Re: Strange float bug?

Thanks everybody for the info so far.  Here is what I found in the control word conditions and value:

with no error = 0x037f where 0x033f = (_PC_64 | _EM_INVALID | _EM_ZERODIVIDE | _EM_OVERFLOW | _EM_UNDERFLOW | _EM_INEXACT | _EM_DENORMAL) and I can't seem to figure out where the last bit 6 comes from????

when I get an error = 0x007f which means I lost precision down to _PC_24.  That doesn't seem enough to cause the problem though does it?

The error gets generated practically anyplace if we try to do math, for example:

out.right.x = 1.0f - (yy + zz);

where out.right.x is a vector, yy = 0.0f and zz = 0.0f.

thus I'm getting:

-nan = 1.0f - (0.0f + 0.0f);

Given the control word and the line above, is this the expected behavior?

Thanks,
Brett

----- Original Message ----- 
From: "Simon O'Connor" <[email protected]>
To: <[email protected]>
Sent: Monday, March 08, 2004 12:17 AM
Subject: RE: [GD-Windows] Strange float bug?


> 
> Hi Brett,
> 
> 
> Personally, almost every NaN I've ever encountered has been due to something
> earlier on such as passing out of domain values to trig functions, divisions
> by zero, square roots of negatives etc. 
> 
> However, the fact that resetting the FPU control word to its default fixes
> the problem hints that something else is changing it behind your back. What
> value is the control word set to when the NaN occurs (_controlfp(0,0))?
> 
> 
> The "something" is likely to be a third party library...
> 
> ...What immediately springs to mind is if you're using Direct3D and also
> using double precision floating point anywhere (probably unwittingly - evil
> things as you'll know from your PS2 experience :o)
> 
> IDirect3D*::CreateDevice() changes the control word so that the FPU uses
> 24bit mantissa precision (_PC_24) rather than the default 53bit precision
> (_PC_53). It also (re)masks FP exceptions (which is the _controlfp()
> default, but it'll re-mask them if you'd unmasked them). It does this for
> performance reasons.
> 
> If you use double precision values in your code with _PC_24 you'll likely
> get denormals, under/over flows and generally unexpected behaviour. If you
> force the control word back to default, you might get unexpected behaviour
> from D3D too.
> 
> If you really need double precision or exceptions unmasked, you can pass the
> D3DCREATE_FPU_PRESERVE flag to CreateDevice() - it will impact performance
> slightly though since it makes D3D save, change and restore the control word
> for every call that might change the control word.
> 
> 
> Unmasking FP exceptions can be handy during debugging for trapping things
> like domain errors at source too IMO.
> 
> 
> Cheers,
> 
> Simon O'Connor
> Programmer @ Acclaim
> & Microsoft DirectX MVP 
> 
> 
> > -----Original Message-----
> > From: [email protected] 
> > [mailto:[email protected]] On 
> > Behalf Of Brett Bibby
> > Sent: 07 March 2004 13:39
> > To: [email protected]
> > Subject: [GD-Windows] Strange float bug?
> > 
> > We have been tracking down the wierdest bug for several days. 
> > Our code was crashing randomly and then we finally found a 
> > repeatable case where an assert on an identity matrix when we 
> > check the orthonormalness of it and we chanced upon a line of 
> > code that returned -nan that was simply x = (1.0f - (a + b)). 
> >  Anyway, I suddenly remembered a GDC talk about Windows 
> > denormals, did a quick google and pasted a _controlfp( 
> > _CW_DEFAULT, 0xfffff ) into our code and presto it now works.
> > 
> > We make games for PS2 and GCN and are Windows amateurs, but I 
> > hate the idea of not understanding what went wrong and how to 
> > properly handle this.  Has anybody ever encountered something 
> > like this before?
> > 
> > Brett
> > 
> > 
> > 
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