[PD] [bob~] denormals issue?

Thu Sep 22 01:10:21 CEST 2016

 From a few posts ago it looks like -O3 is what causes denormals to 
stick around, but I don't really know what I'm talking about. RK4 (which 
is what bob~ uses to solve the system) will never 'naturally' bring the 
filter state exactly back to 0 unless forced to in some way so if the 
denormals aren't handled elegantly, they will abound with 0 input to the 
filter no matter what (once some input has gone into the filter that 
is). My guess is that a slow decay doesn't push the filter states down 
into that region.

It is stupid and certainly sub-optimal, but maybe:

x = (x<DENORMAL_THRESH ? 0 : x);

where DENORMAL_THRESH is some very, very small normal number? This would 
have to be done on the negative side of 0, too. 2x compares is probably 
not as bad as the potential 100x slowdown (according to Wikipedia) that 
denormal arithmetic might cause.

It should be the case that the same compiler flags (for plain C anyway) 
produce, within limits, the same behavior across different compilers but 
that probably isn't true. This would be an interesting check. I'd like 
to know if leaving off -03 has the same effect with gcc.

On 9/21/2016 3:26 PM, katja wrote:
> On Linux i386 bob~ produces subnormals as well and very high cpu load.
> It is not the slowly decaying signal that other filters tend to give,
> but a quick decay to a fixed small number.
>
> It's weird indeed when subnormals cause different CPU load depending
> on how they are compiled. Since MinGW is a gcc port I would think it
> uses the same math implementation which produces such high CPU load
> for bob~ subnormals on my system.
>
> On Thu, Sep 22, 2016 at 12:01 AM, Christof Ressi <christof.ressi at gmx.at> wrote:
>> I tried your patch with the [bob~] object shipped with the Windows binaries. I clearly get subnormals! It's actually no wonder because there isn't any protection against subnormals in the code (at least I couldn't spot it).
>> But the weird thing is: the [bob~] I compiled myself would also show subnormals in your patch but the CPU load is not affected...
>>
>>> Gesendet: Mittwoch, 21. September 2016 um 23:47 Uhr
>>> Von: katja <katjavetter at gmail.com>
>>> An: "Christof Ressi" <christof.ressi at gmx.at>
>>> Cc: "pd-list at lists.iem.at" <pd-list at lists.iem.at>
>>> Betreff: Re: Re: Re: [PD] [bob~] denormals issue?
>>>
>>> Without -O flags you get debug-level and all function inlining is
>>> disabled, depending on the code it can make a huge difference indeed.
>>> But Pd is probably compiled with at least -O2. So the flags don't make
>>> much difference. The compiler? Doesn't Miller compile with MinGW
>>> nowadays, I don't know. MinGW brings its own standard C libs, which
>>> may implement math functions differently than MS. But regarding
>>> denormals I guess they both respect the IEEE 754 standard.
>>>
>>> You can check if you really have subnormals using attached patch
>>> denorm-test.pd you. The patch tests lop~, change it to bob~.
>>>
>>> On Wed, Sep 21, 2016 at 11:18 PM, Christof Ressi <christof.ressi at gmx.at> wrote:
>>>> the SSE optimizations don't seem to matter at all. skipping -ffast-math gives a slight overall CPU rise, while skipping -O3 gives me huge CPU rise (20 bob~ filters are already to much for one core). Even when skipping all of those flags, the denormals issue is still not present.
>>>>
>>>> Maybe it has something to do with the compiler?
>>>>
>>>>> Gesendet: Mittwoch, 21. September 2016 um 22:47 Uhr
>>>>> Von: katja <katjavetter at gmail.com>
>>>>> An: "Christof Ressi" <christof.ressi at gmx.at>, "pd-list at lists.iem.at" <pd-list at lists.iem.at>
>>>>> Betreff: Re: Re: [PD] [bob~] denormals issue?
>>>>>
>>>>> I'm curious to know if the flags do flush denormals on your processor.
>>>>> Forgot to mention that '-O3 -ffast-math' are also set,
>>>>> platform-independent. So if you have a chance to try which flag does
>>>>> something... It's just curiosity.
>>>>>
>>>>> On Wed, Sep 21, 2016 at 10:34 PM, Christof Ressi <christof.ressi at gmx.at> wrote:
>>>>>> Hi Katja,
>>>>>>
>>>>>>> Even if your test reveals a beneficial effect from compiler flags,
>>>>>>> it is better when denormals are detected and flushed in the C code.
>>>>>> definitely! Maybe using the PD_BIGORSMALL macro on each filter state at the end of the DSP routine does the trick, just like in all the other recursive filters in Pd.
>>>>>>
>>>>>>
>>>>>>
>>>>>>> Von: katja <katjavetter at gmail.com>
>>>>>>> An: "Christof Ressi" <christof.ressi at gmx.at>
>>>>>>> Cc: pd-list <pd-list at iem.at>, "Miller Puckette" <msp at ucsd.edu>
>>>>>>> Betreff: Re: [PD] [bob~] denormals issue?
>>>>>>>
>>>>>>> Hi Christof,
>>>>>>>
>>>>>>> Makefile.pdlibbuilder passes flags '-march=pentium4 -msse -msse2
>>>>>>> -mfpmath=sse' for optimization to the compiler on Windows. You could
>>>>>>> try compiling without (some of) these flags to see if they are
>>>>>>> responsible for the different behavior. Makefile-defined optimization
>>>>>>> flags can be overriden with argument CFLAGS given on command line.
>>>>>>>
>>>>>>> The effect of optimization flags on denormals varies per processor
>>>>>>> type, unfortunately. When we had denormals on Raspberry Pi ARMv6 they
>>>>>>> wouldn't go away no matter what flags, is what I remember. Even if
>>>>>>> your test reveals a beneficial effect from compiler flags, it is
>>>>>>> better when denormals are detected and flushed in the C code. Anyway,
>>>>>>> it is still interesting to know what makes the difference.
>>>>>>>
>>>>>>> Katja
>>>>>>>
>>>>>>> On Wed, Sep 21, 2016 at 12:32 AM, Christof Ressi <christof.ressi at gmx.at> wrote:
>>>>>>>> Hmmm... I compiled [bob~] myself with MinGW and pd-lib-builder and I noticed two things:
>>>>>>>> 1) the CPU rise is gone
>>>>>>>> 2) it needs only half the CPU. I put 20 [bob~] objects in a switched subpatch and measured the CPU load. The DLL which comes with the Windows binaries needs 15%, while my own DLL needs only 7%! That's quite a deal...
>>>>>>>>
>>>>>>>> Christof
>>>>>>>>
>>>>>>>> PS: I attached the DLL in case you wanna try it yourself.
>>>>>>>>
>>>>>>>>
>>>>>>>>> Gesendet: Samstag, 17. September 2016 um 22:58 Uhr
>>>>>>>>> Von: "Christof Ressi" <christof.ressi at gmx.at>
>>>>>>>>> An: pd-list at iem.at, "Miller Puckette" <msp at ucsd.edu>
>>>>>>>>> Betreff: [PD] [bob~] denormals issue?
>>>>>>>>>
>>>>>>>>> Hi Miller,
>>>>>>>>>
>>>>>>>>> feeding audio into [bob~] and then going to zero will increase the CPU load by ca. 6%. Clearing the filter or adding a tiny amount of noise brings the CPU load back to its usual level immediately, so I guess it's a problem with denormals.
>>>>>>>>> My Pd load meter won't really show the increase, but it's clearly visibly on Process Explorer.
>>>>>>>>>
>>>>>>>>> See my attached patch. Tried with Pd 0.47.1, Lenovo Thinkpad L440, Windows 7.
>>>>>>>>>
>>>>>>>>> Christof_______________________________________________
>>>>>>>>> Pd-list at lists.iem.at mailing list
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>>>>>>>>>
>>>>>>>> _______________________________________________
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