On Tue, Sep 12, 2017 at 2:09 PM, Jason Ekstrand <ja...@jlekstrand.net> wrote: > On Tue, Sep 12, 2017 at 10:12 AM, Ian Romanick <i...@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 > > --Jason > > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/mesa-dev > _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
