No, maybe you misunderstand the problem here? Given a specific program, yes it is measurable and predictable. A compiler has to deal with arbitrary programs,

No, maybe you misunderstand the problem here? Given a specific program, yes it is measurable and predictable. A compiler has to deal with arbitrary programs, actually - small fragments of arbitrary programs at a time. Even if you could have an oracle giving you very precise measurements for the program fragment the compiler is dealing with, it won't hold up in practice. Consider a small representative example. Suppose the compiler is compiling a function, and it did the best possible job. Now, inlining this function at two callsites could have completely different second order effects like further optimizations, register pressure. There are partial solutions of course, people have spent their entire careers dealing with this specific problem. Rest assured that whatever you can come up with after thinking for a minute has been tried in 1970.