[Mesa-dev] [RFC] NIR serialization

Tue Sep 12 18:09:24 UTC 2017

On Tue, Sep 12, 2017 at 10:12 AM, Ian Romanick <idr at freedesktop.org> wrote:

> On 09/11/2017 11:17 PM, Kenneth Graunke wrote:
> > On Monday, September 11, 2017 9:23:05 PM PDT Ian Romanick wrote:
> >> On 09/08/2017 01:59 AM, Kenneth Graunke wrote:
> >>> On Thursday, September 7, 2017 4:26:04 PM PDT Jordan Justen wrote:
> >>>> On 2017-09-06 14:12:41, Daniel Schürmann wrote:
> >>>>> Hello together!
> >>>>> Recently, we had a small discussion (off the list) about the NIR
> >>>>> serialization, which was previously discussed in [RFC] ARB_gl_spirv
> and
> >>>>> NIR backend for radeonsi.
> >>>>>
> >>>>> As this topic could be interesting to more people, I would like to
> >>>>> share, what was talked about so far (You might want to read from
> bottom up).
> >>>>>
> >>>>> TL;DR:
> >>>>> - NIR serialization is in demand for shader cache
> >>>>> - could be done either directly (NIR binary form) or via SPIR-V
> >>>>> - Ian et al. are working on GLSL IR -> SPIR-V transformation, which
> >>>>> could be adapted for a NIR -> SPIR-V pass
> >>>>> - in NIR representation, some type information is lost
> >>>>> - thus, a serialization via SPIR-V could NOT be a glslang alternative
> >>>>> (otoh, the GLSL IR->SPIR-V pass could), but only for spirv-opt (if
> the
> >>>>> output is valid SPIR-V)
> >>>>
> >>>> Ian,
> >>>>
> >>>> Tim was suggesting that we might look at serializing nir for the i965
> >>>> shader cache. Based on this email, it sounds like serialized nir would
> >>>> not be enough for the shader cache as some GLSL type info would be
> >>>> lost. It sounds like GLSL IR => SPIR-V would be good enough. Is that
> >>>> right?
> >>>>
> >>>> I don't think we have a strict requirement for the GLSL IR => SPIR-V
> >>>> path for GL 4.6, right? So, this is more of a 'nice-to-have'?
> >>>>
> >>>> I'm not sure we'd want to make i965 shader cache depend on a
> >>>> nice-to-have feature. (Unless we're pretty sure it'll be available
> >>>> soon.)
> >>>>
> >>>> But, it would be nice to not have to fallback to compiling the GLSL
> >>>> for i965 shader cache, so it would be worth waiting a little bit to be
> >>>> able to rely on a SPIR-V serialization of the GLSL IR.
> >>>>
> >>>> What do you suggest?
> >>>>
> >>>> -Jordan
> >>>
> >>> 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?
> >
> > In addition to the old pipeline:
> >
> > - GLSL from the app -> GLSL IR -> NIR -> i965 IR
> >
> > GL_ARB_spirv and OpenGL 4.6 add a second pipeline:
> >
> > - SPIR-V from the app -> NIR -> i965 IR
> >
> > Both of those absolutely have to work.  But these:
> >
> > - GLSL -> GLSL IR -> NIR -> SPIR-V -> NIR -> i965 IRs
> > - GLSL -> GLSL IR -> SPIR-V -> NIR -> i965 IRs
> >
> > aren't required to work, or even be supported.  It makes a lot of sense
> > to support them - both for testing purposes, and as an alternative to
> > glslang, for a broader tooling ecosystem.
> >
> > The thing that concerns me is that if you use SPIR-V for the cache, you
> > need these paths to not just work, but be _indistinguishable_ from one
> > another:
> >
> > - GLSL -> GLSL IR -> NIR -> ...
> > - GLSL -> GLSL IR -> NIR -> SPIR-V, then SPIR-V -> NIR -> ...
> >
> > Otherwise the original compile and partially-cached recompile might have
> > different properties.  For example, if the the SPIR-V step messes with
> > variables or instruction ordering a little, it could trip up the loop
> > unroller so the original compiler gets unrolled, and the recompile from
> > partial cache doesn't get unrolled.  I don't want to have to debug that.
>
> That is a very compelling argument.  If we want Mesa to be an
> alternative to glslang, I think we would like to have that property, but
> it's not a hard requirement for that use case.
>

I also find that argument rather compelling.  The SPIR-V -> NIR pass is
*not* a simple pass.  It does piles of lowering and things on-the-fly as
well as creating temporary variables for various things.  The best we could
hope to guarnatee would be that NIR -> SPIR-V -> NIR -> vars_to_ssa -> CSE
is idempotent.  Even that might be a bit of a stretch.

> > One could avoid this by making the original compile always go through
> > SPIR-V, and just drop glsl_to_nir altogether, so both take the same
> > paths.  But...it's kind of an unnecessary step in the common case...
>
> We may eventually partially do that, but that shouldn't block (any)
> other work.  In the short term it would likely add compile overhead that
> many would find unacceptable... by virtue of being non-zero.
>
> > Just serializing/reading back the NIR and resuming the compile from the
> > exact same IR would also solve that problem.
> >
> > Or, just being -really- careful with the translator, I guess...
> >
> >> 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.
> >
> > That is pretty nice.  I don't recall it being that painful, but, not
> > reinventing things is kind of nice too...
>
> Maybe the right answer is to share some things from SPIR-V (e.g., the
> way it describes I/O) to reduce duplication, but serialize NIR
> instructions and control flow in a "native" format.
>

I strongly suspect that there is some overestimation of how much code
nir_serialize would actually be here.  I just looked at nir_clone.c and
it's 781 lines.  It could probably drop by 50-100 LOC if we didn't expose
helpers for also cloning of single functions, variables, and constants.  I
would expect nir_serialize to be able to handle serialization and
deserialization in about 2x the code of nir_clone.c.  By comparison, SPIR-V
-> NIR is 8159 LOC and counting (that doesn't include generated anything)
and I would expect GLSL -> SPIR-V to be similarly sized.

--Jason
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