sequenced-before modification order consistency

Question

from http://en.cppreference.com :

Relaxed ordering Atomic operations tagged std::memory_order_relaxed are not synchronization operations, they do not order memory. They only guarantee atomicity and modification order consistency. For example, with x and y initially zero,

// Thread 1:
r1 = y.load(memory_order_relaxed); // A
x.store(r1, memory_order_relaxed); // B
// Thread 2:
r2 = x.load(memory_order_relaxed); // C 
y.store(42, memory_order_relaxed); // D

is allowed to produce r1 == r2 == 42 because, although A is sequenced-before B and C is sequenced before D, nothing prevents D from appearing before A in the modification order of y, and B from appearing before C in the modification order of x.

Question: What it is the thing that confer to the above code the property A is sequenced-before B and C is sequenced before D?

EDIT:

int A, B;

void foo()
{
    A = B + 1; (A)
    B = 0; (B)
}

lead to

$ gcc -O2 -S -masm=intel foo.c
$ cat foo.s
        ...
        mov     eax, DWORD PTR B
        mov     DWORD PTR B, 0
        add     eax, 1
        mov     DWORD PTR A, eax
        ...

under GCC 4.6.1 with -02 option

so we clearly see that (A) and (B) have been switched

Answer 1

The sequenced-before (not the same as happens-before) relationships are not specific to multithreading. They happen in single threaded programs as well. Any expression that ends with a semicolon is sequenced-before the next, so in this case A is sequenced before B and C before D because each of them is a full-expression.

From the Standard 1.9 Program execution 14:

Every value computation and side effect associated with a full-expression is sequenced before every value computation and side effect associated with the next full-expression to be evaluated.

You can find an explanation here:

Order of evaluation

Answer 2

"Sequenced-before" applies to the visible observable behaviour of your program. The compiler is free to reach that behaviour in any way it wants; it can reorder writes to memory or eliminate them completely, as long as the visible behaviour is the same.