[kbsletten]

They're not saying that C++ compilers do the best possible register allocation, they're saying that C++ compilers generate a register allocation that works and doesn't contain bugs. Technically, spilling everything to memory and loading only what you need to issue instructions is "getting it right" by this definition. No compiler strives to get the "optimal" anything in the general case, but we do expect them to strive to be "correct" in all cases. The language we use determines which properties are included in our idea of "correctness".