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Re: [Qemu-devel] [PATCH] ppc: Three floating point fixes
From: |
Aleksandar Markovic |
Subject: |
Re: [Qemu-devel] [PATCH] ppc: Three floating point fixes |
Date: |
Mon, 19 Aug 2019 08:44:58 +0200 |
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
- [Qemu-devel] [PATCH] ppc: Three floating point fixes, Paul A. Clarke, 2019/08/16
- Re: [Qemu-devel] [PATCH] ppc: Three floating point fixes, Aleksandar Markovic, 2019/08/16
- Re: [Qemu-devel] [PATCH] ppc: Three floating point fixes, Aleksandar Markovic, 2019/08/17
- Re: [Qemu-devel] [PATCH] ppc: Three floating point fixes, Richard Henderson, 2019/08/18
- Re: [Qemu-devel] [PATCH] ppc: Three floating point fixes, Aleksandar Markovic, 2019/08/18
- Re: [Qemu-devel] [PATCH] ppc: Three floating point fixes, David Gibson, 2019/08/19
- Re: [Qemu-devel] [PATCH] ppc: Three floating point fixes,
Aleksandar Markovic <=
- Re: [Qemu-devel] [Qemu-ppc] [PATCH] ppc: Three floating point fixes, Paul Clarke, 2019/08/19
- Re: [Qemu-devel] [Qemu-ppc] [PATCH] ppc: Three floating point fixes, David Gibson, 2019/08/20
- Re: [Qemu-devel] [Qemu-ppc] [PATCH] ppc: Three floating point fixes, Peter Maydell, 2019/08/20
- Re: [Qemu-devel] [Qemu-ppc] [PATCH] ppc: Three floating point fixes, David Gibson, 2019/08/22
Re: [Qemu-devel] [PATCH] ppc: Three floating point fixes, Richard Henderson, 2019/08/18
Re: [Qemu-devel] [PATCH] ppc: Three floating point fixes, David Gibson, 2019/08/19