Index: docs/UsersManual.rst =================================================================== --- docs/UsersManual.rst +++ docs/UsersManual.rst @@ -41,6 +41,7 @@ variants depending on base language. - :ref:`C++ Language ` - :ref:`Objective C++ Language ` +- :ref:`OpenCL C Language `: v1.0, v1.1, v1.2, v2.0. In addition to these base languages and their dialects, Clang supports a broad variety of language extensions, which are documented in the @@ -1973,6 +1974,361 @@ is provided or target does not support TLS, code generation for threadprivate variables relies on OpenMP runtime library. +.. _opencl: + +OpenCL Features +=============== + +Clang can be used to compile OpenCL kernels for execution on a device +(e.g. GPU). It is possible to compile the kernel into a binary (e.g. for AMD or +Nvidia targets) that can be uploaded to run directly on a device (e.g. using +`clCreateProgramWithBinary +`_) or +into generic bitcode files loadable into other toolchains. + +Compiling to a binary using the default target from the installation can be done +as follows: + + .. code-block:: console + + $ echo "kernel void k(){}" > test.cl + $ clang test.cl + +Compiling for a specific target can be done by specifying the triple corresponding +to the target, for example: + + .. code-block:: console + + $ clang -target nvptx64-unknown-unknown test.cl + $ clang -target amdgcn-amd-amdhsa-opencl test.cl + +Compiling to bitcode can be done as follows: + + .. code-block:: console + + $ clang -c -emit-llvm test.cl + +This will produce a generic test.bc file that can be used in vendor toolchains +to perform machine code generation. + +Clang currently supports OpenCL C language standards up to v2.0. + +OpenCL Specific Options +----------------------- + +Most of the OpenCL build options from `the specification v2.0 section 5.8.4 +`_ are available. + +Examples: + + .. code-block:: console + + $ clang -cl-std=CL2.0 -cl-single-precision-constant test.cl + +Some extra options are available to support special OpenCL features. + +.. option:: -finclude-default-header + +Loads standard includes during compilations. By default OpenCL headers are not +loaded and therefore standard library includes are not available. To load them +automatically a flag has been added to the frontend (see also OpenCL Header +section): + + .. code-block:: console + + $ clang -Xclang -finclude-default-header test.cl + +Alternatively ``-include`` or ``-I`` followed by the path to the header location +can be given manually. + + .. code-block:: console + + $ clang -I/lib/Headers/opencl-c.h test.cl + +In this case the kernel code should contain ``#include `` just as a +regular C include. + +.. option:: -cl-ext + +Disables support of OpenCL extensions. All OpenCL targets provide a list +of extensions that they support. Clang allows to amend this using the ``-cl-ext`` +flag with a comma-separated list of extensions prefixed with ``'+'`` or ``'-'``. +The syntax: ``-cl-ext=<(['-'|'+'][,])+>``, where extensions +can be either one of `the OpenCL specification extensions +`_ +or any known vendor extension. Alternatively, ``'all'`` can be used to enable +or disable all known extensions. +Example disabling double support for the 64-bit SPIR target: + + .. code-block:: console + + $ clang -cc1 -triple spir64-unknown-unknown -cl-ext=-cl_khr_fp64 test.cl + +Enabling all extensions except double support in R600 AMD GPU can be done using: + + .. code-block:: console + + $ clang -cc1 -triple r600-unknown-unknown -cl-ext=-all,+cl_khr_fp16 test.cl + +.. _opencl_fake_address_space_map: + +.. option:: -ffake-address-space-map + +Overrides the target address space map with a fake map. +This allows adding explicit address space IDs to the bitcode for non-segmented +memory architectures that don't have separate IDs for each of the OpenCL +logical address spaces by default. Passing ``-ffake-address-space-map`` will +add/override address spaces of the target compiled for with the following values: +``1-global``, ``2-local``, ``3-constant``, ``4-generic``. The private address +space is represented by the absence of an address space attribute in the IR (see +also :ref:`the section on address space attribute `). + + .. code-block:: console + + $ clang -ffake-address-space-map test.cl + +Some other flags used for the compilation for C can also be passed while +compiling for OpenCL, examples: ``-c``, ``-O<1-4|s>``, ``-o``, ``-emit-llvm``, etc. + +OpenCL Targets +-------------- + +OpenCL targets are derived from the regular Clang target classes. The OpenCL +specific parts of the target representation provide address space mapping as +well as a set of supported extensions. + +Specific Targets +^^^^^^^^^^^^^^^^ + +There is a set of concrete HW architectures that OpenCL can be compiled for. + +- For AMD target: + + .. code-block:: console + + $ clang -target amdgcn-amd-amdhsa-opencl test.cl + +- For Nvidia architectures: + + .. code-block:: console + + $ clang -target nvptx64-unknown-unknown test.cl + + +Generic Targets +^^^^^^^^^^^^^^^ + +- SPIR is available as a generic target to allow portable bitcode to be produced + that can be used across GPU toolchains. The implementation follows `the SPIR + specification `_. There are two flavors available + for 32 and 64 bits. + + .. code-block:: console + + $ clang -target spir-unknown-unknown test.cl + $ clang -target spir64-unknown-unknown test.cl + + All known OpenCL extensions are supported in SPIR targets. + +- x86 is used by some implementations that are x86 compatible and currently + remains for backwards compatibility (with older implementations prior to + SPIR target support). For "non-SPMD" targets which cannot spawn multiple + work-items on the fly using hardware, which covers practically all non-GPU + devices such as CPUs and DSPs, additional processing is needed for the kernels + to support multiple WI execution. For this, a 3rd party toolchain, such as for + example `POCL `_ can be used. + + This target does not provide multiple memory segments and, therefore, the fake + address space map can be added using the :ref:`-ffake-address-space-map + ` flag. + +OpenCL Header +------------- + +By default Clang will not include standard headers and therefore OpenCL builtin +functions and some types (i.e. vectors) are unknown. The default CL header is, +however, provided in the Clang installation and can be enabled by passing the +``-finclude-default-header`` flag to the Clang frontend. + + .. code-block:: console + + $ echo "bool is_wg_uniform(int i){return get_enqueued_local_size(i)==get_local_size(i);}" > test.cl + $ clang -Xclang -finclude-default-header -cl-std=CL2.0 test.cl + +Because the header is very large and long to parse, PCH (:doc:`PCHInternals`) +and modules (:doc:`Modules`) are used internally to improve the compilation +speed. + +To enable modules for OpenCL: + + .. code-block:: console + + $ clang -target spir-unknown-unknown -c -emit-llvm -Xclang -finclude-default-header -fmodules -fimplicit-module-maps -fmodules-cache-path= test.cl + +OpenCL Metadata +--------------- + +Clang uses metadata to provide additional OpenCL semantics in IR needed for +backends and OpenCL runtime. + +Each kernel will have function metadata attached to it, specifying the arguments. +Kernel argument metadata is used to provide source level information for querying +at runtime, for example using the `clGetKernelArgInfo +`_ +call. + + +Note that ``-cl-kernel-arg-info`` enables more information about the original CL +code to be added e.g. kernel parameter names will appear in the OpenCL metadata +along with other information. + +The IDs used to encode the OpenCL's logical address spaces in the argument info +metadata follows the SPIR address space mapping as defined in the SPIR +specification `section 2.2 +`_ + +OpenCL-Specific Attributes +-------------------------- + +OpenCL support in Clang contains a set of attribute taken directly from the +specification as well as additional attributes. + +See also :doc:`AttributeReference`. + +nosvm +^^^^^ + +Clang supports this attribute to comply to OpenCL v2.0 conformance, but it +does not have any effect on the IR. For more details reffer to the specification +`section 6.7.2 +`_ + + +opencl_hint_unroll +^^^^^^^^^^^^^^^^^^ + +The implementation of this feature mirrors the unroll hint for C. +More details on the syntax can be found in the specification +`section 6.11.5 +`_ + +convergent +^^^^^^^^^^ + +To make sure no invalid optimizations occur for single program multiple data +(SPMD) / single instruction multiple thread (SIMT) Clang provides attributes that +can be used for special functions that have cross work item semantics. +An example is the subgroup operations such as `intel_sub_group_shuffle +`_ + + .. code-block:: c + + // Define custom my_sub_group_shuffle(data, c) + // that makes use of intel_sub_group_shuffle + r1 = … + if (r0) r1 = computeA(); + // Shuffle data from r1 into r3 + // of threads id r2. + r3 = my_sub_group_shuffle(r1, r2); + if (r0) r3 = computeB(); + +with non-SPMD semantics this is optimized to the following equivalent code: + + .. code-block:: c + + r1 = … + if (!r0) + // Incorrect functionality! The data in r1 + // have not been computed by all threads yet. + r3 = my_sub_group_shuffle(r1, r2); + else { + r1 = computeA(); + r3 = my_sub_group_shuffle(r1, r2); + r3 = computeB(); + } + +Declaring the function ``my_sub_group_shuffle`` with the convergent attribute +would prevent this: + + .. code-block:: c + + my_sub_group_shuffle() __attribute__((convergent)); + +Using ``convergent`` guarantees correct execution by keeping CFG equivalence +wrt operations marked as ``convergent``. CFG ``G´`` is equivalent to ``G`` wrt +node ``Ni`` : ``iff ∀ Nj (i≠j)`` domination and post-domination relations with +respect to ``Ni`` remain the same in both ``G`` and ``G´``. + +noduplicate +^^^^^^^^^^^ + +``noduplicate`` is more restrictive with respect to optimizations than +``convergent`` because a convergent function only preserves CFG equivalence. +This allows some optimizations to happen as long as the control flow remains +unmodified. + + .. code-block:: c + + for (int i=0; i<4; i++) + my_sub_group_shuffle() + +can be modified to: + + .. code-block:: c + + my_sub_group_shuffle(); + my_sub_group_shuffle(); + my_sub_group_shuffle(); + my_sub_group_shuffle(); + +while using ``noduplicate`` would disallow this. Also ``noduplicate`` doesn't +have the same safe semantics of CFG as ``convergent`` and can cause changes in +CFG that modify semantics of the original program. + +``noduplicate`` is kept for backwards compatibility only and it considered to be +deprecated for future uses. + +.. _opencl_addrsp: + +address_space +^^^^^^^^^^^^^ + +Clang has arbitrary address space support using the ``address_space(N)`` attribute, +where ``N`` is an integer number in the range ``0`` to ``16777215`` (``0xffffffu``). + +An OpenCL implementation provides a list of standard address spaces using +keywords: ``private``, ``local``, ``global``, and ``generic``. In AST and IR +local, global, or generic will be represented by the address space attribute +with the corresponding unique number. Note that private does not have any +corresponding attribute added and, therefore, is represented by the absence of +an address space number. The specific IDs for an address space do not have to +match between the AST and the IR. Typically in the AST address space numbers +represent logical segments while in the IR they represent physical segments. +Therefore, machines with flat memory segments can map all AST address space +numbers to the same physical segment ID or skip address space attribute +completely while generating the IR. However, if the address space information +is needed by IR passes e.g. to improve alias analysis, it is recommended +to keep it and only lower to reflect physical memory segments in the late machine +passes. + +OpenCL builtins +--------------- + +There are some standard OpenCL functions that are implemented as Clang builtins: + +- All pipe functions from `section 6.13.16.2/6.13.16.3 + `_ of + the OpenCL v2.0 kernel language specification. ` + +- Address space qualifier conversion functions ``to_global``/``to_local``/``to_private`` + from `section 6.13.9 + `_. + +- All the ``enqueue_kernel`` functions from `section 6.13.17.1 + `_ and + enqueue query functions from `section 6.13.17.5 + `_. + .. _target_features: Target-Specific Features and Limitations Index: www/index.html =================================================================== --- www/index.html +++ www/index.html @@ -15,10 +15,10 @@

clang: a C language family frontend for LLVM

-

The goal of the Clang project is to create a new C, C++, Objective C and - Objective C++ front-end for the LLVM - compiler. You can get and build the source - today.

+

The goal of the Clang project is to create a new C based language + front-end: C, C++, Objective C/C++, OpenCL C and others for the + LLVM compiler. You can + get and build the source today.

Features and Goals