[phire]

The only step that SPIR-V replaces is parsing the GLSL to an AST tree, and that's only a small part of the total time to compile a shader. Usually the bottleneck is Register allocation or scheduling.

Back when Vulkan was developed, there were a bunch of OpenGL drivers out there which had random AST parsing bugs (Dolphin even has a bunch of workarounds for them); So a large chunk of the motivation for SPIR-V was avoiding the need for every driver to implement their own GLSL parser and the associated bugs.

The problem for Dolphin is not the complexity of the shader, but the quantity.

Shaders in modern games are usually written manually (or authored in a shader node editor by an artist), so it's rare for a game to have more than a few thousand total. Better games might only have a few dozen for the entire game.

But because gamecube/Wii games configure the TEV pixel pipeline though a dynamic API, some games use that API in a pattern where Dolphin can find itself generating hundreds of shaders per second. Some games even manage to generate new shaders continually as you play, because they append junk state to their pixel pixeline state which dolphin doesn't detect as a duplicate.

[Jasper_]

Shaving off the frontend costs is not going to be nothing. I don't know if Dolphin is still using FXC/D3DCompile or if they've switched to DXC, but FXC is infamously slow, even for very simple shaders. Dolphin's shaders are medium-complexity IIRC, so I'd expect removing the frontend to be a decent win.

The driver PSO compilers aren't amazing but they're also not terrible. Most games do some form of hash-n-cache for PSO compilation and while stutters are still an issue, it's not the worst in the world. With the frontend gone, I'd expect ~50 shaders per second to be roughly stutter-free.

Being smarter about specialization is probably a good idea -- having a blend between "GPU interpreter" and "full specialized pipeline" is where I think you should head. Several of the weirder TEV features could probably be moved to branching on dynamic buffer contents.

Not to mention using newer features like bindless to merge draw calls. I always wanted to do that but got too busy before I stopped working on Dolphin :)

[phire]

> so I'd expect removing the frontend to be a decent win

I did some testing before working on ubershaders, and my modified build which cached the bytecode output of FXC/D3DCompile (whatever dolphin was using at the time) didn't reduce the stuttering by enough to be worth the effort of optimising the frontend.

My conclusion was that it's simply wasn't worth any effort to optimise for slightly smaller stutters, as they were still very perceivable to users. And Hybrid Ubershaders can hide any compile delays without any issue.

And this testing was with FXC/D3DCompile which does a bunch of optimisations. The fact that SPIR-V comes in (potentially) unoptimised means any vulkan compiler has to send it though all optimisation passes. Though I have been very tempted to do dead code removal before submitting the shaders, partly to make the shaders more readable to humans and partly to reduce the amount of code going though the various compiler passes.

> Being smarter about specialization is probably a good idea -- having a blend between "GPU interpreter" and "full specialized pipeline" is where I think you should head.

Yeah, that was always next on the list. Start with just ubershaders and then incrementally specialise on a background thread for the correct balance of shaders.

Dolphin's current specialised shaders are no-where near fully specialized. Need to go further by baking some of the constants and lookup textures into the shader.