[OpenCL][Docs] Describe tablegen BIFs declaration

[Anastasia]

Summary

Added documentation for the fast builtin function declaration with -fdeclare-opencl-builtins.

[svenvh]

Thanks for working on this!

clang/docs/OpenCLSupport.rst

118

119

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132

We should also mention that this is (still) an experimental flag, and that behavior of the flag may change in the future.

[Anastasia]

Added suggestions from Sven.

[Anastasia]

clang/docs/OpenCLSupport.rst

132

Not against it but it is not strictly necessary - all experimental features in clang can change in many ways normally.

[svenvh]

LGTM, thanks!

clang/docs/OpenCLSupport.rst

 =====================
 
 Clang provides the following new WIP features for the developers to experiment
 and provide early feedback or contribute with further improvements.
 Feel free to contact us on `cfe-dev
 <https://lists.llvm.org/mailman/listinfo/cfe-dev>`_ or via `Bugzilla
 <https://bugs.llvm.org/>`__.
 
+Fast builtin function declarations
+----------------------------------
+
+In addition to regular header includes with builtin types and functions using
+``-finclude-default-header`` explained in :doc:`UsersManual`, clang
+supports a fast mechanism to declare builtin functions with
+``-fdeclare-opencl-builtins``. This does not declare the builtin types and
+therefore it has to be used in combination with ``-finclude-default-header``
+if full functionality is required.
+
+**Example of Use**:
+
+    .. code-block:: console
+ 
+      $ clang -Xclang -finclude-default-header test.cl
+
+Note that this is a frontend-only flag and therefore it requires the use of
+flags that forward options to the frontend, e.g. ``-cc1`` or ``-Xclang``.
+
+As this feature is still in experimental phase some changes might still occur
+on the command line interface side.
+
 C++ libraries for OpenCL
 ------------------------
 
 There is ongoing work to support C++ standard libraries from `LLVM's libcxx
 <https://libcxx.llvm.org/>`_ in OpenCL kernel code using C++ for OpenCL mode.
 
 It is currently possible to include `type_traits` from C++17 in the kernel
 sources when the following clang extensions are enabled

clang/docs/UsersManual.rst

 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=<path to the generated module> test.cl
 
+Another way to circumvent long parsing latency for the OpenCL builtin
+declarations is to use mechanism enabled by ``-fdeclare-opencl-builtins`` flag
+that is available as an experimental feature (see more information in
+:doc:`OpenCLSupport`).
+
 OpenCL Extensions
 -----------------
 
 Most of the ``cl_khr_*`` extensions to OpenCL C from `the official OpenCL
 registry <https://www.khronos.org/registry/OpenCL/>`_ are available and
 configured per target depending on the support available in the specific
 architecture.