[Mesa-dev] [PATCH 00/11] radeonsi: shader buffer support (atomic counters, ssbo)

[Nicolai Hähnle]

Hi,

since shader images have laid most of the foundation, here are shader buffers
now. This is the last extension missing for OpenGL 4.2 (we still need to turn
on GLSL 4.2, but I think that only involves flipping a bit).

As with shader images, this extension needs bleeding edge LLVM - this time,
important patches have not landed upstream yet, and if you want to try this
code you'll need my LLVM branch at https://cgit.freedesktop.org/~nh/llvm/log/?h=images

(For those following along at home, the necessary LLVM patches for shader
images have already landed upstream.)

In principle, there are two alternative implementations for shader buffers:
using LLVM IR  pointers with LLVM-native load/store instructions directly, or
using intrinsics that operate on GCN buffer descriptors. This implementation
uses the second approach. A brief comparison between the two approaches:

1. The pointer approach would use FLAT memory instructions on CI+, which
   operate on 64 bit pointers rather than 128 bit buffer descriptors. This
   would reduce SGPR memory pressure slightly.

2. LLVM understands pointers for alias analysis, so it's possible that it
   would generate somewhat better code if we were to use pointers in the
   IR.

3. The buffer load/store intructions have built-in bounds checks. Bounds
   checks are required for an honest implementation of the ARB_robustness
   extension, which we claim to support.

The last point makes it obvious that the implementation really needs to use
buffer intrinsics, but it'd be interesting to know how big the difference
in code quality is versus something that uses pointers. To get the best of
both worlds, we should really find a way to teach LLVM's alias analysis
about what those buffer descriptors mean. For now, this current approach is
the right way to do it.

Please review!

Thanks,
Nicolai