Have you thoroughly & carefully reviewed all the information on the Keil site about function pointers and C51...?
function pointers and the '51 typically is a result of knowing that "this is how it is done" based on some PC experience.

I am, in no way stating that this applies to the OP (how would I know), but there are many that utterly refuse to accept that C on a '51 is not the same as C on a PC.

I recall a case in the old days when uCs did not exist and minicomputers were the rage. We had this $500,000 unit that did not perform any better than our 'economy' $200,000 unit. The obvious cause was that the software overhead killed the speed and thus the 'fast' unit could run no faster than the 'economy' unit. I spent a few months and did get to the goal of hardware controlled speed i.e. the hardware ran as fast as it could. I then took over another task and the guy that took over from me immediately stated "this is NOT how it is done" and started working. Alas after a couple of months the result was the same as when I started. Fortunately I had made sure my 'epos' was saved outside anybodys control.

Erik