It's conceivable, if you made mutexes compiler/language intrinsic, but as long as you're calling pthread_mutex_lock, the compiler has to assume that that pthread library, which is linked dynamically, is interchangeable, and can do anything it likes to memory. That includes mutating x
That hasn't inhibited optimizations for a long time. Disassemble a call to printf("Hello world") in optimized clang output and look at what it turns into.
Yes if the library is covered by a standard just like the language is, then the compiler can make assumptions. Also threads are a language feature in C and C++ now.
