diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -2241,7 +2241,10 @@
 Clang supports the
 ``__attribute__((amdgpu_flat_work_group_size(<min>, <max>)))`` attribute for the
 AMDGPU target. This attribute may be attached to a kernel function definition
-and is an optimization hint.
+and is an optimization hint. It is mandatory to use this attribute in some 
+situations. Because when the attribute is absent, the compiler assumes the 
+default maximum workgroup size of 256 but nowadays the workgroup size can legally go 
+to 1024. 
 
 ``<min>`` parameter specifies the minimum flat work-group size, and ``<max>``
 parameter specifies the maximum flat work-group size (must be greater than