[Mesa-dev] [RFC] NIR serialization

Fri Sep 15 00:58:14 UTC 2017

On Thu, Sep 14, 2017 at 12:40 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:

> On Tue, Sep 12, 2017 at 2:09 PM, Jason Ekstrand <jason at jlekstrand.net>
> wrote:
> > 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.
>
> I just finished typing up a nir_serialize implementation, although I
> haven't debugged it yet. It wound up being 1150 lines, including
> comments and whatnot:
> https://cgit.freedesktop.org/~cwabbott0/mesa/commit/?h=nir-serialize&id=
> 2bacd646460328940c5021d1bdaced09a45ed947
>

Taking the lock... I've got a test harness written for it and am working on
fixing some of the bugs. :-)
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