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 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.

-- 
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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