The address of the function is taken, for example:

inline int g()
{
    return 1;
}
int (*fp)() = &g;

The function cannot be inlined, for example, a recursive function:

inline unsigned int fact(unsigned int n) {
    return n < 2 ? 1 : n * fact(n - 1);
}

The heuristic used by the compiler decides that it is better not to inline the function. This heuristic is influenced by -Ospace and -Otime. If you use -Otime, the compiler inlines more functions. You can override this heuristic by declaring a function with __forceinline. For example:

__forceinline int g()
{
    return 1;
}

See also --forceinline for more information.