[clang] [HIP] Document func ptr and virtual func (PR #68126)

[Artem Belevich via cfe-commits]

================
@@ -176,3 +176,65 @@ Predefined Macros
    * - ``HIP_API_PER_THREAD_DEFAULT_STREAM``
      - Alias to ``__HIP_API_PER_THREAD_DEFAULT_STREAM__``. Deprecated.
 
+Function Pointers Support in Clang with HIP
+===========================================
+
+Function pointers' support varies with the usage mode in Clang with HIP. The following table provides an overview of the support status across different use-cases and modes.
+
+.. list-table:: Function Pointers Support Overview
+   :widths: 25 25 25
+   :header-rows: 1
+
+   * - Use Case
+     - ``-fno-gpu-rdc`` Mode (default)
+     - ``-fgpu-rdc`` Mode
+   * - Defined and used in the same TU
+     - Supported
+     - Supported
+   * - Defined in one TU and used in another TU
+     - Not Supported
+     - Supported
+
+In the ``-fno-gpu-rdc`` mode, the compiler calculates the resource usage of kernels based only on functions present within the same Translation Unit (TU). This mode does not support the use of function pointers defined in a different TU due to the possibility of incorrect resource usage calculations, leading to undefined behavior.
+
+On the other hand, the ``-fgpu-rdc`` mode allows the definition and use of function pointers across different TUs, as resource usage calculations can accommodate functions from disparate TUs.
+
+Virtual Function Support in Clang with HIP
+==========================================
+
+In Clang with HIP, support for calling virtual functions of an object in device or host code is contingent on where the object is constructed.
----------------
Artem-B wrote:

We should also mention how mixing host and device virtual functions is expected to work.

AFAICT, vtable populates wrong-side virtual functions with nullptr: https://godbolt.org/z/4f6hvjMcj



https://github.com/llvm/llvm-project/pull/68126