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fpga_io_pipes_design.rst

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IO pipes design

Requirements

  • Device shall be able to distinguish kernel-to-kernel pipes and I/O pipes;
  • No changes shall be in SYCL pipe specification;
  • I/O pipe namings/IDs are provided by a vendor in a separated header, like:

Links

Summary

This document describes a design of I/O pipes implementation in SYCL compiler. It includes changes in SYCL library, clang and SPIR-V to LLVM IR translator. It adds extra attribute '__attribute__((io_pipe_id(ID)))' that generates metadata, attached to a pipe storage declaration. By this metadata a backend can recognize I/O pipe and distinguish different I/O pipes from each other.

There is another notable solution which was proposed by Ronan (see the link above): don't make any compiler/library changes - just make backend recognizing I/O pipe by demangling it's name. This proposal wasn't picked, because it will make backend support of the feature more difficult. So far we already have two devices' backends that support SYCL_INTEL_data_flow_pipes extension (Intel FPGA HW and Intel FPGA emulator) and in the future this number may increase. So efforts put in compiler implementation shall be payed off even more.

clang

Need to implement additional attribute, that possible to attach to pipe storage declaration. The attribute shall accept a compile-time known integer argument (the pipe ID). With the attribute applied, clang generates a metadata attached the to pipe storage declaration, that contains the I/O pipe ID (argument).

llvm-spirv translator

Need to implement additional decoration, that saves the I/O pipe ID information, that can be collected from a metadata attached to the pipe storage object.

SYCL implementation in headers

Following the spec, we need to add two more classes for pipes:
  • 'kernel_readable_io_pipe'
  • 'kernel_writeable_io_pipe'

with the same member functions and fields as it is already done for pipe class.

The attribute should be attached to a pipe storage declaration in the headers and it would be looking like: .. code:: cpp

static constexpr int32_t ID = name::id; static constexpr struct ConstantPipeStorage m_Storage __attribute__((io_pipe_id(ID))) = {m_Size, m_Alignment, m_Capacity};

where 'name' is some class used in pipe type construction, like:

When specific io_pipe_def.h is included in a user's code and 'name' is mapped to the pipe name defined in this header (for example 'ethernet_pipe_id' structure defined above) 'name::id' returns the actual I/O pipe ID (compile-time known integer constant) that is passed as the attribute's argument and used to indentify the I/O pipe in some backend.