RE: [GD-Windows] Strange float bug?

[Simon O'C. <si...@sc...>]

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.=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))?


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=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
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