On Mon, May 22, 2017 at 6:45 PM, Rob Clark <robdcl...@gmail.com> wrote:
> 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.. > Serializing NIR would also potentially be useful for us for caching purposes. We could probably get quite a bit of benefit today if we just cached the optimized but not yet 100% lowered NIR.
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