The cl_intel_required_subgroup_size extension is no longer authored in plain text. Please refer to the HTML extension specification instead, which may be found here: https://www.khronos.org/registry/OpenCL/extensions/intel/cl_intel_required_subgroup_size.html The asciidoc source for this extension specification may be found here: https://github.com/KhronosGroup/OpenCL-Docs/blob/main/extensions/cl_intel_required_subgroup_size.asciidoc