Re: [Qemu-devel] Re: QEMU regression problems - Update FPU

[Aurelien Jarno <aurelien@aurel32.net>]

On Wed, Feb 23, 2011 at 10:45:25AM +0100, Laurent Desnogues wrote:
> On Wed, Feb 23, 2011 at 9:16 AM, Peter Maydell <peter.maydell@linaro.org> wrote:
> > On 18 February 2011 07:12, Gerhard Wiesinger <lists@wiesinger.com> wrote:
> >> Issue 1.) with FPU still present
> >> I tracked down the problematic code and it is a rounding error from double
> >> precision to 64bit floats: Any ideas how to fix such an issue in general?
> >>
> >> QEMU result in ST0: 0.42925860786976457 (wrong emulated)
> >> KVM result in ST0:  0.42925860786975449 (correct)
> >
> > This is an error when running QEMU in TCG mode, right?
> > At the moment x86 is the odd-one-out in that it doesn't
> > use CONFIG_SOFTFLOAT for its FPU emulation, so somebody
> > has made an explicit choice of preferring speed over
> > accuracy, and I am unsurprised that there are rounding
> > errors as a result.
> 
> Even if you were using SoftFloat, you'd probably still get wrong
> results given that this test seems to test trigonometric instructions
> which aren't implemented in SoftFloat, but are relying on libm.
> 

Actually that's the reason why i386 doesn't use softfloat, as all the
trigonometric use libm, and the bridge between softfloat and libm is not
working correctly (plenty of type abuse).

The solution is probably to implement them in softfloat, but it's not
something trivial. exp2 and log2 are already in softfloat for float32
type, the same way could be use for other trigonometric functions.

-- 
Aurelien Jarno	                        GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net