Re: [glean-dev] catching FP exceptions in C++

[Brian P. <bri...@tu...>]

Keith Whitwell wrote:
> Brian Paul wrote:
> 
>> Allen Akin wrote:
>>
>>> On Tue, Nov 08, 2005 at 04:08:19PM -0700, Brian Paul wrote:
>>> | I'm going to be writing a new glean test to see how OpenGL handles 
>>> | infinite/nan/denorm/etc floating point values.
>>>
>>> You're a braver man than I am. :-)
>>>
>>> | My experience with exception handling in C++ is pretty slim and I'm 
>>> | not even sure if there's a standard way to handle them.
>>>
>>> I'm not either.  I've checked my printed references and done a few web
>>> searches, and I haven't found anything that claims to work universally.
>>>
>>> I suspect the most portable thing we could do would be to use the C99
>>> standard IEEE exception mechanism.  (See "man fetestexcept", etc.)  It
>>> depends on the ANSI C signal-catching utilities, but nothing
>>> *nix-specific as far as I can tell.
>>>
>>> However, I don't have any experience with it, so I don't know for sure
>>> that it'll do the job.
>>
>>
>>
>> Thanks, Allen.  I wasn't even aware of those functions.
>>
>> I'm checking in what I have so far.  There's no attempt at trapping 
>> exceptions yet.
>>
>> Both Mesa and NVIDIA's driver pass the test in that FP exceptions 
>> aren't raised by default.  If I enable FP exceptions in Mesa with 
>> export MESA_DEBUG=FP things blow up as soon as I do this assignment:
>>
>> 285:        v[1][0] = make_signaling_nan();
>>
>> I guess the various make_*() functions should take a pointer to the 
>> destination float and store the bit pattern as an integer to avoid 
>> blowing up in Glean before we even get into OpenGL.
> 
> 
> It does make me question whether having the traps enabled is going to 
> work - if you can't even handle the values without provoking an 
> exception, it would mean that all the gl entrypoints will also provoke 
> the exceptions.  You may get out of glean, but you'll get caught in the 
> libGL.so dispatch functions...

I was thinking about that too.

Here's a sneaky approach: only enable exceptions before calling glEnd. 
  In Mesa (and probably other OpenGLs) we don't actually do anything 
with the vertex data until glEnd, or some other state-change command.

If exceptions are enabled at that point, only the exceptions generated 
inside OpenGL should get caught.

-Brian