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Re: [Qemu-ppc] [Qemu-devel] PowerPC Decimal Floating Point

From: Tom Musta
Subject: Re: [Qemu-ppc] [Qemu-devel] PowerPC Decimal Floating Point
Date: Fri, 14 Feb 2014 16:22:07 -0600
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0

On 2/14/2014 3:22 PM, Richard Henderson wrote:
> On 02/11/2014 11:38 PM, Alexander Graf wrote:
>>>> My proposal is to incorporate the libdecnumber component of libdfp
>>>> (http://www.eglibc.org/cgi-bin/viewvc.cgi/libdfp/trunk/) in a manner 
>>>> analogous to how
>>>> softfloat is used for binary floating point.  So, for example, the helper 
>>>> for the dadd
>>>> instruction would look something like the following:
>>>>   - map FPSCR state to a decContext.
>>>>   - convert the contents of the source FPRs to decNumbers 
>>>> (decimal64ToNumber).
>>>>   - call decNumberAdd
>>>>   - convert the resultant decNumber to DPD (decimal64FromNumber)
>>>>   - update FPSCR per the decContext.status and result.
>>>> Comments?
>> I think that approach makes a lot of sense, but let's ask Richard and Peter 
>> as well.
> Reasonable, as far as the implementation details go.
> I am a teeny bit concerned which version of libdecnumber is considered most
> "upstream".  Ordinarily I'd point to the copy in gcc, but that is of course
> GPLv3 + GCC runtime exception.  I see the version from eglibc you quote above
> is LGPLv2.1.  I also see that the last change is 18 months old.  On the other
> hand, there is only one non-autoconf change in the gcc sources in the same 
> period.
> The original import is from IBM sources.  Is there somewhere "upstream" at IBM
> that we ought to be importing from instead?


I will check with folks inside of IBM here that I have the latest and greatest.

FWIW ... I have about 20 instructions prototyped and running in a test harness
(not QEMU) and comparing results against actual P7/P8 hardware.  So far, this
looks pretty good.  I haven't yet worked through any details of importing the 
into QEMU.

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