Re: [GD-Windows] Strange float bug?

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

with no error =3D 0x037f where 0x033f =3D (_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 =3D 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 =3D 1.0f - (yy + zz);

where out.right.x is a vector, yy =3D 0.0f and zz =3D 0.0f.

thus I'm getting:

-nan =3D 1.0f - (0.0f + 0.0f);

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

Thanks,
Brett

----- Original Message -----=20
From: "Simon O'Connor" <si...@sc...>
To: <gam...@li...>
Sent: Monday, March 08, 2004 12:17 AM
Subject: RE: [GD-Windows] Strange float bug?

>=20
> Hi Brett,
>=20
>=20
> 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.=20
>=20
> 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))?
>=20
>=20
> The "something" is likely to be a third party library...
>=20
> ...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)
>=20
> 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.
>=20
> 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.
>=20
> 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.
>=20
>=20
> Unmasking FP exceptions can be handy during debugging for trapping =
things
> like domain errors at source too IMO.
>=20
>=20
> Cheers,
>=20
> Simon O'Connor
> Programmer @ Acclaim
> & Microsoft DirectX MVP=20
>=20
>=20
> > -----Original Message-----
> > From: gam...@li...=20
> > [mailto:gam...@li...] On=20
> > Behalf Of Brett Bibby
> > Sent: 07 March 2004 13:39
> > To: Gam...@li...
> > Subject: [GD-Windows] Strange float bug?
> >=20
> > We have been tracking down the wierdest bug for several days.=20
> > Our code was crashing randomly and then we finally found a=20
> > repeatable case where an assert on an identity matrix when we=20
> > check the orthonormalness of it and we chanced upon a line of=20
> > code that returned -nan that was simply x =3D (1.0f - (a + b)).=20
> >  Anyway, I suddenly remembered a GDC talk about Windows=20
> > denormals, did a quick google and pasted a _controlfp(=20
> > _CW_DEFAULT, 0xfffff ) into our code and presto it now works.
> >=20
> > We make games for PS2 and GCN and are Windows amateurs, but I=20
> > hate the idea of not understanding what went wrong and how to=20
> > properly handle this.  Has anybody ever encountered something=20
> > like this before?
> >=20
> > Brett
> >=20
> >=20
> >=20
> > -------------------------------------------------------
> > This SF.Net email is sponsored by: IBM Linux Tutorials Free=20
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