"diakgcn" is the Diablo GCN kernel, it is designed for GPUs built on GCN technology (Radeon HD7000 serries)
CG miner auto-selects the best kernel for your hardware and rarely gets it wrong so just use the default kernel and you'll be fine.
Ah, OK. At one time there was a diagnostic kernel probably started with diag* instead of diak*. Thanks for the correction.
Sam
diakgcn is the Diapolo Kernel for GCN. It has nothing to do with diablo and pretty much never outperformed any other kernel so it is never selected automatically by cgminer.