clarify subgroup barrier behavior in non-uniform control flow (#525)

Author: bashbaug

env/validation_rules.asciidoc

@@ -117,6 +117,11 @@ And, the memory-order constraint in _Memory Semantics_ must be one of:
        supporting {CL_DEVICE_ATOMIC_ORDER_SEQ_CST} in
        {CL_DEVICE_ATOMIC_FENCE_CAPABILITIES}.
 
+In all OpenCL environments, for the *Barrier Instruction* *OpControlBarrier*,
+when the _Scope_ for _Execution_ is *Subgroup*, behavior is undefined unless
+all invocations in the subgroup execute the same dynamic instance of the
+instruction.
+
 In an OpenCL 1.2 environment,
 for the *Atomic Instructions*, the _Scope_ for _Memory_ must be *Device*,
 and the memory-order constraint in _Memory Semantics_ must be *Relaxed*.