[Mesa-dev] [RFC PATCH 03/26] glsl: introduce new base types for bindless samplers/images

[Ilia Mirkin]

On Wed, Apr 19, 2017 at 5:47 AM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> By the way, this is also how Ilia's example would be implemented. When
> inlined, it could become something like:
>
>   layout (bound_sampler) uniform sampler2D u_tex;
>   in sampler2D in_tex;
>
>   void main()
>   {
>     ...
>     sampler2D tmp = foo ? u_tex : in_tex;
>     vec4 color = texture2D(tmp, ...);
>   }
>
> This should compile. At the GLSL level, the type is always sampler2D. At the
> TGSI and LLVM IR level, u_tex is converted to a 64-bit value at the point of
> the ?: operation. To keep the craziness low in TGSI, there should probably
> be some S2U64 instruction for explicitly converting old-style
> samplers/images to 64-bit values, even though most cases of explicit
> conversion between sampler types and uvec2 are actually no-ops in TGSI.

OK, so I think this is an important question. My understanding is that
this shouldn't compile. Furthermore, my understanding is that

layout (bound_sampler) uniform sampler2D a;

void main() {
  sampler2D tmp = a;
}

should also not compile. We should try to get a ruling on that from
the existing impls. If that is not the case, then this needs a
rethink, at least on my part :) It would force all the textures to
become bindless when that happens. This would obviously be supportable
on NVIDIA (it's all bindless), but there are two instructions for
loading stuff from textures, one which is "semi-bound", one which is
fully "indirect". I'm not sure why those two would exist if there
weren't some advantage to using the "semi-bound" one when possible.
[What I mean by semi-bound is that it reads the handle from a
dedicated constbuf, you pass in the offset into that constbuf. The
fully indirect one, you just pass in the handle to the op directly.]

Cheers,

  -ilia