[llvm-dev] [RFC] Upstreaming a proper SPIR-V backend

Tue Mar 2 15:18:26 PST 2021

On Tue, Mar 2, 2021 at 4:40 PM Mehdi AMINI via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Tue, Mar 2, 2021 at 3:07 AM Trifunovic, Konrad via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>>
>> A very good question. I was actually expecting it 😊
>>
>> So, at the moment, it does not integrate into MLIR SPIRV backend and we
>> have not thought about it. I guess You are referring to having a SPV
>> dialect in MLIR and using a 'serialize' option to produce a SPIR-V binary?
>>
>> I agree that developing two backends in parallel is a bit redundant. If
>> SPIR-V LLVM backend becomes a production quality it means actually it could
>> consume any LLVM IR (provided it does conform to some SPIR-V restrictions).
>> By any LLVM IR input I mean: it should be irrelevant whether it is
>> produced by a clang, MLIR to LLVM IR lowering or just some other front-end
>> that produces LLVM IR.
>
> The biggest 'impedance mismatch' that I currently see is that SPV MLIR
>> dialect is now targeted mostly at Vulkan, while LLVM SPIR-V backend targets
>> compute. Besides instruction set, the fundamental difference is a memory
>> model.
>> So if we want to unify those, we should actually make SPIR-V LLVM backend
>> able to produce Vulkan dialect of SPIR-V as well.
>>
>> My answer is a bit elusive, but I totally agree with Your proposal: we
>> should work towards having a one solution, and, LLVM SPIR-V backend seems
>> like a more universal one (since it sits lower in the compiler stack).
>> My proposal would be to include some MLIR -> LLVM-IR translated code in
>> the testing so to have this final goal in mind.
>>
>
> Something you're missing here, and maybe Lei clarified but I'll reiterate:
> the SPIRV dialect in MLIR is equivalent to what your GlobalISel pass will
> produce. It can actually round-trip to/from the SPIRV binary format. So it
> is sitting lower than your backend in my view.
> I can't figure out a situation where it would make sense to go from MLIR
> SPIRV dialect to LLVM to use this new backend, but I may miss something
> here...
>
> It would be really great to find a common path here before duplicating a
> lot of the same thing in the lllvm-project monorepo, for example being able
> to target the MLIR dialect from GlobalISel, or alternatively converting the
> MIR to it right after would be an interesting thing to explore.
> I haven't seen it, but there was a talk last Sunday on this topic:
> https://llvm.org/devmtg/2021-02-28/#vm1
>

This sort of problem seems like just one of those unfortunate consequences
of MLIR being effectively an "LLVM IR 2.0 -- Generic Edition", but not yet
actually layered underneath LLVM where it really wants to be. I think it
doesn't really make sense to tie *this* project to those long-term goals of
layering MLIR under LLVM-IR, given the extremely long timescale that is
likely to occur in. The "proper" solution probably won't be possible any
time soon.

So, in the meantime, we could implement a special-case hack just for SPIRV,
to enable lowering it to MLIR-SPIRV dialect. But, what's the purpose? It
wouldn't really help move towards the longer term goal, I don't think? And
if someone does need that at the moment, they can just feed the SPIRV
binary format back into the existing MLIR SPIRV dialect, right?

PS: one more thought: SPIR-V does come with a set of builtin/intrinsic
>> functions that expose the full capabilities of target architecture (mostly
>> GPU). This set of intrinsics is actually a dialect in its own. So this is
>> LLVM IR + SPIR-V specific intrinsics and their semantics that fully define
>> the SPIR-V dialect at LLVM IR level. I believe this idea could be used in
>> MLIR path: MLIR -> LLVM-IR with SPIR-V intrinsics (let's call it a LLVM IR
>> SPIR-V dialect) -> SPIR-V binary (generated by a backend). So the idea of
>> 'SPIR-V dialect' still exists, it is just now expressed at the LLVM IR
>> level.
>
>
>> regards,
>> konrad
>>
>> > From: Renato Golin <rengolin at gmail.com>
>> > Sent: Tuesday, March 2, 2021 11:12 AM
>> > To: Trifunovic, Konrad <konrad.trifunovic at intel.com>
>> > Cc: llvm-dev at lists.llvm.org; Paszkowski, Michal <
>> michal.paszkowski at intel.com>; Bezzubikov, Aleksandr <
>> aleksandr.bezzubikov at intel.com>; Tretyakov, Andrey1 <
>> andrey1.tretyakov at intel.com>
>> > Subject: Re: [llvm-dev] [RFC] Upstreaming a proper SPIR-V backend
>> >
>> > On Tue, 2 Mar 2021 at 09:36, Trifunovic, Konrad via llvm-dev <mailto:
>> llvm-dev at lists.llvm.org> wrote:
>> > Hi all,
>> >
>> > We would like to propose this RFC for upstreaming a proper SPIR-V
>> backend to LLVM:
>> >
>> > Hi,
>> >
>> > Perhaps a parallel question: how does that integrate with MLIR's SPIRV
>> back-end?
>> >
>> > If this proposal goes through and we have a production-quality SPIRV
>> back-end in LLVM, do we remove MLIR's own version and lower to LLVM, then
>> to SPIRV? Or do we still need the MLIR version?
>> >
>> > In a perfect world, translating to LLVM IR then to SPIRV shouldn't make
>> a difference, but there could be some impedance mismatch between MLIR->LLVM
>> lowering that isn't compatible with SPIRV?
>> >
>> > But as a final goal, if SPIRV becomes an official LLVM target, it would
>> be better if we could iron out the impedance problems and keep only one
>> SPIRV backend.
>> >
>> > cheers,
>> > --renato
>> >
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