Question on IPA on GM107

Question on IPA on GM107

[Fernando Sahmkow]

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So I'm trying to track an special value in IPA instruction generation.
https://cgit.freedesktop.org/mesa/mesa/tree/src/gallium/drivers/nouveau/codegen/nv50_ir_emit_gm107.cpp#L2561

Register on 0x14 (20) is set to some source on "insn->op == OP_PINTERP"

I have found while emulation that such value can be set sometimes to
FragCoord.w, I don't however know what that value is and how to represent
it on glsl. Do you guys know where does that value come from and what it
means?

-- 
Atentamente,
*Fernando A. Sahmkow*
*Móvil*: +584242280286
*Correo*: fsahmkow27-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org

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Re: Question on IPA on GM107

[Ilia Mirkin]

On Mon, Nov 12, 2018 at 6:11 PM Fernando Sahmkow <fsahmkow27@gmail.com> wrote:
>
> So I'm trying to track an special value in IPA instruction generation.
> https://cgit.freedesktop.org/mesa/mesa/tree/src/gallium/drivers/nouveau/codegen/nv50_ir_emit_gm107.cpp#L2561
>
> Register on 0x14 (20) is set to some source on "insn->op == OP_PINTERP"
>
> I have found while emulation that such value can be set sometimes to FragCoord.w, I don't however know what that value is and how to represent it on glsl. Do you guys know where does that value come from and what it means?

It's for perspective division. If you're doing perspective
interpolation, you have to pass in 1/w as that argument (and set the
interpolation mode to "mul", which iirc doesn't cause anything to be
printed in nvdisasm).

Note that there's additional subtlety here ... the hardware will do
different things depending on what's in the shader header, as well as
on the global "shade model" which can be set to "flat". However if
you're converting to GLSL,

IPA = perspective interp
IPA.PASS = linear interp (i.e. "noperspective" in GLSL)
IPA.CONSTANT = flat interp (i.e. "flat" in GLSL)
IPA.SC = we use this for gl_Color. I *think* it's only this which is
affected by the global "shade model" setting. But don't quote me on
this.

Now, the contents of the shader header will affect the barycentric
coordinates that are computed and are made available to the IPA logic,
so you can't have too much disagreement. However if the shader header
says "flat", then flat interp is what you'll get no matter the IPA.*
mode. We use this feature to deal with a situation where you e.g. have

noperspective gl_Color
gl_SecondaryColor

and then glShadeModel(GL_FLAT) -- this is supposed to only affect the
gl_SecondaryColor, since the gl_Color has an explicit interpolation
mode, so we just mess with the shader header. You can see some of this
at work at

https://cgit.freedesktop.org/mesa/mesa/tree/src/gallium/drivers/nouveau/nvc0/nvc0_shader_state.c#n104

Cheers,

  -ilia
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