Re: [hatari-devel] FPU update

[Andreas Grabher <andreas.grabher@xxxxxxxxxxxx>]

Am 11.02.2017 um 08:38 schrieb Thomas Huth <th.huth@xxxxxx>:

> Am Wed, 8 Feb 2017 12:40:02 +0000
> schrieb Douglas Little <doug694@xxxxxxxxxxxxxx>:
> 
>> Hi,
>> 
>> I'm not sure if there are any remaining problems in 6888x emulation.
>> I am
>>> thinking about what could be tested, but most things have been
>>> tested through Douglas' great test utility and some things have
>>> been tested by Toni separately.
>>> I think 68040 might be incorrect for some conditions, especially
>>> related to nonmaskable overflow and underflow exceptions (inexact
>>> bit set, where it shouldn't). But it that makes more sense to test
>>> and fix that while adding 68040 arithmetic exception handling.
>>> So what is next? We need a SoftFloat routine to convert between
>>> extended precision and packed decimal floats! This is commonly
>>> used, but quite faulty with native floats. Any ideas?
>>> 
>> 
>> I'll need to get a recent build with new changes brought into Hatari
>> soon so I can see all the progress.
>> 
>> I'm quite interested to see if the HW-backed float support still has
>> relevant bugs (other than precision differences which I accept should
>> remain). I mean things like (0.5+/-epsilon) rounding up or down in
>> different rounding modes, or common condition flags not being set, or
>> being set inappropriately - that's the kind of thing that can cause
>> breakage.
> 
> You could also try to compile with USE_SOFT_LONG_DOUBLE ... maybe that
> helps with your precision problems, too?
> 
> Thomas
> 
USE_SOFT_LONG_DOUBLE has been removed a while before (2014). It never did anything else than preserve full size data in FPU registers on systems, that have no extended precision floats (not all bits fit in double precision and are effectively lost). This was only useful, if data was copied to and from memory using FPU registers without any arithmetic operations. It did not affect precision of calculations.