It gets harder when you want to do link time optimization. Without it, the approach is inferior to ahead of time optimization with some programmer input... (Degeneralization and devirtualization rather than specialization.)
It's fine if it's inferior to ahead-of-time optimization with programmer input, because you don't have time for a programmer to optimize the kernel code anew every time you spawn a process or open a pipe or a socket. It just needs to be faster than what the kernel is doing now when you call read() or write() or whatever, without being too much slower when you open the file (etc.)
Like, ahead-of-time optimization with programmer input might take a few hours to a few days, and that's not really an acceptable execution time for the open() system call.
