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

[Iago Toral]

On Thu, 2015-10-01 at 02:30 -0400, Ilia Mirkin wrote:
> On Thu, Oct 1, 2015 at 2:24 AM, Iago Toral <itoral at igalia.com> wrote:
> > On Thu, 2015-10-01 at 02:18 -0400, Ilia Mirkin wrote:
> >> On Thu, Oct 1, 2015 at 2:12 AM, Iago Toral <itoral at igalia.com> wrote:
> >> > However, I think this can be a problem in your case, because you can't
> >> > remap the block index if you don't know how many blocks in UniformBlocks
> >> > before the one you are processing are of a different type (i.e. UBOs if
> >> > this is an SSBO or the other way around). And you cannot know how many
> >> > blocks you have to count because the index into the array instance
> >> > blocks is not constant... We can probably fix this by grouping UBOs and
> >> > SSBOS together in the array right before we flow into the backends.
> >>
> >> Slightly annoying but non-fatal. I think that just the remapping table
> >> is enough -- the indexing is always done relative to a base index, and
> >> as long as these arrays are contiguous (which they kinda have to be),
> >> it shouldn't matter what the offset is. i.e. if the list contains u0
> >> u1 s0 s1 u2 u3, and i want to index on u2/3, as long as i know that u2
> >> is the base, I can use its index.
> >
> > But they are not contiguous, that's why I say that we would need to
> > group them. In shader code you can in theory have something like:
> >
> > layout(std140, binding=2) buffer SSBO1 {
> >    vec4 v0;
> >    vec4 v1;
> > } ssbo1[3];
> >
> > layout(std140, binding=3) uniform UBO {
> >    vec4 v0;
> >    vec4 v1;
> > } ubo[2];
> >
> > layout(std140, binding=5) buffer SSBO2 {
> >    vec4 v0;
> >    vec4 v1;
> > } ssbo2[2];
> >
> > and we would add them in that order to the list, which I think would be
> > a problem for you in the case you mention.
> 
> Wouldn't ubo[0] and ubo[1] get sequential entries in that list? So
> when accessing ubo[n], if I look up the remapping of ubo[0] in that
> table, I should be sure that ubo[1] is adjacent to it, no?

Ah, you meant to have a remap table for the start of each interface
block... yeah, I guess that would work, blocks in each array will be
contiguous in UniformBlocks.

Iago

> 
>   -ilia
>