[Mesa-dev] primitive id handling in mesa (and state tracker)

[Roland Scheidegger]

So, primitive id in llvmpipe doesn't really work, or rather it works but
it can't do much useful with the way mesa/st sets this up at least for
some tests. In particular all of piglit
glsl-1.50-geometry-primitive-id-restart fail (result is always 0).
We actually have both system value prim id and ordinary input prim id
(both using TGSI_SEMANTIC_PRIMID) in gallium. There's some halfway
support for both, but in particular ordinary input prim id in gs will
not work in draw/gallivm at all. This is both due to tgsi_scan_shader()
not recognizing it in gs (only in fs), nor is it wired up in other places.
I actually made the test work in llvmpipe by some horrendous hacks (fix
the mentioned tgsi_scan_shader() issue, and hack up the gs_input
fetching to just fetch the associated system value instead), but that's
not a proper fix.
So I am wondering, why isn't this a system value in mesa in the first
place, just like vertex ids, invocation ids and the like? Looks like it
really is such a value to me.
Somehow I guess we should avoid having that as both system value and
ordinary input in gallium...

Roland