Returns the number of work-items in the subgroup. This value is no more than the maximum subgroup size and is implementation-defined based on a combination of the compiled kernel and the dispatch dimensions. This will be a constant value for the lifetime of the subgroup.
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Copyright © 2007-2013 The Khronos Group Inc. 
Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and/or associated documentation files (the
"Materials"), to deal in the Materials without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Materials, and to
permit persons to whom the Materials are furnished to do so, subject to
the condition that this copyright notice and permission notice shall be included
in all copies or substantial portions of the Materials.