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

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Sep 29, 2016 at 12:55:23PM -0400, Programmingkid wrote:
> 
> On Sep 29, 2016, at 11:41 AM, Peter Maydell wrote:
> 
> > On 28 September 2016 at 21:17, David Gibson <david@gibson.dropbear.id.au> wrote:
> >> 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.
> > 
> > Also even if you have float support in both frontend and backend
> > you still need to fall back to fully-emulated for the runtime
> > corner cases (like where tininess before/after rounding makes a
> > difference or where you need to care about minutiae of the
> > floating point exception flags, etc). It's not impossible
> > but it is a very large amount of technically complicated work.
> 
> 
> This project sounds like it should have its own web page. Maybe even
> its own Google Summer of Code entry. I created a mindmap of
> this project. The picture is attached to this email. This is
> just a start. Please let me know what should be added or changed.

TBH, I think this is rather bigger than a GSoC project.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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