To the best of my knowledge, current C compilers typically convert C programs (which obviously don't have a concept of a reference) into SSA form as one of the first steps after parsing. It's, to me, not obvious that this couldn't be done with C++ references either. (Yes, this would make writing a decent C++ compiler harder; but it's already so monstrously hard that this optimization, which is already in all major compilers anyway, would be a significant additional burden.)