[PD] Raspberry Pi does denormals

[Miller Puckette]

OK.. but try the 0.44 build on my site - the one from Raspian is quite old :)

M

On Sun, Jan 20, 2013 at 09:28:30PM +0100, katja wrote:
> Miller, the vanilla Pd which can be installed from Raspbian with
> apt-get or Synaptic does have the subnormals problem, as can be
> checked with a test patch attached with my first post. When an input
> signal to [lop~] is shut off, CPU load increases substantially. Output
> values go down in the order of 1e-44, subnormal range. I was working
> on reverb algo's showing the same problem, and compiled with option
> -ffastmath / --fast-math to see if that would turn on RunFast mode,
> but it didn't.
> 
> I'm not familiar with ARM and it's coprocessors, but from Intel I do
> know that gcc doesn't implement certain specified optimization options
> (notably SSE versions) unless you also mention a processor type that
> can handle it . A similar case could be with Rpi's vfpv2; it can do
> RunFast mode but gcc doesn't implement it, until you find a way to
> specify vfpv2 (vfpv1 can't do RunFast). Miller, if you succeeded in
> getting automatic flush-to-zero on the Pi, it may be related to other
> flags which you've set. Arch flags which I've set so far are
> -march=armv6 and -mfpu=vfp. Option -mfpu=vfpv2 is not allowed. I would
> be happy to do further testing with compiler options, if you know
> some. The big-or-small checks are rather expensive for RPi, that's
> what I've found.
> 
> Katja
> 
> 
> On Sun, Jan 20, 2013 at 8:24 PM, Miller Puckette <msp at ucsd.edu> wrote:
> > Hi all...
> >
> > I think it's possible to get flush-to-zero behavior on the Pi (ARMv6) by
> > calling gcc with --fast-math.  At any rate what I found was that, if I
> > compiled without --fast-math, when numbers got small (e.g., when a
> > reverberator decays down past 10^-38 or so), the patch would suddenly jump
> > in CPI usage as if it were trappnig to the kernel (as it does for i386).
> > But when I added --fast-math the problem went away.
> >
> > On i386 and x86_64, I believe that one can't get flush-to-zero (at least in
> > the "normal" non-SSE floating point instructions) so there's no choice but
> > to use a macro such as PD_BADFLOAT to protect against that.  So in m_pd.h the
> > PD_BADFLOAT macro is only "turned on" for Intel.
> >
> > However I've been mistaken many times about all this in the past and won't
> > be surprised if I'm mistaken again.
> >
> > cheers
> > Miller
> >
> > On Sun, Jan 20, 2013 at 11:12:28AM -0500, Hans-Christoph Steiner wrote:
> >>
> >> I think this is what you want, from 'man gcc'.  Its interesting to note that
> >> the NEON mode, which provides SIMD, also does not do denormals:
> >>
> >> -mfpu=name
> >> -mfpe=number
> >> -mfp=number
> >>     This specifies what floating point hardware (or hardware emulation) is
> >>     available on the target.  Permissible names are: fpa, fpe2, fpe3, maverick,
> >>     vfp, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16,
> >>     neon, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16 and neon-vfpv4.  -mfp and
> >>     -mfpe are synonyms for -mfpu=fpenumber, for compatibility with older
> >>     versions of GCC.
> >>
> >>     If -msoft-float is specified this specifies the format of floating point
> >>     values.
> >>
> >>     If the selected floating-point hardware includes the NEON extension (e.g.
> >>     -mfpu=neon), note that floating-point operations will not be used by GCC's
> >>     auto-vectorization pass unless -funsafe-math-optimizations is also
> >>     specified.  This is because NEON hardware does not fully implement the IEEE
> >>     754 standard for floating-point arithmetic (in particular denormal values
> >>     are treated as zero), so the use of NEON instructions may lead to a loss of
> >>     precision.
> >>
> >>
> >> .hc
> >>
> >> On 01/20/2013 06:54 AM, katja wrote:
> >> > I was assuming, or maybe just hoping? that Raspberry Pi (and ARM
> >> > devices in general) would not suffer from Denormal's disease like
> >> > Intel processors do. But guess what: Pi's float coprocessor is IEEE
> >> > 754 compliant and does all denormals by default (can check with
> >> > attached denorm-test.pd). Bummer! As if one would use an ARM device to
> >> > calculate the size of a Majorana particle, rather than doing simple
> >> > dsp. Do we really need to enable PD-BIGORSMALL() checks for this poor
> >> > little processor? There seems to be something called 'RunFast mode'
> >> > for Pi's float processor vfpv2, but I see no way how to enable this
> >> > via gcc. Option -ffast-math is allowed but doesn't do the trick. Can't
> >> > find an option to set vfpv2 specifically, in gcc docs.
> >> >
> >> > Katja
> >> >
> >> >
> >> >
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> >>
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