Compiler optimizations are one of the primary culprits in making it difficult to reason about lock-free programs. Semantics-preserving optimizations in a single-threaded context are not necessarily semantics-preserving in a multi-threaded, lock-free context.

For example, if you're writing a spin-lock, the compiler may lift a read of the lock value out of a loop because, assuming a single thread, the value will never change. This can result in a non-terminating spin-lock. For more see Linux's ACCESS_ONCE.

The example you gave is unfortunate but the consequences of optimizing loops carelessly can be serious.