[AMDGPU] Document the AMDGPU LLVM attributes

[kzhuravl]

looks good to me

docs/AMDGPUUsage.rst

``singlethread`` Only synchronizes with, and participates in modification
                       and seq_cst total orderings with, other operations (except
                       image operations) running in the same thread for all
                       address spaces (for example, in signal handlers).
      ================ ==========================================================
 
 AMDGPU Intrinsics
 -----------------
 
-The AMDGPU backend implements the following intrinsics.
+The AMDGPU backend implements the following LLVM IR intrinsics.
 
 *This section is WIP.*
 
 .. TODO
    List AMDGPU intrinsics
 
+AMDGPU Attributes
+-----------------
+
+The AMDGPU backend supports the following LLVM IR attributes.
+
+  .. table:: AMDGPU LLVM IR Attributes
+     :name: amdgpu-llvm-ir-attributes-table
+
+     ======================================= ==========================================================
+     LLVM Attribute                          Description
+     ======================================= ==========================================================
+     "amdgpu-flat-work-group-size"="min,max" Specify the minimum and maximum flat work group sizes that
+                                             will be specified when the kernel is dispatched. Generated
+                                             by the ``amdgpu_flat_work_group_size`` CLANG attribute [CLANG-ATTR]_.
+     "amdgpu-implicitarg-num-bytes"="n"      Number of kernel argument bytes to add to the kernel
+                                             argument block size for the implicit arguments. This
+                                             varies by OS and language (for OpenCL see
+                                             :ref:`opencl-kernel-implicit-arguments-appended-for-amdhsa-os-table`).
+     "amdgpu-max-work-group-size"="n"        Specify the maximum work-group size that will be specifed
+                                             when the kernel is dispatched.
+     "amdgpu-num-sgpr"="n"                   Specifies the number of SGPRs to use. Generated by
+                                             the ``amdgpu_num_sgpr`` CLANG attribute [CLANG-ATTR]_.
+     "amdgpu-num-vgpr"="n"                   Specifies the number of VGPRs to use. Generated by the
+                                             ``amdgpu_num_vgpr`` CLANG attribute [CLANG-ATTR]_.
+     "amdgpu-waves-per-eu"="m,n"             Specify the minimum and maximum number of waves per
+                                             execution unit. Generated by the ``amdgpu_waves_per_eu``
+                                             CLANG attribute [CLANG-ATTR]_.
+     ======================================= ==========================================================
+
 Code Object
 ===========
 
 The AMDGPU backend generates a standard ELF [ELF]_ relocatable code object that
 can be linked by ``lld`` to produce a standard ELF shared code object which can
 be loaded and executed on an AMDGPU target.
 
 Header

 .. [AMD-ROCm] `ROCm: Open Platform for Development, Discovery and Education Around GPU Computing <http://gpuopen.com/compute-product/rocm/>`__
 .. [AMD-ROCm-github] `ROCm github <http://github.com/RadeonOpenCompute>`__
 .. [HSA] `Heterogeneous System Architecture (HSA) Foundation <http://www.hsafoundation.com/>`__
 .. [ELF] `Executable and Linkable Format (ELF) <http://www.sco.com/developers/gabi/>`__
 .. [DWARF] `DWARF Debugging Information Format <http://dwarfstd.org/>`__
 .. [YAML] `YAML Ain't Markup Language (YAML™) Version 1.2 <http://www.yaml.org/spec/1.2/spec.html>`__
 .. [OpenCL] `The OpenCL Specification Version 2.0 <http://www.khronos.org/registry/cl/specs/opencl-2.0.pdf>`__
 .. [HRF] `Heterogeneous-race-free Memory Models <http://benedictgaster.org/wp-content/uploads/2014/01/asplos269-FINAL.pdf>`__
+.. [CLANG-ATTR] `Attributes in Clang <http://clang.llvm.org/docs/AttributeReference.html>`__