[llvm-dev] [SPIR-V] SPIR-V in LLVM

Nicholas Wilson via llvm-dev llvm-dev at lists.llvm.org
Mon May 1 01:01:12 PDT 2017

I note that there was a talk recently at EuroLLVM SPIR-V and LLVM about and so I want to get this message out soon so as to avoid duplicated effort.

I have an up to date backend for SPIR-V on an up to date fork (~2-3 weeks behind) of LLVM, transplanted and “modernised” from Khronos’ SPIRV-LLVM that I plan on integrating into LLVM trunk. While it is usable in it’s current form there are several issues.

SPIRV-LLVM’s SPIRV support is not a real backend and does not live in /lib/Target/SPIRV. This has been solved, SPIRV is now a proper target(s). I have not copied the tests across yet, and they will need to be updated in light of the changes made/being made below.
It only support’s the OpenCL “flavour” of SPIRV, but not the Vulkan. I plan on eventually supporting this but its priority is less than that of mainline integration.
Intrinsics (for both Core and OpenCL extension instructions, Vulkan to follow) are done through C++ "Itanium with extensions” mangling. I plan to convert these to a table-gen format and cull the associated mangling code..
Likewise the instruction format is currently done through a home-brew table format, which I am in the process of converting to table-gen. The core instructions are almost complete, but I haven’t started on the OpenCL instructions.
As noted in the talk the textual representation is different from the the reference implementation. I see no advantage of the current format over the reference implementation’s format, infact theirs is much easier to read, This is a low priority.
The instruction table-gen tables contain a significant number of critical non-instruction tables (there are no registers) and therefore a proper table-gen backend needs to be written to accomodate for this. Required.
There are some LLVM “infrastructure” things (TargetInfo, MCTargetDesc) that I have stubs in order to get LLVM to compile, I have no idea what I’m doing and those were mostly adapted from Sparc and RISCV. Due to the lack of a Dyld and the fact that SPIR-V is  intermediate format, there is no point of a jit, Simple optimisations (CSE,DCE, some very simple inlining) would be nice to have, as would DebugInfo support.

The code is available at my GitHub:
SPIRV backend https://github.com/thewilsonator/llvm-target-spirv
LLVM https://github.com/thewilsonator/llvm/tree/compute
should you want to inspect/offer advice.

I am very busy until about July, but I just want to put this out here to gather interest / feedback / contributors.

Nicholas Wilson
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