[cfe-dev] [llvm-dev] [RfC] A proposal of adding SPIR-V Toolchain in Clang
Tom Stellard via cfe-dev
cfe-dev at lists.llvm.org
Wed Sep 12 17:14:41 PDT 2018
On 09/12/2018 05:04 PM, Richard Smith wrote:
> On Wed, 12 Sep 2018 at 16:52, Tom Stellard via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
> On 09/12/2018 02:32 PM, Matthias Braun wrote:
> >
> >
> >> On Sep 11, 2018, at 7:39 PM, Tom Stellard via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> >>
> >> On 09/11/2018 12:50 PM, Richard Smith via llvm-dev wrote:
> >>> On Mon, 10 Sep 2018 at 18:47, Nicholas Wilson via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> <mailto:llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>> wrote:
> >>>
> >>> I was going to wait until Neil Trevett got back to me about becoming a SPIR-V TSG advisor but this seems like just as good an opportunity. Please see the previous discussion [1] if you have not already, there were many relevant points made.
> >>>
> >>> First, I’d like to note that while clang may be the primary motivator for many of the readers here it is not the only frontend that would like to make use of proper SPIR-V support in LLVM. In particular, the current implementation of builtins as Itanium with extensions mangled C++ is much more difficult to use (even if those frontends have a C++ mangler, the extensions make it unusable), compared to intrinsics which is _the_ way backends for LLVM expose builtins. This is an absolute requirement for me for SPIR-V support in upstream LLVM.
> >>>
> >>> I’d much rather have this as a target than as an external library, but if it means I get intrinsics faster then I’m all for it.
> >>>
> >>>
> >>> +1. What would be the justification for using an external binary for this rather than treating it like any other LLVM backend?
> >>>
> >>
> >> This has been discussed in the past, but I don't think SPIR-V
> >> is a good fit for an LLVM backend. It is very similar to LLVM
> >> IR and it seems like overkill to write a whole backend just to
> >> do a simple translation. Not to mention the fact that I don't
> >> see how it's possible with the current backend infrastructure
> >> to preserve type information for complex types like structs all
> >> the way through the codegen pipeline.
> >
> > Note that even when not using lib/CodeGen (which is indeed a bad fit here), you should still be able to implement the TargetMachine interface (and return nullptr for most of the methods in there) so it behaves like the other LLVM backend on the outside API.
> >
>
> In the past one of the arguments against doing this is that when you
> have a target that doesn't use the common legalization/lowering framework
> then it makes changes to the IR that much more burdensome, because
> you now have one more thing to update in addition to SelectionDAG,
> GlobalISel, and FastISel. And then what happens if we end up with
> multiple targets like this?
>
>
> We have to pay that cost regardless of whether it's part of maintaining llvm-spirv or part of maintaining a SPIR-V backend, though, don't we?
>
This all depends on if llvm-spirv becomes an official sub-project or not, which
is a whole other discussion, but if it does then yes the cost would be the same.
-Tom
> I do think having some having some kind of TargeMachine wrapper
> around an IR pass(es) would be a good technical solution, though, if
> we can find a way to keep the support burden minimal, especially
> since the LLVM IR -> SPIR-V translation code already exists.
>
> -Tom
>
> > - Matthias
> >
> >>
> >> -Tom
> >>
> >>
> >>>
> >>> Could you please link the thread mentioned?
> >>>
> >>> Thanks,
> >>> Nic
> >>>
> >>> P.S. Feel free to use the tablegen descriptions of the SPIR-V format from [2]
> >>>
> >>> [1]: http://lists.llvm.org/pipermail/llvm-dev/2017-May/112538.html
> >>> [2]: https://github.com/thewilsonator/llvm-target-spirv
> >>>
> >>>> On 10 Sep 2018, at 11:10 pm, Anastasia Stulova via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> <mailto:llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>> wrote:
> >>>>
> >>>> Hello,
> >>>>
> >>>> Since 2015 Khronos has switched to the new portable intermediate format SPIR-V, which has replaced the original SPIR. The advantage is that it offers higher portability across different toolchains. There was a talk about it at a Dev Meeting:
> >>>> http://llvm.org/devmtg/2017-03//2017/02/20/accepted-sessions.html#17
> >>>>
> >>>> LLVM currently only supports SPIR format for OpenCL in Clang. Several Khronos vendors (ARM, AMD, Intel, Xilinx, Codeplay and others) are interested in adding support for SPIR-V, which should gradually replace the old SPIR once products are no longer shipped with the old format. Here is the detailed description:
> >>>> https://github.com/KhronosGroup/SPIRV-LLVM-Translator/wiki/SPIRV-Toolchain-for-Clang
> >>>>
> >>>> To summarize, the idea is to add a SPIR-V target triple to Clang that can be used to generate a SPIR-V binary for OpenCL code. There was a separate thread regarding generation of SPIR-V binary and the community suggested that a translator from LLVM IR to SPIR-V can be used as an external tool, called llvm-spirv. This can be invoked similar to such tools as ptxas and fatbinary for the CUDA toolchain:
> >>>> http://lists.llvm.org/pipermail/llvm-dev/2018-February/121440.html
> >>>>
> >>>> An example of how Clang can be used to target SPIR-V:
> >>>>
> >>>> clang -c test.cl <http://test.cl> <http://test.cl> -target spirv[32|64]-unknown-unknown -o test.spv
> >>>>
> >>>> This will result in the following Clang actions:
> >>>>
> >>>> (1) clang -cc1 -triple spirv[32|64]-unknown-unknown test.cl <http://test.cl> <http://test.cl> -emit-llvm-bc -o test.bc
> >>>>
> >>>> (2) llvm-spirv test.bc -o test.spv
> >>>>
> >>>> SPIR-V generation is essential for completion of OpenCL C++ support in Clang, as newer OpenCL standards require frontend invocation to be performed offline, producing the SPIR-V binary that can be then loaded at application execution time. Besides that, it will also allow Clang to be used as a complete standalone tool to generate portable binaries that can then be consumed by different proprietary toolchains. In addition, this will open a path to the LLVM backends for various languages and frontends that already generate SPIR-V.
> >>>>
> >>>> A more detailed explanation of the complete design proposal is given in this Wiki page:
> >>>> https://github.com/KhronosGroup/SPIRV-LLVM-Translator/wiki/SPIRV-Toolchain-for-Clang
> >>>>
> >>>> Looking forward to any feedback about the proposal or possible collaborations,
> >>>>
> >>>> Thanks!
> >>>>
> >>>> Anastasia
> >>>> _______________________________________________
> >>>> LLVM Developers mailing list
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> >>>
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