I'm skeptical. I doubt that the whole of qsort gets inlined (it's big), and the best the compiler can do is to clone qsort to const-propagate the function point

I'm skeptical. I doubt that the whole of qsort gets inlined (it's big), and the best the compiler can do is to clone qsort to const-propagate the function pointer for it to get inlined. In my similar experiments with std::sort with a function pointer only gcc does this with -O3.