On Wed, Feb 23, 2011 at 9:16 AM, Peter Maydell email@example.com wrote:
On 18 February 2011 07:12, Gerhard Wiesinger firstname.lastname@example.org 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.