The GCC Wiki says this about the memory model synchronization mode [Acquire/Release][1]: > To make matters a bit more complex, the interactions of non-atomic > variables are still the same. Any store before an atomic operation > must be seen in other threads that synchronize. For example: > > | -Thread 1- > | y = 20; > | x.store (10, memory_order_release); > | > | -Thread 2- > | if (x.load(memory_order_acquire) == 10) > | assert (y == 20); > > Since 'y' is not an atomic variable, the store to 'y' _happens-before_ > the store to 'x', so the assert cannot fail in this case. The > optimizers must still limit the operations performed on shared memory > variables around atomic operations. Now, what if I make 'y' an atomic variable (without imposing _happens-before_ restrictions)? | -Thread 1- | y.store (20, memory_order_relaxed); | x.store (10, memory_order_release); | | -Thread 2- | if (x.load(memory_order_acquire) == 10) | assert (y.load (memory_order_relaxed) == 20); Can the assert fail? Are there fewer requirements for atomic variables than for non-atomic variables? Or is the Wiki's restriction to non-atomic variables gratuitous and misleading here? [1]: https://gcc.gnu.org/wiki/Atomic/GCCMM/AtomicSync#Acquire.2BAC8-Release -- Regards, Dietmar Schindler
