Relation between bytecode instructions and processor operations

Question

Java specification guarantees primitive variable assignments are always atomic (expect for long and double types.

On the contrary, Fetch-and-Ad

Answer 1

Even if the i++ would translate into an X86 Fetch-And-Add instruction would change nothing because the memory mentionned in the Fetch-And-Add instruction refers to the local memory registres of the CPU and not to the general memory of the device/application. On a modern CPU, this property will extend to the local memory caches of the CPU and can even extend to the various caches used by the different cores for a multicores CPU but in the case of a multithreading application; there is absolutely no garanty that this distribution will extend to the copy of the memory used by the threads themselves.

In clear, in a multithread application, if a variable can be modified by different threads running at the same time then you must use some synchronisation mecanism provided by the system and you cannot rely on the fact that the instruction i++ occupies a single line of java code to be atomic.