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

[Peter Maydell <peter.maydell@linaro.org>]

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.

thanks
-- PMM