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106 lines
4.3 KiB
ReStructuredText
106 lines
4.3 KiB
ReStructuredText
=============================================
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SYCL Compiler and Runtime architecture design
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=============================================
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.. contents::
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:local:
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Introduction
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============
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This document describes the architecture of the SYCL compiler and runtime
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library. More details are provided in
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`external document <https://github.com/intel/llvm/blob/sycl/sycl/doc/CompilerAndRuntimeDesign.md>`_\ ,
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which are going to be added to clang documentation in the future.
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Address space handling
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======================
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The SYCL specification represents pointers to disjoint memory regions using C++
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wrapper classes on an accelerator to enable compilation with a standard C++
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toolchain and a SYCL compiler toolchain. Section 3.8.2 of SYCL 2020
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specification defines
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`memory model <https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#_sycl_device_memory_model>`_\ ,
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section 4.7.7 - `address space classes <https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#_address_space_classes>`_
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and section 5.9 covers `address space deduction <https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#_address_space_deduction>`_.
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The SYCL specification allows two modes of address space deduction: "generic as
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default address space" (see section 5.9.3) and "inferred address space" (see
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section 5.9.4). Current implementation supports only "generic as default address
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space" mode.
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SYCL borrows its memory model from OpenCL however SYCL doesn't perform
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the address space qualifier inference as detailed in
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`OpenCL C v3.0 6.7.8 <https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_C.html#addr-spaces-inference>`_.
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The default address space is "generic-memory", which is a virtual address space
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that overlaps the global, local, and private address spaces. SYCL mode enables
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explicit conversions to/from the default address space from/to the address
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space-attributed type and implicit conversions from the address space-attributed
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type to the default address space. All named address spaces are disjoint and
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sub-sets of default address space.
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The SPIR target allocates SYCL namespace scope variables in the global address
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space.
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Pointers to default address space should get lowered into a pointer to a generic
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address space (or flat to reuse more general terminology). But depending on the
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allocation context, the default address space of a non-pointer type is assigned
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to a specific address space. This is described in
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`common address space deduction rules <https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#subsec:commonAddressSpace>`_
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section.
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This is also in line with the behaviour of CUDA (`small example
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<https://godbolt.org/z/veqTfo9PK>`_).
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``multi_ptr`` class implementation example:
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.. code-block:: C++
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// check that SYCL mode is ON and we can use non-standard decorations
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#if defined(__SYCL_DEVICE_ONLY__)
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// GPU/accelerator implementation
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template <typename T, address_space AS> class multi_ptr {
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// DecoratedType applies corresponding address space attribute to the type T
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// DecoratedType<T, global_space>::type == "__attribute__((opencl_global)) T"
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// See sycl/include/CL/sycl/access/access.hpp for more details
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using pointer_t = typename DecoratedType<T, AS>::type *;
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pointer_t m_Pointer;
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public:
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pointer_t get() { return m_Pointer; }
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T& operator* () { return *reinterpret_cast<T*>(m_Pointer); }
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}
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#else
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// CPU/host implementation
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template <typename T, address_space AS> class multi_ptr {
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T *m_Pointer; // regular undecorated pointer
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public:
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T *get() { return m_Pointer; }
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T& operator* () { return *m_Pointer; }
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}
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#endif
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Depending on the compiler mode, ``multi_ptr`` will either decorate its internal
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data with the address space attribute or not.
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To utilize clang's existing functionality, we reuse the following OpenCL address
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space attributes for pointers:
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.. list-table::
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:header-rows: 1
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* - Address space attribute
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- SYCL address_space enumeration
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* - ``__attribute__((opencl_global))``
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- global_space, constant_space
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* - ``__attribute__((opencl_local))``
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- local_space
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* - ``__attribute__((opencl_private))``
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- private_space
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.. code-block:: C++
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//TODO: add support for __attribute__((opencl_global_host)) and __attribute__((opencl_global_device)).
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