x86 processors have a LOCK instruction prefix, which makes some instructions atomic including increment. Increment is nontrivial to make atomic, because there are two memory accesses: read X and write back X+1. It's a bit slow, because it has to inform every other core in the system, "Hey, I just read this memory address and I'm about to write something back to it, so don't don't use it until I'm done." C++ has functions to do this like std::atomic_fetch_add, but they're clunky to use so people often forget.