[Mesa-dev] Questions about arrays in GLSL 1.50 interface blocks

[Ian Romanick]

On 08/02/2013 07:52 PM, Paul Berry wrote:
> 1. Is it ok to declare an unsized array in an interface block?  In other
> words, is this ok?
>
>      out blk {
>          vec4 foo[];
>      };
>
> The nVidia Linux driver allows this, but Mesa currently doesn't.  AFAICT
> the spec says it's ok.  In fact, the spec defines the built-in interface
> block gl_PerVertex this way:
>
>      out gl_PerVertex {
>          vec4 gl_Position;
>          float gl_PointSize;
>          float gl_ClipDistance[];
>      };
>
> So it seems like it ought to be allowed for user-defined interface
> blocks too.
>
> 2. If you declare an unsized array in an interface block, is it legal to
> redeclare that interface block later and specify an array size?  E.g.:
>
>      out blk {
>          vec4 foo[];
>      };
>      out blk {
>          vec4 foo[2];
>      };
>
> I can't find any text in the spec saying whether this is allowed, but it
> seems reasonable that if the answer to question 1 is yes, then this
> should be allowed too, since it's parallel to what's allowed outside of
> interface blocks.
>
> 3. How about redeclaring a named interface block and specifying a size?
>
>      out blk {
>          vec4 foo[];
>      } ifc_name;
>      out blk {
>          vec4 foo[2];
>      } ifc_name;
>
> It seems to me that this should have the same answer as question 2, but
> nVidia disallows this (it gives a compile error: declaration of
> "ifc_name" conflicts with previous declaration at ...).
>
> 4. How about if you redeclare an interface block that has some arrays
> and some non-arrays?  E.g.:
>
>      out blk {
>          vec4 foo[];
>          vec4 bar;
>      };
>      out blk {
>          vec4 foo[2];
>          vec4 bar;
>      };
>
> It seems to me that this should also have the same answer as question 2,
> but nVidia rejects this with a compile error too (declaration of "bar"
> conflicts with previous declaration at ...).
>
> 5. How about if you redeclare an interface block and give it different
> members?  E.g.:
>
>      out blk {
>          vec4 foo[];
>          vec4 bar;
>      };
>      out blk {
>          vec3 baz;
>      };
>
> This seems like it should definitely give an error, since the spec says
> "Matched block names within an interface ... must match in terms of
> having the same number of declarations with the same sequence of types
> and the same sequence of member names, as well as having the same
> member-wise layout qualification ...".
>
> But the nVidia driver allows this.
>
>
> I'm baffled as to what behaviour I should implement in Mesa.  At the
> very least I'm pretty sure I have to allow gl_PerVertex to be
> redeclared, since (a) that's the only way to give the gl_ClipDistance
> input a size in a geometry shader, and (b) redeclaring gl_PerVertex is
> explicitly permitted by GLSL 4.10 section 7.1.1 (and I'm pretty sure the
> text there is meant as a clarification, rather than a new rule).
>
> But for anything beyond that, the spec seems open to interpretation.
> And I don't feel like I can use nVidia's implementation for guidance,
> because of the inconsistencies and non-compliant behaviour I noted above.

I definitely think NVIDIA has some bugs. :)

I spent most of the train ride home last night digging through the GLSL 
specs and trying to remember any relevant Khronos discussions.  I could 
only find one bit of text that even hinted at anything.  The bottom of 
page 50 (page 56 of the PDF) of the GLSL 4.40 spec says:

     "A block name is allowed to have different definitions in
     different interfaces within the same shader, allowing, for
     example, an input block and output block to have the same
     name."

That /suggests/ that you can't redeclare a block for the same interface, 
but I don't feel very comfortable relying on that.  I'll submit a bug.

> Any thoughts?
>
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