[Mesa-dev] [PATCH 4/5] glsl: Optionally lower TCS gl_PatchVerticesIn to a uniform.

[Kenneth Graunke]

On Wednesday, June 8, 2016 7:45:43 PM PDT Alejandro Piñeiro wrote:
> On 02/06/16 23:09, Kenneth Graunke wrote:
> > i965 has no special hardware for this, so the best way to implement
> > this is to pass it in via a uniform.
> 
> Is this comment accurate? On the next patch you implement the i965 bits,
> and you only do the lowering on gen8+, mentioning that you would like to
> do the same on gen7. So for gen7 it is implemented, but an uniform is
> not used. Or am I missing something?

i965 hardware doesn't provide gl_PatchVerticesIn in the TCS thread
payload, as one might expect.  Currently, we implement it as a
compile-time constant, and recompile the shader whenever it changes
(on all generations).

Another way to implement it is to pass it in as a uniform.  This is
arguably better because it doesn't require recompiling the shader
whenever you change the number of vertices in your input patch.

The next patch makes it use a uniform and stop recompiling on Gen8+.

However, on Gen7, we also have to loop over all the input vertices
and emit a URB write message (input release) for every pair.  Because
we know this at compile time, it's easy to emit the right number of
URB write messages.  So I left it as is for now (still recompiling).

I could instead rework that to use an actual DO..WHILE loop in the
shader itself, and then a uniform would work.
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