C Efficiency Question

[utzoo!decvax!harpo!seismo!dmy]

The "register" storage class is useful for improving code efficiency.
However, if the compiler is smart enough to use all available registers,
then putting the "wrong" variables into registers should actually
decrease efficiency.  Does anyone have single examples or general rules
about when usage of "register" is actually less efficient?
(Mail to me [ harpo!seismo!dmy ] , I'll summarize on the net.)

--dmy--