[Mesa-dev] Thoughts on fp64 for GLES?

[Stéphane Marchesin]

On Wed, Feb 13, 2019 at 10:29 AM Elie Tournier <tournier.elie at gmail.com> wrote:
>
>
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> On Wednesday, 13 February 2019, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
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>> On Wed, Feb 13, 2019 at 12:47 PM Elie Tournier <tournier.elie at gmail.com> wrote:
>> >
>> > On Fri, Jan 25, 2019 at 11:52:56AM -0800, Stéphane Marchesin 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 :)
>> > >
>> > > >
>> > > > 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.
>> >
>> > I spent a bit of time on the project recently.
>> > My thinking so far:
>> > * FP64 is bad . But everyone knows that. :)
>> > * Using the current soft fp64 require to emulate int64.
>> > * Soft fp64 and int64 involve function call which is, iiuc, not really
>> > supported in TGSI.
>> > * Soft fp64 is tied to NIR. Some pass/hack need to be port to GLSLIR.
>> >
>> > So the project will require a lot of work.
>>
>> But what's the alternative? Let's say you make a spec to expose
>> "proper" fp64 in GLES. No one outside mesa will implement this (why
>> bother). Certainly not the Adreno/Mali proprietary stacks of the
>> world.
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>
> I'm not saying that we should get an extension.
> My point was, it's a lot of work.
>>
>>
>> And if you are on a stack that implements this in GLES, you might as
>> well be using desktop GL anyways...
>>
>> So going back to the original -- what use-case are you trying to cover
>> that's not already covered some other way?
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>
> iiuc, Stephane want to run GL desktop on top of GLES.
> In order to expose a bigger version of GL, he need fp64 support.


Yes, at a high level, softfp64 seems to solve the problem we have. If
a TGSI lowering pass is too complex, could we do it as a GLSL lowering
pass?

Stéphane

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>>   -ilia