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

[Programmingkid]

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?