[llvm-dev] [RfC] A proposal of adding SPIR-V Toolchain in Clang

[Richard Smith via llvm-dev]

On Mon, 10 Sep 2018 at 18:47, Nicholas Wilson via llvm-dev <
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?


> 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> 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 -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 -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
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