for any
normal audio DSP code.
The only use case where I'd consider not using it is for precision
measurement applications.
If using it leads to unexpected results that means that the algorithm
used is itself numerically unstable. A classic example of this is the
'biquad' and similar structures when used for filters that operate
mainly on low frequencies. Unfortunately, that sort of code is
everywhere, cut and pasted from one bad example to the next.
Strange that this turns up right now. Earlier today I was implementing
an IIR filter originally designed in MatLab which uses doubles for
everything. When the coefficients were truncated to 32-bit floats,
the filter became unstable. This doesn't mean that such a filter
needs double precision. It just indicates bad design.
Ciao, While testing some mixed floating point and
integer calculations I found a
quite surprising difference when this compiler option was set (gcc 6.x). It was
clearly different at only 100 iterations and got dramatically worse with
larger counts.
...
In the guitarix project we've disabled -ffast-math several years ago,
when
I remember right it was at gcc3, as it could lead to different
un-reproduciable calculations. Last option I've disabled on gcc8 now, is
-ffinite-math-only, this one leads to nan's and inf's in several cases,
which been as well not reproducible.