[Mesa-dev] [RFC] NIR serialization

[Eero Tamminen]

Hi,

On 12.09.2017 09:55, Jordan Justen wrote:
> On 2017-09-11 21:44:32, Timothy Arceri wrote:
>> On 12/09/17 14:23, Ian Romanick wrote:
>>> On 09/08/2017 01:59 AM, Kenneth Graunke wrote:
>>>>
>>>> We shouldn't use SPIR-V for the shader cache.
>>>>
>>>> The compilation process for GLSL is: GLSL -> GLSL IR -> NIR -> i965 IRs.
>>>> Storing the content at one of those points, and later loading it and
>>>> resuming the normal compilation process from that point...that's totally
>>>> reasonable.
>>>>
>>>> Having a fallback for "some things in the cache but not all the variants
>>>> we needed" suddenly take a different compilation pipeline, i.e. SPIR-V
>>>> -> NIR -> ... seems risky.  It's a different compilation path that we
>>>> don't normally use.  And one you'd only hit in limited circumstances.
>>>> There's a lot of potential for really obscure bugs.
>>>
>>> Since we're going to expose exactly that path for GL_ARB_spirv / OpenGL
>>> 4.6, we'd better make sure it works always.  Right?
>>>
>>> One nice thing about SPIR-V is that all of the handling of uniform
>>> layouts, initial uniform values, attribute locations, etc. is already
>>> serialized.  If I'm not mistaken, that was one of the big pain points
>>> for all of the existing on-disk storage methods.  All of that has been
>>> sorted out for SPIR-V, and we have to make it work anyway.
>>
>> Correct these are the main issues for the fallback path, however this is
>> only used by i965 (exactly because an intermediate cache is missing).
>> Using SPIR-V as the intermediate cache means we still need to convert to
>> NIR and run all the opts, so I don't really see the advantage of caching
>> to SPIR-V over NIR.
> 
> For shader cache, hopefully we'll normally have the final program in
> the cache, which means the 're-run opt passes' is probably not a big
> factor. But, it still seems a fair point.
> 
> I think the biggest advantage of having either nir or spir-v would be
> not having to fallback to running the glsl compiler, right?

Shader cache is performance optimization aimed at reducing compile times.

When I earlier profiled it, 2/3 of the shader compilation work is done 
at linking stage.

Caching done at higher level than the linked binary shaders, may be of 
questionable performance value, because the caching itself has also some 
cost (extra code, disk access cache lookups etc).

Some profiling showing that doing caching at NIR/SPIRV level could 
measurable help performance (compared to caching overhead), would be 
appreciated.

However, if the main purpose of caching NIR/SPIRV is something else than 
performance[1], then that's a different matter.  It just should be clear 
what is the aim of this change.


> Maybe we'll see which becomes available first? :)


	- Eero

[1] Such as:
* better code validation, or
* ability to replace / tweak the cached files to manually test impact of 
compiler optimizations before implementing them