[Mesa-dev] SSBO's in UniformBlocks list?

[Ilia Mirkin]

On Wed, Sep 30, 2015 at 3:18 AM, Iago Toral <itoral at igalia.com> wrote:
> On Wed, 2015-09-30 at 02:34 -0400, Ilia Mirkin wrote:
>> On Wed, Sep 30, 2015 at 2:26 AM, Iago Toral <itoral at igalia.com> wrote:
>> > On Tue, 2015-09-29 at 11:19 -0400, Ilia Mirkin wrote:
>> >> On Tue, Sep 29, 2015 at 4:33 AM, Iago Toral <itoral at igalia.com> wrote:
>> >> > Hi ilia,
>> >> >
>> >> > On Tue, 2015-09-29 at 03:53 -0400, Ilia Mirkin wrote:
>> >> >> Hi Samuel, and any other onlookers,
>> >> >>
>> >> >> I was wondering why the decision was made to stick SSBO's onto the
>> >> >> same list as constbufs. Seems like they're entirely separate entities,
>> >> >> no? Perhaps I'm missing something?
>> >> >
>> >> > The reason for this was that there is a lot of code in the compiler to
>> >> > handle uniform blocks and all the rules for them and we needed the same
>> >> > treatment for SSBOs, so that seemed like a reasonable way forward to
>> >> > reuse a lot of the code in the compiler front end. I think the only
>> >> > place where we needed to make explicit distinctions is when we check for
>> >> > resource limits, since these are different for UBOs and SSBOs of course.
>> >> > Although UBOs and SSBOs are separate entities they have a lot of
>> >> > similarities too, so that did not look like a terrible idea, considering
>> >> > the benefits.
>> >>
>> >> My concern is around indexing... now the per-stage indices are in the
>> >> combined UBO/SSBO space -- how do I tease out the individual ones?
>> >> Easy enough when you can loop over NumUniformBlocks and just count the
>> >> right type, but what about in the shader, where I get the buffer index
>> >> in a ir_rvalue?
>
> By the way, in i965 this is not a problem either, we have access to the
> gl_shader struct from the compiler backend, so if we need to translate
> from the shared index space to a separate space we have NumUniformBlocks
> available to do that. From your words I get that you can't access this
> information from the compiler backend, right? In that case, wouldn't it
> be possible to translate the index during the GLSL IR -> TGSI
> conversion?

Actually a shader should be available at GLSL -> TGSI translation time
as well. Somehow I didn't connect that in my mind with the relevant
info being available. So I guess this moves from "impossible" to
"annoying", since we have to do a list scan and fix up all the UBO
logic as well. Probably easiest to just compute a remapping table and
stick it somewhere that will last at link time. I'm wondering if this
will affect indirect array accesses (ARB_gs5 ubo indexing -- does that
exist on ssbo btw?), but I think that should be fine.

  -ilia