Re: [hatari-devel] Native FPU emulation

[Toni Wilen <twilen@xxxxxxxxxx>]

> > Hi,
> >
> > while trying to debug an application, i just noticed that Hatari does 
> > not seem to use long doubles when using native emulation 
> > (USE_LONG_DOUBLE is not defined). Wouldn't it make sense to use at 
> > least the long double format when available, to be closer to the m68k 
> > format, even if that will not be as accurate as the softfloat emulation?
> >
> > Also, in uae_cpu/newcpu.h, USE_LONG_DOUBLE is defined, but to a zero 
> > value. However in other places ifdef is used, which will still be true.
> >
> > And if i understand the code correctly, the softfloat emulation solely 
> > uses the fpdata.fpx member, while the native emulation only uses 
> > fpdata.fp. When switching the emulation type at runtime, i can't see 
> > any attempt to convert from one field to the other (unless i missed 
> > something). Instead, it seems to reset the FPU registers to all NANs. 
> > Is that intended?
> >
> > And the file cpu/md-fpp.h does not seem to be used at all.
>
> Hi
>
> I might be wrong but IIRC Toni removed the long double because the extra 
> cpu overhead was not worth the small accuracy gain.
>
> For this and the other points of your message, Toni might give more 
> detailed answer when he read this.

It is not removed.

It is not used (but should still work if USE_LONG_DOUBLE=1) in WinUAE 
because MSVC does not support long doubles (long double is simply alias 
to double. Just don't ask.).

WinUAE uses alternative method where every long double FPU function 
calls is redirected to "wrapper" x86/x64 assembly function that does the 
calculation. It uses softfloat (fpx) FPU register variables.  Host FPU 
variable (double/long double) is not used in either 80-bit mode.

It is also runtime configurable in WinUAE (host 64-bit, host 80-bit or 
softfloat 80-bit). 64-bit is the default because there are very few 
software that needs 80-bit accuracy.

Hopefully this helped.