Not sure what numbers you are talking about. If you use qsort from the C library the comparison function will not be inlined, but if you provide your own, this is no problem.
If just "providing my own" would help why wouldn't the stdlib benefit too? You're going to have to spell out what you think can actually work here if you want me to believe there's "no problem".
It would also, but nobody cares enough because qsort is already fast enough for most things, and if you cared it is simply enough to do yourself. Are you doubting that C compilers can devirtualize function calls? Here is a small example that illustrates this. The compiler dervirtualizes all calls than folds the result: https://godbolt.org/z/E6cMMr8vx