Re: [fluid-dev] FPE Exceptions (was Re: Problem in fluidsynth (Ubuntu 11.10) still in Ubuntu 12.04)

[David Henningsson]

2012-05-28 15:56, Pedro Lopez-Cabanillas skrev:
> Hi,
>
> On Wednesday 23 May 2012, David Henningsson wrote:
> > FluidSynth upstream) didn't make a bug fix release either. Perhaps we
> > should do that some time soon...so that the fixes will go into 12.10 at
> > least...
> I'm not sure if only fixing that particular bug justifies a release. It
> doesn't harm to do it, of course. But we may face very similar bugs in the
> future. Let me explain.
>
> I've found the bug you are talking about some time ago, while I was working
> with fluidsynth for a Nokia device having an ARM processor:
> http://sourceforge.net/apps/trac/fluidsynth/ticket/100
>
> This bug was solved by David with this patch:
> http://sourceforge.net/apps/trac/fluidsynth/changeset/435
>
> The problem was very specific to lowering the polyphony, but the cause was not
> uncommon and had plagued FluidSynth for a long time. The root problem is
> denormals in floating point calculations:

When you hit an "overflow" situation, i e, when we start to kill voices, 
the engine starts swapping rvoice and overflow_rvoice pointers. This is 
because we can't kill the rvoice directly in the synth thread, because 
it might be currently used by the rendering thread. So we have an 
additional rvoice for every "standard" rvoice, which is only being used 
in overflow situations.

Now, as the overflow_rvoices were not initialised correctly, so they 
could contain garbage, invalid volume envelopes, and so on. This in turn 
could very well have caused denormal number problems (as the garbage 
could have contained denormal numbers), but I don't see the denormal 
numbers as being the root cause for this particular bug.

--
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic