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

["Alex Bennée" <alex.bennee@linaro.org>]

Programmingkid <programmingkidx@gmail.com> 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 <programmingkidx@gmail.com> 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