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

Tue Mar 2 11:02:05 PST 2021

Chiming in mostly from the perspective of MLIR SPIR-V support. More
comments inlined, but first some general comments. :)

As I understand it, SPIR-V is actually a mix of multiple things. It is
first and foremost 1) a binary format for encoding GPU executables that
cross the toolchain and hardware driver boundaries. Then it's 2) an
intermediate level language for expressing such GPU executables. It is also
3) a flexible and extensible spec with all sorts of capability and
extension mechanisms in order to support the needs of multiple APIs and
hardware features. It's unclear to me what a production-quality SPIR-V LLVM
backend would entail; but to actually support various use cases SPIR-V can
support (OpenCL, OpenGL, Vulkan; shader/kernel; various levels of
extensions; etc.), it looks to me that we need a story for all the above
points, where the IR aspect (2) is actually just facet. My understanding
over LLVM is it's mostly focusing on 2): we have a very coherent single IR
threading through the majority layers of the compiler stack and the IR
focuses very much as a means for compiler transformations (i.e., no
instruction versioning etc.). There isn't much native modelling for most
points for 1) and 3) (which makes sense as LLVM IR is a compiler IR). So to
make it work, one would need to shorehore through existing LLVM mechanisms
(e.g., using intrinsics for various GPU related builtins, using metadata
for SPIR-V decorations?, etc.), unless we want to evolve LLVM
infrastructure to have native support for the missing SPIR-V mechanisms,
which I think might be too much to take on. This is just general
mechanisms, not mentioning the different semantics between different SPIR-V
consumers (e.g., shader vs. kernel and what that means over
memory/execution model, etc.) that needs to be sorted out too.. Just
supporting a certain use case of what SPIR-V supports is certainly simpler
though as we can bake in assumptions and avoid some infrastructure needs
for the full generality.

That's why I think using MLIR as the infrastructure to build general
support for SPIR-V is more preferable as we control everything there and
can feel free to model all SPIR-V concepts in the most native way. For
example we can feel free to define all SPIR-V ops natively, including all
ops introduced by SPIR-V extensions and extended instruction sets. We can
support versions/extensions/capabilities natively and integrate it with the
target environment to automatically filter out
<https://github.com/llvm/llvm-project/blob/37eca08e5bcfbe926176412a4a0acf5b963da7e6/mlir/lib/Dialect/SPIRV/Transforms/SPIRVConversion.cpp#L690-L755>
CodeGen patterns generating ops not available on the target, etc. To me,
MLIR's open dialect/op/type/etc. system is a perfect fit for the open
SPIR-V spec with many capabilities/extensions/etc. For example we can even
make the SPIR-V dialect itself open to allow out-of-tree extensions and
development and such.

With that said, I understand that software development has many reality
concerns (like existing codebase, familiarity with different components,
etc.) and we have many different use cases, which may mean that different
paths make sense. So please don't take this as a negative feedback in
general. It's just that to me it's unclear how we can unify here right now.
Even when the time arrives for unification, I'd believe going through MLIR
is better to have general SPIR-V support. :)

On Tue, Mar 2, 2021 at 6:07 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,
> e.g. in Vulkan we have logical adressing mode).
> 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.
>

I think we have an assumption here: LLVM itself should support all
mechanisms and use cases SPIR-V can support, if to make LLVM a layer before
SPIR-V. I think there is a huge gap here.

>
> 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.

To be specific, Vulkan compute is the most well supported use case right
now. But there are interests from the community to push on Vulkan graphics
and OpenCL.

> 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).
>

I actually believe the opposite, because of the reasons I listed at the
very beginning. To me SPIR-V also stays at a higher level than LLVM. (But
again, depending on what subset we are talking about.)

My proposal would be to include some MLIR -> LLVM-IR translated code in the
> testing so to have this final goal in mind.
>
> 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.

Not sure this is the prefered way, given that we can define SPIR-V ops
easily in MLIR in its own dialect with native support for various aspects.

> 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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