On Mon, May 22, 2017 at 9:33 PM, Rob Clark <robdcl...@gmail.com> wrote: > On Mon, May 22, 2017 at 9:15 PM, Timothy Arceri <tarc...@itsqueeze.com> wrote: >> On 23/05/17 10:44, Marek Olšák wrote: >>> >>> On Tue, May 23, 2017 at 12:07 AM, Timothy Arceri <tarc...@itsqueeze.com> >>> wrote: >>>> >>>> On 23/05/17 05:02, Marek Olšák wrote: >>>>> >>>>> >>>>> On Sun, May 21, 2017 at 12:48 PM, Nicolai Hähnle <nhaeh...@gmail.com> >>>>> 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 > BR, > -R > >> IMO 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 >> mesa-dev@lists.freedesktop.org >> https://lists.freedesktop.org/mailman/listinfo/mesa-dev _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
