Strorage-class specifier 'register' is not supported.
Refer to OpenCL Spec Section 6.8(g)
What is the behavior with register? Please report if you are not getting proper error message.
In case you just mean arrays that sit in registers rather than private DRAM backed memory:
In general the register file is not indexable, but a private array with static or simple indexing within an unrollable loop should convert to registers in many cases. You can see when by using the stream kernel analyzer.