Question about in-line expansion of subprograms with "cc"

[Paul C. Clements]

The man entry for "cc" on the Suns says the following:

     Filenames ending in .il are taken to be inline expansion code
     template files; these are used to expand calls to selected
     routines in-line when the -O option is in effect.  

But that's ALL it says.   Does anyone know exactly what a "code template
file" is supposed to contain?   Is there one per subprogram?   One for
everything?   Are particular file names assumed?   Do I need to #include
them?   Link them?   And what are the semantics of the in-line expansion?  
Do I have to worry about collisions between variable names in the called
and calling modules?   And so forth.

Thanks in advance,
P. C. Clements