[Mesa-dev] A question about nir_lower_wpos_ytransform

[Jason Ekstrand]

On Mon, May 7, 2018 at 9:37 AM, Kenneth Graunke <kenneth at whitecape.org>
wrote:

> On Monday, May 7, 2018 8:53:34 AM PDT Jason Ekstrand wrote:
> > On Mon, May 7, 2018 at 8:02 AM, Alejandro Piñeiro <apinheiro at igalia.com>
> > wrote:
> >
> > > Hi Jason,
> > >
> > > as part of the ARB_gl_spirv work, we are doing the linking based on the
> > > nir shader that comes from spirv_to_nir. On some cases,
> > > nir_lower_wpos_ytransform introduces a new uniform,
> > > "gl_FbWposYTransform". So as the uniform is there when we start to do
> > > the linking, it is treated as any other uniform, being added to the
> > > UniformStorage, accessible through the OpenGL API and so on. Something
> > > that I understand that we don't want. On GLSL this is not a problem
> > > because the linking is done based on MESA/IR, then the IR is converted
> > > to NIR, and the lowering adds the uniform based on the NIR shader,
> > > really after the linking.
> > >
> > > The first solution I thought was just mark the uniform with a special
> > > location [1]. It would need to be a negative value, not -1 that is
> > > already reserved for not-assigned locations (we still have some cases
> > > for those, even if in general ARB_gl_spirv makes explicit location
> > > mandatory). So it would be a new negative value, like lets say -5. So
> > > then on the linking, if the uniform location has this value, the
> uniform
> > > is skipped.
> > >
> > > After checking that this solution worked (although I only tested with
> > > some tests), I started to search for an alternative, as this solution
> > > seemed too hacky.
> >
> >
> > It does seem a bit hacky, yes.
> >
> >
> > > But this lowering needs to be run before nir_lower_io,
> > > and nir_lower_io is part of the uniform lowering. So I didn't see
> really
> > > reasonable to do a nir_lower_io without nir_lower_wpos_ytransform, do
> > > the linking, and then execute again nir_lower_io/nir_lower_wpos_
> transform.
> > >
> >
> > This is all a bit sticky.  One observation is that the newly added
> uniform
> > makes usage of the token system.  Are tokens no longer used with
> SPIR-V?  I
> > guess they are kind-of an old mechanism to handle the crufty ARB program
> > parameters.  However, you could have -5 mean "It uses a token"
> >
> > Another option would be to use a system value instead of a uniform.  We
> can
> > then convert it to a uniform in the back-end.  We have a few places where
> > we do things like this for compute shaders.
> >
> > I don't have a lot of good ideas or strong opinions, I'm afraid. :-(
> Maybe
> > Ken has some?
> >
> > --Jason
>
> In GLSL IR, we use var->how_declared == ir_var_hidden to hide internally
> created variables from API introspection.  Why not just port something
> similar over to the newer NIR code?
>

I wasn't aware of that.  If we're going to do linking in NIR, something
like that is probably needed.

--Jason
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