Re: TCG Floating Point Support (Work in Progress)

[Matt]

Clarification: In my previous message, I talked a lot about x87
emulation; I want to make clear that x87 is merely my motivator. The
eventual goal of this TCG FP support is not only to enable fast x87
emulation, but to be generic and robust enough that other QEMU targets
could be modified to utilize it for accelerated floating point
emulation, and be able to be implemented for various TCG targets where
possible, with x86-64 and AArch64 being my priorities.

Thanks,
Matt

On Wed, Sep 29, 2021 at 10:39 PM Matt <mborgerson@gmail.com> wrote:
>
> Hello--
>
> I'm excited to share that I have been developing support for TCG
> floating point operations; specifically, to accelerate emulation of
> x86 guest code which heavily exercises the x87 FPU for a game console
> emulator project based on QEMU. So far, this work has shown great
> promise, demonstrating some dramatic performance improvements in
> emulation of x87 heavy code.
>
> The feature works in concert with unaccelerated x87 FPU helpers, and
> also allows total soft float helper fallback if the user discovers
> some issue with the hard float implementation. For the TCG target,
> I've opted to implement it for x86-64 hosts using SSE2, although this
> could be extended to support full 80b double extended precision with
> host x87 support. I'm also in early development of an implementation
> for AArch64 hosts.
>
> There are still some significant tasks to be done, like proper
> handling of exception flags, edge cases, and testing, to name a few.
> Once in a slightly more mature state, I do think this feature would
> make a natural addition to upstream QEMU and plan to submit it for
> consideration.
>
> I'm writing to the mailing list now to inform FPU maintainers and any
> other interested parties that this work is happening, to solicit any
> early feedback, and to extend an invitation to anyone interested in
> collaborating to expedite its upstreaming.
>
> My initial TCG FP work can be found here:
> https://github.com/mborgerson/xemu/pull/464/commits
>
> Thanks,
> Matt