If compilers could do this even with hours of time to spend, that would be absolutely revolutionary. I program in a high level language (C#) and my whole day is usually moving things to the stack instead of the heap, devirtualizing calls, making SoA/AoS changes etc. I doubt we'll ever be in a position where compilers can make these decisions well enough. Much less jit compilers and dynamic runtimes that need to do it with time constraints.