Re: [Qemu-ppc] [Qemu-devel] [PATCH] ppc: Three floating point fixes

[Aleksandar Markovic]

19.08.2019. 08.30, "David Gibson" <address@hidden> је написао/ла:
>
> On Sun, Aug 18, 2019 at 10:59:01PM +0200, Aleksandar Markovic wrote:
> > 18.08.2019. 10.10, "Richard Henderson" <address@hidden> је
> > написао/ла:
> > >
> > > On 8/16/19 11:59 PM, Aleksandar Markovic wrote:
> > > >> From: "Paul A. Clarke" <address@hidden>
> > > ...
> > > >>   ISA 3.0B has xscvdpspn leaving its result in word 1 of the target
> > > > register,
> > > >>   and mffprwz expecting its input to come from word 0 of the source
> > > > register.
> > > >>   This sequence fails with QEMU, as a shift is required between those
> > two
> > > >>   instructions.  However, the hardware splats the result to both word 0
> > > > and
> > > >>   word 1 of its output register, so the shift is not necessary.
> > > >>   Expect a future revision of the ISA to specify this behavior.
> > > >>
> > > >
> > > > Hmmm... Isn't this a gcc bug (using undocumented hardware feature),
> > given
> > > > everything you said here?
> > >
> > > The key here is "expect a future revision of the ISA to specify this
> > behavior".
> > >
> > > It's clearly within IBM's purview to adjust the specification to document
> > a
> > > previously undocumented hardware feature.
> > >
> >
> > By no means, yes, the key is in ISA documentation. But, the impression that
> > full original commit message conveys is that the main reason for change is
> > gcc behavior. If we accepted in general that gcc behavior determines QEMU
> > behavior, I am afraid we would be on a very slippery slope - therefore I
> > think the commit message (and possible code comment) should, in my opinion,
> > mention ISA docs as the central reason for change. Paul, is there any
> > tentative release date of the new ISA specification?
>
> It's not really a question of gcc behaviour, it's a question of actual
> cpu behaviour versus ISA document.  Which one qemu should follow is
> somewhat debatable, but it sounds here like the ISA will be updated to
> match the cpu, which weights it heavily in favour of mimicing the
> actual cpu.
>

This sounds right to me.

Aleksandar

> --
> David Gibson                    | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
>                                 | _way_ _around_!
> http://www.ozlabs.org/~dgibson