Am 12.12.2012 22:03, schrieb clipka:
> Am 12.12.2012 20:56, schrieb clipka:
>> Am 12.12.2012 20:30, schrieb clipka:
>>
>>> I swear I'm gonna find out what's going on here...
>>
>> Apparently there's something fishy happening with the phong highlighting.
>
> Further investigations suggest that the Pi isn't happy with denormalized
> values, at least not in single precision.
>
> I'll try to throw together some simple C test program, and if that
> confirms my observations I'll file it as a bug to the Pi people.

Apparently the problems are related to the -ffast-math compiler option; 
I'll try whether turning that off will help.

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Am 13.12.2012 00:24, schrieb clipka:

> Apparently the problems are related to the -ffast-math compiler option;
> I'll try whether turning that off will help.

Well, no, not really. No success, except that now the color of the 
artifacts is inverted.

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Am 13.12.2012 08:52, schrieb clipka:
> Am 13.12.2012 00:24, schrieb clipka:
>
>> Apparently the problems are related to the -ffast-math compiler option;
>> I'll try whether turning that off will help.
>
> Well, no, not really. No success, except that now the color of the
> artifacts is inverted.

Turning off /all/ optimizations does give the expected results. I don't 
consider this an option though, so I guess we'll have to wait for 
feedback from the raspbian team.

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Am 13.12.2012 14:48, schrieb clipka:

> Turning off /all/ optimizations does give the expected results. I don't
> consider this an option though, so I guess we'll have to wait for
> feedback from the raspbian team.

I've received feedback at last: The problem /may/ be solved with 
gcc-4.7; I'm currently recompiling POV-Ray on my Pi.

Raspbian Wheezy's standard gcc package is still at 4.6, so I had to 
manually install the gcc-4.7 and g++-4.7 packages, and force the 
./configure script to actually use gcc-4.7 via the "CXX=gcc-4.7" option.

We shall see in a couple of hours whether that actually solves the issue.

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clipka <ano### [at] anonymousorg> wrote:
> Am 13.12.2012 14:48, schrieb clipka:
>
> > Turning off /all/ optimizations does give the expected results. I don't
> > consider this an option though, so I guess we'll have to wait for
> > feedback from the raspbian team.
>
> I've received feedback at last: The problem /may/ be solved with
> gcc-4.7; I'm currently recompiling POV-Ray on my Pi.
>
> Raspbian Wheezy's standard gcc package is still at 4.6, so I had to
> manually install the gcc-4.7 and g++-4.7 packages, and force the
> ./configure script to actually use gcc-4.7 via the "CXX=gcc-4.7" option.
>
> We shall see in a couple of hours whether that actually solves the issue.

I haven't tried 3.7 but 3.6.1 on the RPi using the 4.7 versions of the compiler
seem to work fine.

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Am 01.08.2013 18:29, schrieb Craig Lindley:
> clipka <ano### [at] anonymousorg> wrote:
>> Am 13.12.2012 14:48, schrieb clipka:
>>
>>> Turning off /all/ optimizations does give the expected results. I don't
>>> consider this an option though, so I guess we'll have to wait for
>>> feedback from the raspbian team.
>>
>> I've received feedback at last: The problem /may/ be solved with
>> gcc-4.7; I'm currently recompiling POV-Ray on my Pi.
>>
>> Raspbian Wheezy's standard gcc package is still at 4.6, so I had to
>> manually install the gcc-4.7 and g++-4.7 packages, and force the
>> ./configure script to actually use gcc-4.7 via the "CXX=gcc-4.7" option.
>>
>> We shall see in a couple of hours whether that actually solves the issue.
>
> I haven't tried 3.7 but 3.6.1 on the RPi using the 4.7 versions of the compiler
> seem to work fine.

I wouldn't trust it; regarding the 3.7 issues, gcc 4.7 did somewhat 
change the symptoms, but apparently not the underlying problem.

There's something seriously fishy going on (or at least was, last time I 
tried) when the Pi tries to do single-precision vector computations with 
very small (near-zero) numbers involved, and in some inexplicable way it 
is very, /very/ sensitive to the structure of the program that does 
these computations. Stuff like, say, you boil down the error to a small 
program running some very simple computations in a loop 10 times, 
triggering the bug on the 5th iteration; you reduce the loop count to 8 
times, and suddenly the error does /not/ occur anymore, or manifests in 
an entirely different manner. You "unroll" the loop into a sequence of 
identical computations, and the program's behaviour changes yet again. 
You use a different compiler, and the error goes away, but only in that 
small boiled-down program, while still manifesting in the big program 
that you're actually trying to get fixed.

So more likely than not it is just pure chance if the bug doesn't 
manifest in 3.6.1. compiled with gcc 4.7 - and it may actually be still 
lurking in some dark, rarely used corner.

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