[Mesa-dev] [RFC] ARB_gl_spirv and NIR backend for radeonsi

Tue May 23 01:45:17 UTC 2017

On Mon, May 22, 2017 at 9:33 PM, Rob Clark <robdclark at gmail.com> wrote:
> On Mon, May 22, 2017 at 9:15 PM, Timothy Arceri <tarceri at itsqueeze.com> wrote:
>> On 23/05/17 10:44, Marek Olšák wrote:
>>>
>>> On Tue, May 23, 2017 at 12:07 AM, Timothy Arceri <tarceri at itsqueeze.com>
>>> wrote:
>>>>
>>>> On 23/05/17 05:02, Marek Olšák wrote:
>>>>>
>>>>>
>>>>> On Sun, May 21, 2017 at 12:48 PM, Nicolai Hähnle <nhaehnle at 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 :)
>>
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