A while back, there was some discussion about whether using the SRC directive with Keil C51 does or doesn't affect the compiler's optimisation:
http://www.8052.com/forum/read.phtml?id=19821&top=

Advice received from Keil by some correspondents seemed to conflict with the experience of others.

Well, now we seem to have a definitive statement from Keil:
http://www.keil.com/discuss/docs/thread1657.htm

In some earlier versions of C51, using the SRC directive did disable some optimisations, but in newer versions, using the SRC directive does not affect optimisation!