On 23/05/17 11:45, Rob Clark wrote:
On Mon, May 22, 2017 at 9:33 PM, Rob Clark <[email protected]> wrote:On Mon, May 22, 2017 at 9:15 PM, Timothy Arceri <[email protected]> wrote:On 23/05/17 10:44, Marek Olšák wrote:On Tue, May 23, 2017 at 12:07 AM, Timothy Arceri <[email protected]> wrote:On 23/05/17 05:02, Marek Olšák wrote:On Sun, May 21, 2017 at 12:48 PM, Nicolai Hähnle <[email protected]> wrote:Hi all, I've been looking into ARB_gl_spirv for radeonsi. I don't fancy re-inventing the ~8k LOC of src/compiler/spirv, and there's already a perfectly fine SPIR-V -> NIR -> LLVM compiler pipeline in radv, so I looked into re-using that. It's not entirely straightforward because radeonsi and radv use different "ABIs" for their shaders, i.e. prolog/epilog shader parts, different user SGPR allocations, descriptor loads work differently (obviously), and so on. Still, it's possible to separate the ABI from the meat of the NIR -> LLVM translation. So here goes... The Step-by-Step Plan ===================== 1. Add an optional GLSL-to-NIR path (controlled by R600_DEBUG=nir) for very simple VS-PS pipelines. 2. Add GL_ARB_gl_spirv support to Mesa and test it on simple VS-PS pipelines. 3. Fill in all the rest: 3a. GL 4.x shader extensions (SSBOs, images, atomics, ...) 3b. Geometry and tessellation shaders 3c. Compute shaders 3d. Tests I've started with step 1 and got basic GLSL 1.30-level vertex shaders working via NIR. The code is here: https://cgit.freedesktop.org/~nh/mesa/log/?h=nir The basic approach is to introduce `struct ac_shader_abi' to capture the differences between radeonsi and radv. In the end, the entry point for NIR -> LLVM translation will simply be: void ac_nir_translate(struct ac_llvm_context *ac, struct ac_shader_abi *abi, struct nir_shader *nir); Setting up the LLVM function with its parameters is still considered part of the driver.This sounds good.Questions ========= 1. How do we get good test coverage? ------------------------------------ A natural candidate would be to add a SPIR-V execution mode for the piglit shader_runner. That is, use build scripts to extract shaders from shader_test files and feed them through glslang to get spv files, and then load those from shader_runner if a `-spirv' flag is passed on the command line. This immediately runs into the difficulty that GL_ARB_gl_spirv wants SSO linking semantics, and I'm pretty sure the majority of shader_test files don't support that -- if only because they don't set a location on the fragment shader color output. Some ideas: 1. Add a GL_MESA_spirv_link_by_name extension 2. Have glslang add the locations for us (probably difficult because glslang seems to be focused on one shader stage at a time.) 3. Hack something together in the shader_test-to-spv build scripts via regular expressions (and now we have two problems? :-) ) 4. Other ideas?We have plenty of GLSL SSO shader tests in shader-db, but we can only compile-test them. Initially I think we can convert a few shader tests to SSO manually and use those.2. What's the Gallium interface? -------------------------------- Specifically, does it pass SPIR-V or NIR? I'm leaning towards NIR, because then specialization, mapping of uniform locations, atomics, etc. can be done entirely in st/mesa. On the other hand, Pierre Moreau's work passes SPIR-V directly. On the third hand, it wouldn't be the first time that clover does things differently.If you passed SPIR-V to radeonsi and let radeonsi do SPIR-V -> NIR -> LLVM, you wouldn't need the serialization capability in NIR. You can just use SPIR-V as the shader binary and the major NIR disadvantage is gone. Also, you won't have to touch GLSL-to-NIR, and the radeonsi shader cache will continue working as-is. However, I don't know how much GL awareness is required for doing SPIR-V -> NIR in radeonsi. Additional GL-specific information might have to be added to SPIR-V by st/mesa for the conversion to be doable. You probably know better. st/mesa or core Mesa just needs to fill gl_program, gl_shader, and gl_shader_program by parsing SPIR-V and not relying on NIR. I don't know how feasible that is, but it seems to be the only thing needed in shared code. That also answers the NIR vs TGSI debate for the shader cache. The answer is: Neither.Just to list some downsides to this approach, not switching the the GLSL path to also use NIR has the following negatives: 1. We don't get to leverage the large GL test suits and app ecosystem for testing the NIR -> LLVM pass both during development and afterwards. 2. Jason has already said it best so to quote his reply: "There have been a variety of different discussions over the last few years about compiler design choices but we've lacked the ability to get any good apples-to-apples comparisons. This may provide some opportunities to do so." 3. The GLSL IR opts are both slow and not always optimal (possibly transforming the code to something that's harder to opt later), but due to uniform/varying optimisation requirements some optimisations are required *before* we can do validation. With NIR we have an opportunity to do these optimisations in NIR either by building a nir based linker for the final linking stage (uniform/varying validation/location assignment) or by a little bit of back and forth of information between NIR and GLSL IR. This is something that can't really be done with LLVM/Gallium. I was working towards this while at Collabora. 4. We don't get to drop the glsl_to_tgsi pass which is not the most maintenance friendly piece of code. Also currently 10% of cpu is spent in the slow tgsi optimisations during start-up of Deus EX which equals around 50 seconds on my machine. Most of this optimisation is clean-up simply due to how sloppy the glsl_to_tgsi pass is. 5. It's probably arguable but using GLSL -> NIR should result in more shared code paths both between radeonsi/radv and the drivers for other gpus anv/freedreno/vc4. Anyway just a few things to think about.Using GLSL -> NIR for radeonsi won't really change the GLSL linker situation, because there are 12 other drivers consuming only TGSI.Ignoring the software drivers Nouveau is the only one in active development though right?on x86, probably.. although etnaviv hasn't switched to nir and is seeing active development. (And, well, there are a couple arm/SoC gpu's that still need a driver, although I guess if a foss mali driver happens it is a good candidate to skip straight to nir)I guess it's OK to switch only radeonsi to NIR if it improves compile times, but we also have the shader cache, so I don't know if it's worth it just for the faster compilation that takes place only on the first run. It's very hard to justify the massive development effort here.Rob seemed to think wiring up geom/tess support for glsl_to_nir should be straightforward.I guess shader-stage aspect of it should be dead simple.. shader-feature aspect *might* be more involved (but really only to the extent that mesa/st does clever things.. I probably should have split up the patch that added mesa/st glsl->nir support for compute shaders from the part that added ssbo's and atomic counters, because of the way mesa/st lowers atomic counters to ssbo..). Not sure if mesa/st does anything clever with images (like it does w/ lowering atomic counters to atomic ops to ssbo's).. if not that should require nothing additional. Anyways, if there is anything I can do to help on the plumbing nir through mesa/st end of things, let me know.. I'll need it eventually for freedreno. But there are some things I haven't looked at yet just because of features I haven't r/e'd and implemented yet.btw, random thought, but if serializing/deserializing nir is going to be needed for on-disk shader cache (something I haven't had time to look at yet), maybe I should add that near the top of the todo-pile? AFAIU freedreno and vc4 will want that, and if it turns out to be useful for radeonsi too down the road, then bonus-points.. BR, -R
Yep it would also simplify the i965 on-disk cache as we wouldn't need to mess around with the fallback path.
If we can serialise NIR it should be very easy to add a NIR level cache for vc4/freedreno just like r600/nouveau only have a TGSI level cache currently.
BR, -RIMO it would be interesting to be able to play around with the various NIR optimsation passes in conjunction with LLVM and shader-db it could, be useful for comparisons and identifying weaknesses in both compilers. Anyway there is value in either approach I just thought I'd throw some counterpoints out there :) _______________________________________________ mesa-dev mailing list [email protected] https://lists.freedesktop.org/mailman/listinfo/mesa-dev
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