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Date: Wed, 23 Feb 2011 10:45:25 +0100
Message-ID: <AANLkTi=msmzzaSYF0NQD3enbxbppnbdrdPThhUjK6nve@mail.gmail.com>
Subject: Re: [Qemu-devel] Re: QEMU regression problems - Update FPU
From: Laurent Desnogues <laurent.desnogues@gmail.com>
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Roy Tam <roytam@gmail.com>, seabios@seabios.org, qemu-devel@nongnu.org, Kevin O'Connor <kevin@koconnor.net>, Gerhard Wiesinger <lists@wiesinger.com>, Aurelien Jarno <aurelien@aurel32.net>

On Wed, Feb 23, 2011 at 9:16 AM, Peter Maydell <peter.maydell@linaro.org> w=
rote:
> 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 doub=
le
>> 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: =A00.42925860786975449 (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.


Laurent