One reader one writer, int or atomic_int

Question

I know this isn't a new issue, but I got confused after reading about c++11 memory fences;

If I have one reader thread and one writer thread.
Can I use an ordinary int?

    int x = 0; // global
writer    reader
x = 1;    printf("%d\n", x);

Is this behavior is undefined?
May I get an undefined value in the reader thread?
Or it's like using an std::atomic_uint_fast32_t or std::atomic<int>? So the value will get to the reader thread - eventually.

    std::atomic<int x = 0; // global
writer                                    reader
x.store(1, std::memory_order_relaxed);    printf("%d\n", x.load(std::memory_order_relaxed));

Does the answer depends on the platform I'm using? (x86 for example), so loading/storing an ordinary int is one CPU instruction?

If both of the behaviors are similar, should I expect the same performance for both of the types?

Answer 1

In short, never use a plain int to share in a multi-threaded environment.

The problem is not only your CPU, but your compiler's optimiser. gcc can (and will) optimise code like:

while(i == 1) {} into if(i==1) { while(1) {} }. Once it has checked the variable once, it does not have to reload the value again. that's separate from all the other possible issues, of seeing half-written values (which actually won't usually occur on x86 ints).

measuring the effect of atomic is very hard -- in many cases CPUs can highly optimise the accesses, in others they are much slower. You really have to benchmark in practice.