[LLVMdev] [RFC] Proposal for Adding SPIRV Target

[Liu, Yaxun (Sam)]

Hi Mehdi,

Thank you for your comments. My comments are below.

Sam

From: Mehdi Amini [mailto:mehdi.amini at apple.com]
Sent: Wednesday, June 17, 2015 12:43 PM
To: Liu, Yaxun (Sam)
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] [RFC] Proposal for Adding SPIRV Target

Hi Liu,

Thanks for the detailed proposal.


On Jun 17, 2015, at 5:44 AM, Liu, Yaxun (Sam) <Yaxun.Liu at amd.com<mailto:Yaxun.Liu at amd.com>> wrote:

Here is the revised proposal for the LLVM/SPIR-V converter. Please comment. Thanks.

Proposal of Adding SPIRV Target

Background

SPIR-V is a portable binary format for OpenCL kernels and GLSL shaders. A typical use case of SPIR-V is as follows:

1.      An application developer uses Clang to compile an OpenCL kernel source code to a SPIR-V binary which is common for all OpenCL platforms.
2.      The application developer ships the application containing the SPIR-V binary to customers.
3.      A customer runs the application on an OpenCL platform, which loads the SPIR-V binary through an OpenCL API function.
4.      The vendor-specific OpenCL runtime translates SPIR-V to LLVM IR, changes the target triple and data layout to suit the device which will execute the kernel, performs target specific optimizations, generates the ISA and executes the ISA on the device.


Step 4 of your “typical use case” includes "changes the target triple and data layout to suit the device which will execute the kernel”. It implies that SPIR-V is data layout agnostic since you can load it with any data layout, or there are (to be specified) constraint on what a “compatible” data layout is, or you considered that it is up to the OpenCL vendor to figure out what will work or not, with the drawback that any LLVM update can break its use case.

+ For OpenCL, LLVM IR translated from SPIR-V has specific data layouts, which are the data layouts for target spir/spir64. OpenCL vendor’s target data layout are assumed to be consistent with them.


For OpenCL kernels, there is implicit data layout dependence when compiling the source to LLVM. Since SPIR-V is for common OpenCL platforms, a common data layout accepted by different OpenCL vendors is required. We choose the data layout which has been adopted by SPIR 1.2/2.0 for SPIR-V, since it has been successfully used for supporting consumption of SPIR 1.2/2.0 on various OpenCL platforms. For GLSL shaders, it is still under discussion whether to choose the same data layout as OpenCL, or a different data layout, or no data layout at all.

Location

From feedback of the previous version of the proposal, there are several suggestions about the location for the LLVM/SPIR-V converter:

1.      llvm/lib/SPIRV only, adding an option to Clang for outputting SPIR-V. The advantage is ease of use for bi-way translation. However it does not reflect the fact that only LLVM IR with specific target triple and data layout can be translated to SPIR-V.

How important is it to “reflect it”?
The SPIR-V emitter could just assert on the data layout matching what is expected.
+ Putting the converter at llvm/lib/SPIRV may encourage misuse of the converter, i.e., using the converter to convert LLVM IR of arbitrary target, whereas the converter can only convert LLVM IR with spir/spir64 target.


2.      llvm/lib/SPIRV containing the main functionality of bi-way translation between LLVM IR and SPIR-V, llvm/lib/Target/SPIRV containing a thin wrapper as a target machine to allow Clang targeting SPIR-V. The advantage compared with 1 is that it allows a more conventional way of using Clang to produce SPIR-V. However it is subject to the same issue as 1 about not reflecting the requirement on the LLVM IR which can be translated to SPIR-V.

I don’t see why you think there is an issue on the data layout in this case. Currently every target in LLVM has an expected data layout, and if the IR has been processed with a different data layout than what the target expect, it may or may not work.
I believe your “thin wrapper” in llvm/lib/Target/SPIRV would declare a triple and data layout and then makes this requirement explicit.
This looks like the cleaner solution to me, I’m not sure I understand why you are more in favor of option 3?
+ Since the converter is only for converting LLVM IR of a specific target, I prefer to put it at the same location of that target.

Thanks,

—
Mehdi



3.      llvm/lib/Target/SPIRV only. The advantage is that it reflects the requirement on the target triple and data layout for LLVM IR which can be translated to SPIR-V. However putting the SPIR-V to LLVM converter in the same directory is unconventional. Leaving the SPIR-V to LLVM converter out of LLVM source tree is also not desirable since OpenCL vendors need this functionality.

Our proposal is to take approach 3 and keep the bi-way converter in llvm/lib/Target/SPIRV. The functionality of the bi-way converter is exposed through llvm/include/Support/SPIRV.h. A thin wrapper as a target machine is also provided to allow Clang targeting SPIR-V. The rationale is that this directory structure better reflects the nature of SPIR-V. SPIR-V is not an alternative representation for arbitrary LLVM IR. Instead, it is an alternative representation for LLVM IR targeting generic OpenCL or Vulkan platforms. It has its own specific target triple and data layout. Therefore it makes sense for the functionality to be put under llvm/lib/Target. Also, as an alternative representation of LLVM IR, it makes sense to have a bi-way convertor.

Implementation

About the implementation of the converter, although there are suggestions to take the SelectionDAG/MC approach, it seems not a major concern in general. The current implementation uses a shared in-memory representation of SPIR-V, which facilitates supporting bi-way translation. The round-trip translated LLVM IR by the current implementation has passed OpenCL SPIR 1.2 conformance test, which proves the current implementation works. On the other hand, the SelectionDAG/MC approach would require significant tweaking compared to a conventional backend, since SPIR-V is not a low level machine ISA but a high level generic IR. Also, the SelectionDAG/MC approach only supports one-way translation from LLVM to SPIR-V. Therefore unless major concern arises, we will keep the current implementation approach.

Maintenance

The current implementation works by breaking down LLVM IR to instructions and re-constructing them in SPIR-V, and vice versa. Therefore the dependence of the current implementation on LLVM is mainly the C++ API in llvm/include/IR. As such, its dependence on LLVM is similar to typical module passes. Our experience with porting it among LLVM 3.2/3.4/3.6 is that the porting effort is moderate.

Milestones

Currently Clang can compile OpenCL 1.2/2.0 C kernel source to LLVM IR with spir/spir64 target triple, which is compatible with SPIR-V for the supported instructions, data types and builtin functions. Therefore the first milestone of the converter is to support compiling OpenCL 1.2/2.0 kernel to SPIR-V. This is also to lay the foundation for the upcoming OpenCL 2.1 C++ frontend development in Clang. The next milestone would be supporting OpenCL 2.1 C++, which hopefully would be in synch with the frontend development work. In the meantime, as the SPIR-V target becomes stable and is able to support the instructions common to OpenCL and GLSL, hopefully the GLSL frontend work and support for GLSL specific instructions would pick up and finally we would have a SPIR-V target supporting the complete SPIR-V spec.

Testing

We will add lit tests for SPIR-V. Also the converter would be tested by different OpenCL vendors for production quality through comprehensive conformance tests, since SPIR-V is required by OpenCL 2.1.

Logistics

AMD, Intel and some other SPIR WG members would join force for the development of the bi-way converter. Since supporting of SPIR-V is required by OpenCL 2.1, it is expected that OpenCL vendors would continue the maintenance efforts for supporting SPIRV target in LLVM.

Yaxun Liu
AMD

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