I don't understand this part:
> Meshlet has no clear 1:1 lane to vertex mapping, there’s no straightforward way to run a partial mesh shader wave for selected triangles. This is the main reason mobile GPU vendors haven’t been keen to adapt the desktop centric mesh shader API designed by Nvidia and AMD. Vertex shaders are still important for mobile.
I get that there's no mapping from vertex/triangle to tile until after the mesh shader runs. But even with vertex shaders there's also no mapping from vertex/triangle to tile until after the vertex shader runs. The binning of triangles to tiles has to happen after the vertex/mesh shader stage. So I don't understand why mesh shaders would be worse for mobile TBDR.
I guess this is suggesting that TBDR implementations split the vertex shader into two parts, one that runs before binning and only calculates positions, and one that runs after and computes everything else. I guess this could be done but it sounds crazy to me, probably duplicating most of the work. And if that's the case why isn't there an extension allowing applications to explicitly separate position and attribute calculations for better efficiency? (Maybe there is?)
Edit: I found docs on Intel's site about this. I think I understand now. https://www.intel.com/content/www/us/en/developer/articles/g...
Yes, you have to execute the vertex shader twice, which is extra work. But if your main constraint is memory bandwidth, not FLOPS, then I guess it can be better to throw away the entire output of the vertex shader except the position, rather than save all the output in memory and read it back later during rasterization. At rasterization time when the vertex shader is executed again, you only shade the triangles that actually went into your tile, and the vertex shader outputs stay in local cache and never hit main memory. And this doesn't work with mesh shaders because you can't pick a subset of the mesh's triangles to shade.
It does seem like there ought to be an extension to add separate position-only and attribute-only vertex shaders. But it wouldn't help the mesh shader situation.