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Re: target/ppc: bug in optimised vsl/vsr implementation?


From: Alex Bennée
Subject: Re: target/ppc: bug in optimised vsl/vsr implementation?
Date: Wed, 02 Oct 2019 18:38:06 +0100
User-agent: mu4e 1.3.4; emacs 27.0.50

Mark Cave-Ayland <address@hidden> writes:

> On 28/09/2019 18:45, Aleksandar Markovic wrote:
>
> Hi Aleksandar,
>
> Thanks for taking a look at this!
>
>> Mark and Paul (and Stefan),
>>
>> Thanks for spotting this and pinpointing the culprit commit. I guess Stefan 
>> is going
>> to respond soon, but, in the meantime, I took a look at the commit in 
>> question:
>>
>> https://github.com/qemu/qemu/commit/4e6d0920e7547e6af4bbac5ffe9adfe6ea621822
>>
>> I don't have at the moment any dev/test environment handy, but I did manual
>> inspection of the code, and here is what I found (in order of importance, 
>> perceived
>> by me):
>>
<snip>
>
>> Given all these circumstances, perhaps the most reasonable solution would be 
>> to
>> revert the commit in question, and allow Stefan enough dev and test time to 
>> hopefully
>> submit a new, better, version later on.
>
> Given that it has been broken for 3 months now, I don't think we're in any 
> major rush
> to revert ASAP. I'd prefer to give Stefan a bit more time first since he does 
> report
> some substantial speed improvements from these new implementations.

Is the denbcdq instruction exposed in any standard float operations?
Once this is fixed it would be worth adding a testcase (either ppc64
specific or multiarch) so protect it from regression in the future.

>
>
> ATB,
>
> Mark.


--
Alex Bennée



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