AMDGPU: Add some notes about amdgpu-flat-work-group-size

[arsenm]

llvm/docs/AMDGPUUsage.rst

1000

I suppose this first sentence could use clarification away from dispatched

[arsenm]

ping

[yaxunl]

llvm/docs/AMDGPUUsage.rst

1002–1007

Clang always adds this function attribute to the kernel. The implicit default value specified by Clang is 1,256 for OpenCL and 1,1024 for HIP.

[arsenm]

Clarify default is backend default

[yaxunl]

llvm/docs/AMDGPUUsage.rst

1004

if the actual block size or workgroup size exceeds the limit, the behaviour will be undefined. For example, even if there is only one active thread but the thread local id exceeds the limit, the behaviour is undefined.

[scchan]

llvm/docs/AMDGPUUsage.rst

1004

I agree, the nuance here is to refer to the actual work group size at execution time exceeding the limit rather than the number of logical active lanes.

[arsenm]

Reword again. I was trying to express you can't do what vulkan was doing and increasing the set of active lanes beyond the bounds

[yaxunl]

LGTM. Thanks.

[arsenm]

30fd35f59ceb4c00a550b82af767a5b9cf9e252d

llvm/docs/AMDGPUUsage.rst

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]_.
-                                             The implied default value is 1,1024.
+                                             The IR implied default value is 1,1024. Clang may emit this attribute
+                                             with more restrictive bounds depending on language defaults.
+                                             If the actual block or workgroup size exceeds the limit at any point during
+                                             the execution, the behavior is undefined. For example, even if there is
+                                             only one active thread but the thread local id exceeds the limit, the
+                                             behavior is undefined.
 
      "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-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