Do atomics in C++11 cache repeatable reads in a register or are they only atomic?

Do atomics cache repeatable reads in a register ? Or are they just only atomic, i.e. a read may not be split in multiple parts ?

MSVC++, clang++ / clang-cl and g++ don't cache atomic reads without memory ordering:

#include <atomic>

using namespace std;

int x( atomic_int const &ai )
{
    int
        a = ai.load( memory_order_relaxed ),
        b = ai.load( memory_order_relaxed );
    return a + b;
}

g++:

movl    (%rdi), %edx
movl    (%rdi), %eax
addl    %edx, %eax
ret

clang-cl:

mov eax, dword ptr [rcx]
add eax, dword ptr [rcx]
ret

cl:

mov edx, DWORD PTR [rcx]
mov eax, DWORD PTR [rcx]
add eax, edx
ret 0`