[Mesa-dev] Thoughts on fp64 for GLES?

[Jason Ekstrand]

On Fri, Jan 25, 2019 at 1:53 PM Stéphane Marchesin <
stephane.marchesin at gmail.com> wrote:

> On Fri, Jan 25, 2019 at 2:25 AM Gert Wollny <gw.fossdev at gmail.com> wrote:
> >
> > Am Donnerstag, den 24.01.2019, 22:25 -0800 schrieb Stéphane Marchesin:
> > >
> > > Yes, it's for running virgl on top of GLES. To emulate fp64 in GL on
> > > the guest side, we need fp64 on the host...
> >
> > BTW: we could also get it emulated from the guest side. When Elie (in
> > CC)  initially proposed the fp64 emulation series it was for r600 and
> > TGSI was emitted. The created shaders are horribly long and it is
> > certainly not performant, but if it's just for getting OpenGL 4.0
> > exposed it should be good enough.
>
> Yes, Ilia suggested this on IRC yesterday. My impression is that not
> many applications/games need high performance fp64 (it's likely mostly
> compute stuff, which is not our target). I could be wrong though. If
> anyone knows differently, please tell us :)
>

In our experience, we have yet to see an app actually use the extension.
If we do have such apps, it'd be better to do it in hardware when
available.  However, if it's just so that you can claim support, maybe a
GLES extension isn't worth the bother?  I don't have a particularly strong
opinion at the moment beyond "fp64 sounds like the most non-ES thing ever".


> >
> > I'm not sure though how much work it would be to add this to the soft
> > fp64 as it has now landed for NIR, though.
>
> Yes, with virgl not using NIR, I am not sure how much work soft fp64
> will require.
>

The core of the soft fp64 stuff is a library of GLSL functions which
actually implement it.  We just compile them to NIR and then lower fp64
math to function calls and inline.  You could write a lowering pass in
GLSL, TGSI, or a back-end compiler based on those easily enough.

--Jason
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