optimizer seems to give up

[Frank Puck]

We have a very big function calculating some numerical values depending on 
multiple numeric input values.
The problem is the compiler seems to give up on optimizing this function.
We deduce this because of the performance is quite bad and from looking into 
the generated assembler code -- there are stack variables used for 
loop-indexes.
Is there any way to dell the optimizer not to give up?
We tried already breaking this function into multiple functions -- problem 
is that this is not as easy as it sounds
and would make the code quite unreadable -- because of plenty of temporary 
variables are used and would have to be passed to the separate function.
I would much more appreciate if the compiler would not just give up.
At least the compiler should print a warning if he decides to give up 
optimizing.