Register allocation is one of the most basic optimizations that a compiler can do. Some modern cpus can alias stack memory with internal registers, but it is st

Register allocation is one of the most basic optimizations that a compiler can do. Some modern cpus can alias stack memory with internal registers, but it is still not as fast as not spilling at all. You can enjoy -O0 today and the compiler will happily allocate stack slots for all your variables and keep them up to date (which is useful for debugging). But the difference between -O0 and -O3 is orders of magnitude on many programs.