Clarify that clustered subgroup operations may lead to undefined behaviour (#734)

* Clarify that clustered subgroup operations may lead to undefined behaviour

... when the cluster size is not an integer constant that is a power of two.

Signed-off-by: Kevin Petit <kevin.petit@arm.com>
Change-Id: I0116b75808c7fe5df7146a33f6b36698f2c4e6b7

* Update ext/cl_khr_subgroup_extensions.asciidoc

Co-authored-by: Ben Ashbaugh <ben.ashbaugh@intel.com>

* address review comment

Change-Id: I069696608b4d815978677a5c19ff6c405abb0824

Co-authored-by: Ben Ashbaugh <ben.ashbaugh@intel.com>

Author: kpet

ext/cl_khr_subgroup_extensions.asciidoc

@@ -641,8 +641,7 @@ This extension adds support for clustered reductions that operate on a subset of
 This section describes arithmetic operations that are performed on a subset of work items in a subgroup, referred to as a cluster.
 A cluster is described by a specified cluster size.
 Work items in a subgroup are assigned to clusters such that for cluster size _n_, the _n_ work items in the subgroup with the smallest subgroup local IDs are assigned to the first cluster, then the _n_ remaining work items with the smallest subgroup local IDs are assigned to the next cluster, and so on.
-The specified cluster size must be an integer constant expression that is a power-of-two.
-Behavior is undefined if the specified cluster size is greater than the maximum size of a subgroup within the dispatch.
+Behavior is undefined if the specified cluster size is not an integer constant expression, is not a power-of-two, or is greater than the maximum size of a subgroup within the dispatch.
 
 ===== Arithmetic Operations