It’s worse than that. Eventually everybody calls into code that hits hardware. That is the level that the compiler (ironically?) can no longer make guarantees. Registers change outside the scope of the currently running program all the time. Reading a register can cause other registers on a chip to change. Random chips with access to a shared memory bus can modify the memory that the comipler deduced was static. There be dragons everywhere at the hardware layer and no compiler can ever reason correctly about all of them, because, guess what, rev2 of the hardware could swap a footprint compatible chip clone that has undocumented behavior that. So even if you gave all you board information to the compiler, the program could only be verifiably correct for one potential state of one potential hardware rev.