On Thu, Sep 14, 2017 at 12:40 PM, Connor Abbott <[email protected]> wrote:
> On Tue, Sep 12, 2017 at 2:09 PM, Jason Ekstrand <[email protected]> > wrote: > > On Tue, Sep 12, 2017 at 10:12 AM, Ian Romanick <[email protected]> > 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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