bader updated this revision to Diff 340526. bader added a comment. Rebased patch to unblock commit.
This patch had a dependency on D99190 <https://reviews.llvm.org/D99190>, which adds SYCLSupport.rst document. To unblock commit of D99488 <https://reviews.llvm.org/D99488>, I switched the order of these two patches. Now D99488 <https://reviews.llvm.org/D99488> add SYCLSupport.rst document with just two sections: "Introduction" and "Address space handling". D99190 <https://reviews.llvm.org/D99190> will be rebased on top of D99488 <https://reviews.llvm.org/D99488> to add more content to the document. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D99488/new/ https://reviews.llvm.org/D99488 Files: clang/docs/SYCLSupport.rst
Index: clang/docs/SYCLSupport.rst =================================================================== --- /dev/null +++ clang/docs/SYCLSupport.rst @@ -0,0 +1,105 @@ +============================================= +SYCL Compiler and Runtime architecture design +============================================= + +.. contents:: + :local: + +Introduction +============ + +This document describes the architecture of the SYCL compiler and runtime +library. More details are provided in +`external document <https://github.com/intel/llvm/blob/sycl/sycl/doc/CompilerAndRuntimeDesign.md>`_\ , +which are going to be added to clang documentation in the future. + +Address space handling +====================== + +The SYCL specification represents pointers to disjoint memory regions using C++ +wrapper classes on an accelerator to enable compilation with a standard C++ +toolchain and a SYCL compiler toolchain. Section 3.8.2 of SYCL 2020 +specification defines +`memory model <https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#_sycl_device_memory_model>`_\ , +section 4.7.7 - `address space classes <https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#_address_space_classes>`_ +and section 5.9 covers `address space deduction <https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#_address_space_deduction>`_. +The SYCL specification allows two modes of address space deduction: "generic as +default address space" (see section 5.9.3) and "inferred address space" (see +section 5.9.4). Current implementation supports only "generic as default address +space" mode. + +SYCL borrows its memory model from OpenCL however SYCL doesn't perform +the address space qualifier inference as detailed in +`OpenCL C v3.0 6.7.8 <https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_C.html#addr-spaces-inference>`_. + +The default address space is "generic-memory", which is a virtual address space +that overlaps the global, local, and private address spaces. SYCL mode enables +explicit conversions to/from the default address space from/to the address +space-attributed type and implicit conversions from the address space-attributed +type to the default address space. All named address spaces are disjoint and +sub-sets of default address space. + +The SPIR target allocates SYCL namespace scope variables in the global address +space. + +Pointers to default address space should get lowered into a pointer to a generic +address space (or flat to reuse more general terminology). But depending on the +allocation context, the default address space of a non-pointer type is assigned +to a specific address space. This is described in +`common address space deduction rules <https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#subsec:commonAddressSpace>`_ +section. + +This is also in line with the behaviour of CUDA (`small example +<https://godbolt.org/z/veqTfo9PK>`_). + +``multi_ptr`` class implementation example: + +.. code-block:: C++ + + // check that SYCL mode is ON and we can use non-standard decorations + #if defined(__SYCL_DEVICE_ONLY__) + // GPU/accelerator implementation + template <typename T, address_space AS> class multi_ptr { + // DecoratedType applies corresponding address space attribute to the type T + // DecoratedType<T, global_space>::type == "__attribute__((opencl_global)) T" + // See sycl/include/CL/sycl/access/access.hpp for more details + using pointer_t = typename DecoratedType<T, AS>::type *; + + pointer_t m_Pointer; + public: + pointer_t get() { return m_Pointer; } + T& operator* () { return *reinterpret_cast<T*>(m_Pointer); } + } + #else + // CPU/host implementation + template <typename T, address_space AS> class multi_ptr { + T *m_Pointer; // regular undecorated pointer + public: + T *get() { return m_Pointer; } + T& operator* () { return *m_Pointer; } + } + #endif + +Depending on the compiler mode, ``multi_ptr`` will either decorate its internal +data with the address space attribute or not. + +To utilize clang's existing functionality, we reuse the following OpenCL address +space attributes for pointers: + +.. list-table:: + :header-rows: 1 + + * - Address space attribute + - SYCL address_space enumeration + * - ``__attribute__((opencl_global))`` + - global_space, constant_space + * - ``__attribute__((opencl_local))`` + - local_space + * - ``__attribute__((opencl_private))`` + - private_space + + +.. code-block:: + + TODO: add support for `__attribute__((opencl_global_host))` and + `__attribute__((opencl_global_device))`.
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