[OpenCL] Add support of OpenCL C 3.0 __opencl_c_fp64
There already exists cl_khr_fp64 extension. So OpenCL C 3.0
and higher should use the feature, earlier versions still
use the extension. OpenCL C 3.0 API spec states that extension
will be not described in the option string if corresponding
optional functionality is not supported (see 4.2. Querying Devices).
Due to that fact the usage of features for OpenCL C 3.0 must
be as follows:
```
$ clang -Xclang -cl-ext=+cl_khr_fp64,+__opencl_c_fp64 ...
$ clang -Xclang -cl-ext=-cl_khr_fp64,-__opencl_c_fp64 ...
```
e.g. the feature and the equivalent extension (if exists)
must be set to the same values
Reviewed By: Anastasia
Differential Revision: https://reviews.llvm.org/D96524
diff --git a/clang/docs/OpenCLSupport.rst b/clang/docs/OpenCLSupport.rst
index fa719c8..dbfb67f 100644
--- a/clang/docs/OpenCLSupport.rst
+++ b/clang/docs/OpenCLSupport.rst
@@ -339,7 +339,24 @@
.. _opencl_300:
-OpenCL 3.0 Implementation Status
+OpenCL C 3.0 Usage
+================================
+
+OpenCL C 3.0 language standard makes most OpenCL C 2.0 features optional. Optional
+functionality in OpenCL C 3.0 is indicated with the presence of feature-test macros
+(list of feature-test macros is `here <https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_C.html#features>`_).
+Command-line flag :ref:`-cl-ext <opencl_cl_ext>` can be used to override features supported by a target.
+
+For cases when there is an associated extension for a specific feature (fp64 and 3d image writes)
+user should specify both (extension and feature) in command-line flag:
+
+ .. code-block:: console
+
+ $ clang -cc1 -cl-std=CL3.0 -cl-ext=+cl_khr_fp64,+__opencl_c_fp64 ...
+ $ clang -cc1 -cl-std=CL3.0 -cl-ext=-cl_khr_fp64,-__opencl_c_fp64 ...
+
+
+OpenCL C 3.0 Implementation Status
================================
The following table provides an overview of features in OpenCL C 3.0 and their
