[SYCL][Doc] Add design document for SYCL mode

Initial version of the document covers address space handling

Differential Revision: https://reviews.llvm.org/D99488

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))`.