Re: [Qemu-devel] [Qemu-ppc] How to add my implementation of the fmadds instruction to QEMU

[Alex Bennée]

Programmingkid <address@hidden> writes:

> On Sep 29, 2016, at 2:19 PM, Alex Bennée wrote:
>
>>
>> Programmingkid <address@hidden> writes:
>>
>>> On Sep 29, 2016, at 12:17 AM, David Gibson wrote:
>>>
>>>> On Tue, Sep 27, 2016 at 09:58:02AM -0700, Peter Maydell wrote:
>>>>> On 27 September 2016 at 09:51, G 3 <address@hidden> wrote:
>>>>>> The problem with your reasoning is you assume this instruction has to be
>>>>>> 100% correctly implemented. That every single "corner-case" has to be
>>>>>> accounted for.
>>>>>
>>>>> For upstream QEMU we've already made this design decision --
>>>>> emulation accuracy comes first, and speed is secondary.
>>>>> That's why we implement this the way we do.
>>>>
>>>> I think there is a way you could get both speed and accuracy, but it's
>>>> a huge project:
>>>>
>>>> You'd need to add full float awareness to TCG - so floating point TCG
>>>> values and floating point operations as tcp micro-ops, defined
>>>> according to IEEE semantics.  Then you'd need to rewrite the TCG
>>>> frontends in terms of those new ops, at least for target CPUs close
>>>> enough to IEEE semantics for that to work.  And you'd need to rewrite
>>>> the TCG backends to implement those fp ops in terms of host cpu fp
>>>> instructions .. at least when the host has fp behaviour close enough
>>>> to IEEE to make that work, with a fallback to soft float when that's
>>>> not the case.
>>>
>>> Interesting idea. Do you think we would see a large enough increase in speed
>>> to justify this project?
>>
>> It really depends on workload. If you want to run lots of encoding/audio
>> workloads in TCG guests it is certainly something that could be
>> improved.
>>
>> As others have pointed out however it is a fairly big project.
>>
>> --
>> Alex Bennée
>
> Alex Bennée? I was just watching your KVM video about MTTCG! Small world.
>
> I so want audio to play correctly in a PowerPC Mac OS guest. So this
> project might be necessary.
>
> If it is a fairly big project, then I will need to map it out some more.
> I've made a mind map of what I know so far. It is attached to this email.
> Let me know if you can think of anything to add.
>
> http://i.imgur.com/MYkiKGx.png

While I appreciate your target is PPC I think if you are going to
suggest any core floating point TCGOps you will need to survey the
behaviour of the FPUs on all (or at least the most common) TCG targets
and go for instructions that behave the same across a broad range of
targets.

I think if we were to introduce this into the code base we would need to
have a decent range of test cases. I'm talking about making sure we
exercise the whole range of behaviour:

  - min/max rounding behaviour
  - handling of denormalisation
  - signalling and non-signalling NaN behaviour
  - exception generation

Testing is going to be very important for confidence.

--
Alex Bennée